Omega OM-USB-5203 User manual

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Servicing North America:
U.S.A.: Omega Engineering, Inc., One Omega Drive, P.O. Box 4047
Stamford, CT 06907-0047 USA
Toll-Free: 1-800-826-6342 (USA & Canada only)
Customer Service: 1-800-622-2378 (USA & Canada only)
Engineering Service: 1-800-872-9436 (USA & Canada only)
Tel: [phone] Fax: [phone]
e-mail: [email protected]
For Other Locations Visit omega.com/worldwide
omega.com [email protected]
The information contained in this document is believed to be correct, but OMEGA accepts no liability for any errors it contains, and reserves
the right to alter specifications without notice.
WARNING: These products are not designed for use in, and should not be used for, human applications.

3
Table of Contents
Preface
About this User’s Guide.......................................................................................................................5
What you will learn from this user's guide.........................................................................................................5
Conventions in this user's guide .........................................................................................................................5
Where to find more information.........................................................................................................................5
Chapter 1
Introducing the OM-USB-5203.............................................................................................................6
Logging data with the OM-USB-5203.............................................................................................................................. 6
Functional block diagram ...................................................................................................................................7
Chapter 2
Installing the OM-USB-5203.................................................................................................................8
Unpacking...........................................................................................................................................................8
Installing the software ........................................................................................................................................8
Installing the hardware .......................................................................................................................................8
Configuring the hardware...................................................................................................................................8
Configuring data logging options...................................................................................................................................... 8
Chapter 3
Sensor Connections .............................................................................................................................9
Screw terminal pinout.........................................................................................................................................9
Sensor input terminals (C0H/C0L to C7H/C7L)............................................................................................................... 9
CJC sensors......................................................................................................................................................................10
Current excitation output terminals (±I1 to ±I4)..............................................................................................................10
Four-wire, two sensor common terminals (4W01 to 4W67)............................................................................................10
Two sensor common terminals (IC01 to IC67)................................................................................................................10
Digital terminals (DIO0 to DIO7)....................................................................................................................................10
Power output terminals (+5V)..........................................................................................................................................10
Ground terminals (GND).................................................................................................................................................10
Thermocouple connections...............................................................................................................................11
Wiring configuration........................................................................................................................................................11
RTD and thermistor connections......................................................................................................................11
Two-wire configuration...................................................................................................................................................12
Three-wire configuration .................................................................................................................................................13
Four-wire configuration...................................................................................................................................................13
Semiconductor sensor measurements...............................................................................................................14
Wiring configuration........................................................................................................................................................14
Digital I/O connections.....................................................................................................................................15
Configuring the DIO channels to generate alarms ...........................................................................................................15
Chapter 4
Functional Details ...............................................................................................................................16
Thermocouple measurements...........................................................................................................................16
Cold junction compensation (CJC)..................................................................................................................................16
Data linearization.............................................................................................................................................................16
Open-thermocouple detection (OTD) ..............................................................................................................................16
RTD and thermistor measurements ..................................................................................................................17
Data linearization.............................................................................................................................................................17
External components ........................................................................................................................................17

OM-USB-5203 User's Guide
4
Screw terminals................................................................................................................................................................18
USB connector.................................................................................................................................................................18
LED .................................................................................................................................................................................18
CompactFlash memory card slot......................................................................................................................................19
Data logging button .........................................................................................................................................................19
External power supply......................................................................................................................................20
Disconnecting the OM-USB-5203 from the computer.....................................................................................20
Transferring binary data after a logging session...............................................................................................20
Converting binary data after a logging session.................................................................................................20
Chapter 5
Specifications......................................................................................................................................21
Analog input .....................................................................................................................................................21
Channel configurations.....................................................................................................................................21
Compatible sensors...........................................................................................................................................22
Accuracy...........................................................................................................................................................22
Thermocouple measurement accuracy.............................................................................................................................22
Semiconductor sensor measurement accuracy.................................................................................................................23
RTD measurement accuracy............................................................................................................................................23
Thermistor measurement accuracy ..................................................................................................................................24
Throughput rate to PC ......................................................................................................................................25
Digital input/output...........................................................................................................................................25
Temperature alarms..........................................................................................................................................26
Memory ............................................................................................................................................................26
Microcontroller.................................................................................................................................................26
Data Logging....................................................................................................................................................27
Real time clock.................................................................................................................................................27
USB +5V voltage .............................................................................................................................................28
Power................................................................................................................................................................28
USB specifications ...........................................................................................................................................28
Current excitation outputs (Ix+) .......................................................................................................................29
Environmental ..................................................................................................................................................29
Mechanical .......................................................................................................................................................29
Screw terminal connector.................................................................................................................................30

