Kenco KT Series User manual

DDA Interface Manual
“KT” Series Magnetostrictive
Liquid Level Transmitters
Kenco Engineering Company
10001 East 54th Street
Tulsa, Oklahoma 74146
Phone: [phone]
Fax: [phone]
Website: www.kenco-eng.com
Email: [email protected]
79862
(Rev. #)
[5-2-19]

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DDA Interface Manual
Table of contents
1. Terms and Definitions.................................................................3
2. Introduction.................................................................................5
3. Safety Instructions......................................................................5
4. Quick Start-Up Guide..................................................................5
4.1 Before You Begin .................................................................. 5
4.2 Quick Start-Up Procedure ..................................................... 5
5. Display Menu..............................................................................5
5.1 Operation Modes................................................................... 5
5.1.1 Run Mode ...................................................................... 5
5.1.2 Program Mode............................................................... 5
5.2 Display Diagram.................................................................... 6
5.3 Menu Structure..................................................................... 6
6. Alarms.........................................................................................6
7. Error Codes (Faults)....................................................................7
8. DDA Interface..............................................................................7
9. Hardware and Software Environments.......................................8
10. DDA Command Decoder Examples ..........................................8
11. DDA/Host Computer Communication Protocol.........................9
12. DDA Command Definitions .....................................................13
12.1 Special Control Commands........................................... 13
12.2 Level Commands .......................................................... 13
12.3 Temperature Commands............................................... 14
12.4 Multiple Output Commands .......................................... 14
12.5 High-Level Memory Read Commands .......................... 15
12.6 High-Level Memory Write Commands .......................... 16
12.7 Diagnostic/Special Command Set ................................. 18
12.8 DDA Error Codes........................................................... 18
13. Dashboard, Setup Software ..................................... 19
13.1 Installing Dashboard..................................................... 19
13.2 Home Screen ................................................................ 19
13.3  ................................................................ 20
13.4 Signal Settings............................................................. 20
13.5 Level Settings .............................................................. 20
13.6 Temperature Settings................................................... 21
13.7 Flash Settings .............................................................. 21
13.8 Save Settings ............................................................... 22
13.9 Programming the Display ............................................ 22
13.9.1 Data from Device Tab........................................ 22
13.9.2 Calibration......................................................... 22
13.9.3 Factory.............................................................. 22

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DDA Interface Manual
1. Terms and definitions
6A Heavy Oils
‘Generalized Crude Oils’, Correction of Volume to 60 °F against API
Gravity.
6B Light Oils
‘Generalized Products’, Correction of Volume to
60 °F against API Gravity.
6C Chemical
‘Volume Correction Factors (VCF)’ for individual and special applica-
tions, volume correction to 60 °F against thermal expansion coeffi-
cients.
6C Mod
An adjustable temperature reference for defining VCF.
A
API Gravity
The measure of how heavy or light a petroleum liquid is compared to
water. Allowable values are 0 to 100 degrees API for (6A) and 0 to 85
degrees API for (6B).
D
DDA
‘Direct Digital Access’ – The digital protocol for use in intrinsically safe
areas.
Density
Mass divided by the volume of an object at a specific temperature. The
density value should be entered as lb. / cu. ft..
E
Explosion proof
Type of protection based on enclosure in which the parts which can
ignite an explosive gas atmosphere are placed within, and which can
withstand the pressure developed during an internal explosion of an ex-
plosive mixture, and which prevents the transmission of the explosion
to the explosive gas atmosphere surrounding the enclosure.
F
Flameproof
Type of protection based on enclosure in which the parts which can
ignite an explosive gas atmosphere are placed within and which can
withstand the pressure developed during an internal explosion of an ex-
plosive mixture, and which prevents the transmission of the explosion
to the explosive gas atmosphere surrounding the enclosure.
FOUNDATION™ fieldbus
An all digital, serial, two-way communications system that serves as
the base-level network in a plant or factory automation environment.
Developed and administered by the fieldbus FOUNDATION™.
G
GOVI
‘Gross Observed Volume of the Interface’ – The total volume of the tank
occupied by the interface liquid. The GOVI is only given when mea-
suring two liquids and is calculated by subtracting the volume of the
product from the total volume of liquid in the tank (GOVT – GOVP).
GOVP
‘Gross Observed Volume of the Product’ – The total volume of the tank
occupied by the product liquid. When measuring only one liquid, it is
also the total volume of liquid in the tank (GOVT). When measuring two
liquids it is the total volume of liquid in the tank minus the volume of
the interface liquid (GOVT – GOVI).
GOVT
‘Total Gross Observed Volume’ – The total volume of liquid in the tank.
When measuring only one liquid it is equal to the volume of the product
(GOVP). When measuring two liquids it is equal to the volume of the
product and interface liquids (GOVP + GOVI).
GOVU
‘Gross Observed Volume Ullage’ – the difference in volume between the
working capacity of a tank and the total volume in the tank (Working
Capacity – GOVT).
H