5
Preface
About this User’s Guide
What you will learn from this user's guide
This user's guide describes the Omega Engineering OM-USB-5203 data acquisition device and lists device
specifications.
Conventions in this user's guide
For more information
Text presented in a box signifies additional information and helpful hints related to the subject matter you are
reading.
Caution! Shaded caution statements present information to help you avoid injuring yourself and others,
damaging your hardware, or losing your data.
bold text Bold text is used for the names of objects on a screen, such as buttons, text boxes, and check boxes.
italic text Italic text is used for the names of manuals and help topic titles, and to emphasize a word or phrase.
Where to find more information
Additional information about OM-USB-5203 hardware is available on our website at www.omega.com. You
can also contact Omega Engineering by phone, fax, or email with specific questions.
Phone: [phone]
Fax: [phone]
Email: [email protected]

6
Chapter 1
Introducing the OM-USB-5203
The OM-USB-5203 is a USB 2.0 full-speed, temperature input device that is supported under popular
Microsoft®Windows®operating systems. The OM-USB-5203 is fully compatible with both USB 1.1 and USB
2.0 ports.
The OM-USB-5203 provides eight differential input channels that are software-programmable for different
sensor categories including thermocouple, RTDs, thermistors, and semiconductor sensors.
Eight independent, TTL-compatible digital I/O channels are provided to monitor TTL-level inputs,
communicate with external devices, and to generate alarms. The digital I/O channels are software-
programmable for input or output.
With the OM-USB-5203, you can take measurements from four sensor categories:
Thermocouple – types J, K, R, S, T, N, E, and B
Resistance temperature detectors (RTDs) – 2, 3, or 4-wire measurements of 100 Ω platinum RTDs
Thermistors – 2, 3, or 4-wire measurements
Semiconductor temperature sensors – LM35 or equivalent
The OM-USB-5203 provides a 24-bit analog-to-digital (A/D) converter for each pair of differential analog input
channels. Each pair of differential inputs constitutes a channel pair.
You can connect a different category of sensor to each channel pair, but you cannot mix categories among the
channels that constitute a channel pair (although it is permissible to mix thermocouple types).
The OM-USB-5203 provides two integrated cold junction compensation (CJC) sensors for thermocouple
measurements, and built-in current excitation sources for resistive sensor measurements.
An open thermocouple detection feature lets you detect a broken thermocouple. An on-board microprocessor
automatically linearizes the measurement data according to the sensor category.
The OM-USB-5203 features eight independent temperature alarms. Each alarm controls an associated digital
I/O channel as an alarm output. The input to each alarm is one of the temperature input channels. The output of
each alarm is software configurable as active high or low. You set up the temperature threshold conditions to
activate each alarm. When an alarm is activated, the associated DIO channel is driven to the output state.
You can log your sensor measurements to a CompactFlash®memory card. CompactFlash is a removable non-
volatile storage device. A 512 MB CompactFlash memory card is shipped with the device to store your data.
For more information, refer to Logging data with the OM-USB-5203 below.
External power is required for data logging operations
Due to processing limitations, you cannot log data to the memory card when the OM-USB-5203 is connected to
your computer's active USB bus. When operating as a data logger, disconnect the USB cable from the
computer, and connect the external power supply shipped with the device.
The OM-USB-5203 is a standalone plug-and-play device. External power is required for data logging mode
only. All configurable options are software programmable. The OM-USB-5203 is fully software calibrated.
Logging data with the OM-USB-5203
Due to processing limitations, you cannot log data to the memory card when the is connected to an active USB
port. When operating as a data logger, disconnect the USB cable from the computer, and connect the external
power supply shipped with the device.
The OM-USB-5203 has many software-selectable options for setting up data logging.
You can record:
temperature (°C) or raw data from selected input channels
timestamp data
CJC sensor readings

OM-USB-5203 User's Guide Introducing the OM-USB-5203
7
You can specify the number of seconds between samples. You can begin logging data at power up, when you
press the data logging button, or at a specific date and time.
Data is stored on the memory card in binary files. After logging measurements, you can transfer the files to your
computer. You can use InstaCal to convert the files to .csv format for use in Microsoft Excel files, or to .txt
format for use in other applications.
Functional block diagram
OM-USB-5203 functions are illustrated in the block diagram shown here.
Figure 1.OM-USB-5203 functional block diagram

8
Chapter 2
Installing the OM-USB-5203
Unpacking
As with any electronic device, you should take care while handling to avoid damage from static
electricity. Before removing the device from its packaging, ground yourself using a wrist strap or by simply
touching the computer chassis or other grounded object to eliminate any stored static charge.
If any components are missing or damaged, notify Omega Engineering immediately by phone, fax, or e-mail.
Installing the software
Refer to the Software User's Guide for instructions on installing the software on the CD. This booklet is
available in PDF at www.omega.com/manuals/manualpdf/M4803.pdf.
Installing the hardware
To connect the OM-USB-5203 to your system, turn on your computer and connect the USB cable to an
available USB port on the computer or to an external USB hub connected to the computer. Connect the other
end of the USB cable to the USB connector on the device. No external power is required. A green Status LED
turns on when power is applied.
When you connect the device for the first time, a Found New Hardware dialog opens when the operating
system detects the device. The dialog closes after the device is installed.
Configuring the hardware
All hardware configuration options on the OM-USB-5203 are software-selectable. Use InstaCal to set the
thermocouple type for each channel. Configuration options are stored on an isolated microcontroller in
EEPROM – which is non-volatile memory on the OM-USB-5203 – and are loaded on power-up. The factory
default configuration is Type J thermocouple.
Warm up
Allow the OM-USB-5203 to warm up for 30 minutes before taking measurements. This warm up time
minimizes thermal drift and achieves the specified rated accuracy of measurements.
Configuring data logging options
The following data logging options are programmable with software.
select the input channels to log
set the data format
set the start mode
set up alarm conditions
copy and convert saved binary files
delete data files
All data logging options are stored on the OM-USB-5203 in non-volatile memory in EEPROM, and are loaded
on power up.

9
Chapter 3
Sensor Connections
The OM-USB-5203 supports the following temperature sensor types:
Thermocouple – types J, K, R, S, T, N, E, and B
Resistance temperature detectors (RTDs) – 2, 3, or 4-wire measurement modes of 100 Ω platinum RTDs.
Thermistors – 2, 3, or 4-wire measurement modes.
Semiconductor temperature sensors – LM35 or equivalent
Sensor selection
The type of sensor you select will depend on your application needs. Review the temperature ranges and
accuracies of each sensor type to determine which is best suited for your application.
Screw terminal pinout
The OM-USB-5203 has four rows of screw terminals – two rows on the top edge of the housing, and two rows
on the bottom edge. Each row has 26 connections. Between each bank of screw terminals are two integrated
CJC sensors used for thermocouple measurements. Signals are identified in Figure 2.
Figure 2. OM-USB-5203 screw terminal pin numbers
Use 16 AWG to 30 AWG wire for your signal connections.
Tighten screw terminal connections
Fully tighten the screw when making connections to the screw terminals. Simply touching the top of the screw
terminal is not sufficient to make a proper connection.
Sensor input terminals (C0H/C0L to C7H/C7L)
You can connect up to eight temperature sensors to the differential sensor inputs (C0H/C0L to C7H/C7L).
Supported sensor categories include thermocouples, RTDs, thermistors, or semiconductor sensors.
Do not mix sensor categories within channel pairs. It is permitted to mix thermocouple types (J, K, R, S, T, N,
E, and B) within channel pairs, however.

OM-USB-5203 User's Guide Sensor Connections
10
Do not connect two different sensor categories to the same channel pair
The OM-USB-5203 provides a 24 bit A/D converter for each channel pair. Each channel pair can monitor one
sensor category. To monitor a sensor from a different category, connect the sensor to a different channel pair
(input terminals).
CJC sensors
The OM-USB-5203 has two built in high-resolution temperature sensors. One sensor is located on the right side
of the package, and one sensor is located on the left side.
Current excitation output terminals (±I1 to ±I4)
The OM-USB-5203 has four dedicated pairs of current excitation output terminals (±I1 to ±I4). These terminals
have a built-in precision current source to provide excitation for the resistive sensors used for RTD and
thermistor measurements.
Each current excitation terminal is dedicated to one pair of sensor input channels:
I1+ is the current excitation source for channel 0 and channel 1
I2+ is the current excitation source for channel 2 and channel 3
I3+ is the current excitation source for channel 4 and channel 5
I4+ is the current excitation source for channel 6 and channel 7
Four-wire, two sensor common terminals (4W01 to 4W67)
These terminals are used as the common connection for four-wire configurations with two RTD or thermistor
sensors.
Two sensor common terminals (IC01 to IC67)
These terminals are used as the common connection for two-wire configurations with two RTD or thermistor
sensors.
Digital terminals (DIO0 to DIO7)
You can connect up to eight digital I/O lines to the screw terminals labeled DIO0 to DIO7. Each terminal is
software-configurable for input or output.
If a digital bit is set up as an alarm, the bit is configured for output on power-up, and assumes the state defined
by the alarm configuration.
Power output terminals (+5V)
The two +5V output terminals are isolated (500 VDC) from the USB +5V.
Caution! Each +5V terminal is an output. Do not connect to an external power supply to these terminals or
you may damage the OM-USB-5203 and possibly the computer.
Ground terminals (GND)
The six ground terminals (GND) provide a common ground for the input channels and DIO bits, and are isolated
(500 VDC) from the USB GND.

OM-USB-5203 User's Guide Sensor Connections
11
Thermocouple connections
Connect thermocouples to the OM-USB-5203 such that they are floating with respect to GND. A thermocouple
consists of two dissimilar metals that are joined together at one end. When the junction of the metals is heated
or cooled, a voltage is produced that correlates to temperature.
The OM-USB-5203 makes fully-differential thermocouple measurements without the need of ground-
referencing resistors. A 32-bit floating point value in either a voltage or temperature format is returned by
software. An open thermocouple detection feature is available for each analog input which automatically detects
an open or broken thermocouple.
Use InstaCal to select the thermocouple type (J, K, R, S, T, N, E, and B) and one or more sensor input channels
to connect the thermocouple.
Wiring configuration
Connect the thermocouple to the OM-USB-5203 using a differential configuration, as shown in Figure 3.
Figure 3. Typical thermocouple connection
Connect thermocouples to the OM-USB-5203 such that they are floating with respect to GND. The OM-USB-
5203ground pins are isolated from earth ground, so you can connect thermocouple sensors to voltages
referenced to earth ground as long as the isolation between the GND pins and earth ground is maintained.
When thermocouples are attached to conductive surfaces, the voltage differential between multiple
thermocouples must remain within ±1.4 V. For best results, use insulated or ungrounded thermocouples when
possible.
Maximum input voltage between analog input and ground
The absolute maximum input voltage between an analog input and the isolated GND pins is ±24 VDC when the
OM-USB-5203 is powered on or off.
If you need to increase the length of your thermocouple, use the same type of thermocouple wires to minimize
the error introduced by thermal EMFs.
RTD and thermistor connections
A resistance temperature detector (RTD) measures temperature by correlating the resistance of the RTD
element with temperature. A thermistor is a thermally-sensitive resistor that is similar to an RTD in that its
resistance changes with temperature – thermistors show a large change in resistance that is proportional to a
small change in temperature. The main difference between RTD and thermistor measurements is the method
used to linearize the sensor data.
RTDs and thermistors are resistive devices that require an excitation current to produce a voltage drop that can
be measured differentially across the sensor. The device has four built-in current excitation sources (±I1 to ±I4)
for measuring resistive type sensors. Each current excitation terminal is dedicated to one channel pair.
The OM-USB-5203 makes two, three, and four-wire measurements of RTDs (100 Ω platinum type) and
thermistors.

OM-USB-5203 User's Guide Sensor Connections
12
Use InstaCal to select the sensor type and the wiring configuration. Once the resistance value is calculated, the
value is linearized in order to convert it to a temperature value. A 32-bit floating point value in either
temperature or resistance is returned by software.
RTD maximum resistance
Resistance values greater than 660 Ω cannot be measured in RTD mode. The 660 Ω resistance limit includes the
total resistance across the current excitation (±Ix) pins, which is the sum of the RTD resistance and the lead
resistances.
Thermistor maximum resistance
Resistance values greater than 180 kΩ cannot be measured in thermistor mode. The 180 kΩ resistance limit
includes the total resistance across the current excitation (±Ix) pins, which is the sum of the thermistor
resistance and the lead resistance.
Two-wire configuration
The easiest way to connect an RTD sensor or thermistor to the OM-USB-5203 is with a two-wire configuration,
since it requires the fewest connections to the sensor. With this method, the two wires that provide the RTD
sensor with its excitation current also measure the voltage across the sensor.
Since RTDs exhibit a low nominal resistance, measurement accuracy can be affected due to the lead wire
resistance. For example, connecting lead wires that have a resistance of 1 Ω (0.5 Ω each lead) to a 100 Ω
platinum RTD will result in a 1% measurement error.
With a two-wire configuration, you can connect either one sensor per channel pair, or two sensors per channel
pair.
Two-wire, single-sensor
A two-wire single-sensor measurement configuration is shown in Figure 4.
Figure 4. Two-wire, single RTD or thermistor sensor measurement configuration
When you select a two-wire single sensor configuration with InstaCal, connections to C#H and C#L are made
internally.
Two-wire, two sensor
A two-wire, two-sensor measurement configuration is shown in Figure 5.
Figure 5. Two-wire, two RTD or thermistor sensors measurement configuration
When you select a two-wire, two sensor configuration with InstaCal, connections to C#H (first sensor) and
C#H/C#L (second sensor) are made internally.
When configured for two-wire mode, both sensors must be connected to obtain proper measurements.

OM-USB-5203 User's Guide Sensor Connections
13
Three-wire configuration
A three-wire configuration compensates for lead-wire resistance by using a single-voltage sense connection.
With a three-wire configuration, you can connect only one sensor per channel pair. A three-wire measurement
configuration is shown in Figure 6.
Figure 6. Three-wire RTD or thermistor sensor measurement configuration
When you select a three-wire sensor configuration with InstaCal, the OM-USB-5203 measures the lead
resistance on the first channel (C#H/C#L) and measures the sensor itself using the second channel (C#H/C#L).
This configuration compensates for any lead-wire resistance and temperature change in lead-wire resistance.
Connections to C#H for the first channel and C#H/C#L of the second channel are made internally.
Three-wire compensation
For accurate three wire compensation, the individual lead resistances connected to the ±I# pins must be of equal
resistance value.
Four-wire configuration
With a four-wire configuration, connect two sets of sense/excitation wires at each end of the RTD or thermistor
sensor. This configuration completely compensates for any lead-wire resistance and temperature change in lead-
wire resistance.
Connect your sensor with a four-wire configuration when your application requires very high accuracy
measurements. Examples of a four-wire single-sensor measurement configuration are shown in Figure 7 and
Figure 8.
You can configure the OM-USB-5203 with either a single-sensor-per-channel, or a two-sensor–per-channel
pair.
Four-wire, single-sensor
A four-wire, single-sensor connected to the first channel of a channel pair is shown in Figure 7.
Figure 7. Four-wire, single RTD or thermistor sensor measurement configuration
A four-wire, single-sensor connected to the second channel of a channel pair is shown in Figure 8.

OM-USB-5203 User's Guide Sensor Connections
14
Figure 8. Four-wire, single RTD or thermistor sensor measurement configuration
A four-wire, two-sensor measurement configuration is shown in Figure 9.
Figure 9. Four-wire, two RTD or thermistor sensors measurement configuration
When configured for two-wire mode, both sensors must be connected to obtain proper measurements.
Semiconductor sensor measurements
Semiconductor sensors are suitable over a range of approximately -40 °C to 125 °C, where an accuracy of
±2 °C is adequate. The temperature measurement range of a semiconductor sensor is small when compared to
thermocouples and RTDs. However, semiconductor sensors can be accurate, inexpensive and easy to interface
with other electronics for display and control.
The OM-USB-5203 makes high-resolution measurements of semiconductor sensors. The software outputs the
measurement data as a 32-bit floating point value in either voltage or temperature.
Use InstaCal to select the sensor type (LM35, TMP35 or equivalent) and the sensor input channel to connect the
sensor.
Wiring configuration
Connect the semiconductor sensor to the OM-USB-5203 using a single-ended configuration, as shown in Figure
10. The OM-USB-5203device provides +5V and GND pins for powering the sensor.
Figure 10. Semiconductor sensor measurement configuration

OM-USB-5203 User's Guide Sensor Connections
15
Digital I/O connections
You can connect up to eight digital I/O lines to the screw terminals labeled DIO0 to DIO7. You can configure
each digital bit for either input or output. All digital I/O lines are pulled up to +5 V with a 47 kΩ resistor
(default). You can request the factory to configure the resistor for pull-down to ground. You can configure each
digital bit for either input or output.
Caution! If a digital bit is set up as an alarm, the bit will be configured for output on power-up, and assume
the state defined by the alarm configuration.
When you configure the digital bits for input, you can use the OM-USB-5203 digital I/O terminals to detect the
state of any TTL-level input. Refer to the schematic shown in Figure 11. If you set the switch to the +5 V input,
DIO0 reads TRUE (1). If you move the switch to GND, DIO0 reads FALSE (0).
Figure 11. Schematic showing switch detection by digital channel DIO0
Caution! All ground pins are common and are isolated from earth ground. If a connection is made to earth
ground when using digital I/O and conductive thermocouples, the thermocouples are no longer
isolated. In this case, thermocouples must not be connected to any conductive surfaces that may be
referenced to earth ground.
For general information regarding digital signal connections and digital I/O techniques, refer to the Guide to
Signal Connections (available on our web site at www.omega.com/manuals/manualpdf/M4830.pdf).
Configuring the DIO channels to generate alarms
The OM-USB-5203 features eight independent temperature alarms. All alarm options are software configurable.
When a digital bit is configured as an alarm, that bit is configured as an output on the next power cycle and
assumes the state defined by the alarm configuration.
Each alarm controls an associated digital I/O channel as an alarm output. The input to each alarm is one of the
temperature input channels. You set up the temperature conditions to activate an alarm, and the output state of
the channel (active high or low) when activated. When an alarm is activated, its associated DIO channel is
driven to the output state specified.
The alarm configurations are stored in non-volatile memory and are loaded on power up. The temperature
alarms function both in data logging mode and while attached to the USB port on a computer.

16
Chapter 4
Functional Details
Thermocouple measurements
A thermocouple consists of two dissimilar metals that are joined together at one end. When the junction of the
metals is heated or cooled, a voltage is produced that correlates to temperature.
The OM-USB-5203 hardware level-shifts the thermocouple output voltage into the A/D’s common mode input
range by applying +2.5 V to the thermocouple low side at the C#L input. Always connect thermocouple sensors
to the OM-USB-5203 in a floating fashion. Do not attempt to connect the thermocouple low side C#L to GND
or to a ground referencing resistor.
Cold junction compensation (CJC)
When you connect the thermocouple sensor leads to the sensor input channel, the dissimilar metals at the
OM-USB-5203 terminal blocks produce an additional thermocouple junction. This junction creates a small
voltage error term which must be removed from the overall sensor measurement using a cold junction
compensation technique. The measured voltage includes both the thermocouple voltage and the cold junction
voltage. To compensate for the additional cold junction voltage, the OM-USB-5203 subtracts the cold junction
voltage from the thermocouple voltage.
The OM-USB-5203 has two high-resolution temperature sensors that are integrated into the design of the
OM-USB-5203. One sensor is located on the right side of the package, and one sensor is located at the left side.
The CJC sensors measure the average temperature at the terminal blocks so that the cold junction voltage can be
calculated. A software algorithm automatically corrects for the additional thermocouples created at the terminal
blocks by subtracting the calculated cold junction voltage from the analog input's thermocouple voltage
measurement.
Increasing the thermocouple length
If you need to increase the length of your thermocouple, use the same type of thermocouple wires to minimize
the error introduced by thermal EMFs.
Data linearization
After the CJC correction is performed on the measurement data, an on-board microcontroller automatically
linearizes the thermocouple measurement data using National Institute of Standards and Technology (NIST)
linearization coefficients for the selected thermocouple type.
The measurement data is then output as a 32-bit floating point value in the configured format (voltage or
temperature).
Open-thermocouple detection (OTD)
The OM-USB-5203 is equipped with an open-thermocouple detection for each analog input channel. With
OTD, any open-circuit or short-circuit condition at the thermocouple sensor is detected by the software. An
open channel is detected by driving the input voltage to a negative value outside the range of any thermocouple
output. The software recognizes this as an invalid reading and flags the appropriate channel. The software
continues to sample all channels when OTD is detected.
Input leakage current
With open-thermocouple detection enabled, 105 nA (max.) of input leakage current is injected into the
thermocouple. This current can cause an error voltage to develop across the lead resistance of the thermocouple
that is indistinguishable from the thermocouple voltage you are measuring. You can estimate this error voltage
with the following formula:
error voltage = resistance of the thermocouple x 105 nA

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To reduce the error, reduce the length of the thermocouple to lower its resistance, or lower the AWG of the wire
by using a wire with a larger diameter. With open-thermocouple detection disabled, 30 nA (max) of input
leakage current is injected into the thermocouple.
RTD and thermistor measurements
RTDs and thermistors are resistive devices that require an excitation current to produce a voltage drop that can
be measured differentially across the sensor. The OM-USB-5203 measures the sensor resistance by forcing a
known excitation current through the sensor and then measuring (differentially) the voltage across the sensor to
determine its resistance.
After the voltage measurement is made, the resistance of the RTD is calculated using Ohms law – the sensor
resistance is calculated by dividing the measured voltage by the current excitation level (±Ix) source. The value
of the ±Ix source is stored in local memory.
Once the resistance value is calculated, the value is linearized in order to convert it to a temperature value. The
measurement is returned by software as a 32-bit floating point value in a voltage, resistance or temperature
format.
Data linearization
An on-board microcontroller automatically performs linearization on RTD and thermistor measurements.
RTD measurements are linearized using a Callendar-Van Dusen coefficients algorithm (you select DIN,
SAMA, or ITS-90).
Thermistor measurements are linearized using a Steinhart-Hart linearization algorithm (you supply the
coefficients from the sensor manufacturer's data sheet).
External components
The OM-USB-5203 has the following external components, as shown in Figure 12.
Screw terminals
USB connector
LED
CompactFlash slot with memory card
1
Screw terminal pins 1 to 26
4
Data logging button
2
Screw terminal pins 27 to 52
5
CompactFlash memory card
3 LED 6 USB connector
Figure 12. External component locations

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18
Screw terminals
Use the screw terminals for connecting temperature sensors and digital I/O lines. These terminals also provide
ground and power output connections. Refer to the "Sensor Connections" chapter for screw terminal
descriptions.
USB connector
When not logging data, connect the USB cable to a USB port on your computer or to an external USB hub that
is connected to your computer. When connected to an active USB bus, the USB connector provides +5 V power
and communication. The voltage supplied through the USB connector is system-dependent, and may be less
than 5 V. No external power supply is required.
Due to processing limitations, you cannot log data when the device is attached to an active USB bus. For data
logging operations, connect the USB connector to the external power supply.
LED
The LED uses up to 5 mA of current. The function of the LED varies according to whether the OM-USB-5203
is connected to an active USB port, or when the device is logging data and connected to the external power
supply.
Refer to the following table for the function of the LED when the device is connected to an active USB port and
not logging data.
LED function when the OM-USB-5203 is connected to an active USB port
LED Illumination Indication
Steady green
The OM-USB-5203 is connected to a computer or external USB hub.
Blinks continuously
Data is being transferred.
Upon connection, the LED blinks a few times and then remain on (indicates a successful
installation).
Blinks several times
Initial communication is established between the OM-USB-5203 and the computer.
Off
The OM-USB-5203 is not connected to an active USB port.
Refer to the following table for the function of the LED when the device is connected to the external supply and
is logging data. The function of the LED varies according to the selected logging mode.
LED function when the OM-USB-5203 is logging data
Logging mode LED Illumination Indication
Logging off
The LED is off.
The OM-USB-5203 is not logging data,
and/or the device is not powered
Start Logging on Power
Up
The LED turns on when external power is
connected, then blinks each time data is captured.
Blinks when logging data.
Start Logging on Button
The LED stays off until the data logging button is
pressed and held for approximately 1 second. At
that time, the LED turns on and blinks each time
data is captured.
Blinks when logging data.
Start Logging at
Specified Time
The LED is off – blinks on once per second until
the specified date/time to start logging is reached.
At that time, the LED turns on – blinks off each
time data is captured.
Blinks on once per second until specified
data/time to log data occurs. Then it turns
on and blinks each time data is captured.
Any logging mode
Blinks rapidly (250 ms period) and continuously.

The memory card is full.
The memory card was removed during
logging. Insert the memory card again
to stop the device blinking.

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CompactFlash memory card slot
The CompactFlash slot accepts standard memory cards. A 512 MB memory card is shipped with the device. For
extensive data logging, you can insert a higher capacity card of up to 2 GB. You must format the memory card
before logging data for the first time.
Calculating the logging time for a memory card
Use the following formula to calculate the approximate amount of logging time in seconds that a supported
memory card allows.
where
time = total disk time in seconds
NDisk = size of memory card in bytes
NDIO = 1 byte for digital I/O
nChannels = number of channels logged
R = logging rate in seconds
NTS = 6 bytes for timestamp, if enabled
NTemp = 4 bytes for a temperature reading
NCJC = 8 bytes if CJC sensor readings are enabled
The following table provides examples of logging one channel and eight channels of temperature readings at the
max logging rate of 1 S/s to a 512 MB memory card with DIO, timestamp, and CJC sensor logging enabled:
Memory card
(bytes) DIO enabled Number of
channels Logging rate (S/s) Timestamp enabled Temperature reading CJC enabled
512,000,000 1 1 1 6 4 8
512,000,000 1 8 1 6 4 8
Data logging button
Use the data logging button to end a data logging session. The data logging button is also used to start recording
data when the logging mode is set in InstaCal to Start Logging on Button.
To begin recording data, press and hold the button until the LED begins to blink. The first sample is taken
one second after the LED illuminates.
When you first power on the OM-USB-5203, wait at least five seconds before pressing the data logging
button. To achieve rated accuracy, allow the device to warm up for 30 minutes before logging data.
To stop recording data, press and hold the button again until the LED is off.
Caution! To prevent loss of data, always use the button to stop logging. Make sure the data is written to the
memory card before you disconnect the device from the power source.
The device caches log data in volatile memory prior to writing to the memory card. Pressing the data logging
button has no effect when the OM-USB-5203 is connected to an active USB port and not logging data.
External power required for data logging
Due to processing limitations, data logging is not allowed when the OM-USB-5203 is attached to an active USB
bus. The OM-USB-5203 must be connected to the standalone power supply to perform data logging.

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20
External power supply
The 2.5 W USB adapter is used to power the OM-USB-5203 during data logging operations.
Disconnecting the OM-USB-5203 from the computer
You don't need to shut down your computer to disconnect the OM-USB-5203. Refer to the instructions below
when disconnecting the OM-USB-5203 from your computer's USB port.
In most cases, the OM-USB-5203 shows up as a mass storage device in Windows. To avoid losing data, safely
remove the device from Windows before disconnecting it by using the Eject option or the Safely Remove
Hardware icon. Windows displays a notification telling you it's safe to disconnect the device.
Transferring binary data after a logging session
Data is stored on the memory card in binary files. After logging measurements, you can transfer the files to your
computer by reconnecting the OM-USB-5203 to a USB port on your computer or by removing the
CompactFlash card from the OM-USB-5203 and using a card reader connected to your computer.
Note that when installed with firmware version 3 and later the OM-USB-5203 appears as a Mass Storage
Device when connected to a USB port on your computer, so you can copy files using Windows Explorer.
Converting binary data after a logging session
If your OM-USB-5203 is connected to a USB port on your computer, you can use InstaCal or TracerDAQ to
convert the files on the CompactFlash card to .CSV format for use in Microsoft Excel files, or to .TXT format
for use in other applications.
If you transferred binary files to your computer hard drive or removed the CompactFlash card from the device
and are using a card reader connected to a computer, use TracerDAQ to import the files and save them as .CSV
or.TXT format. InstaCal can only convert files when the CompactFlash card is in a device connected to a
computer.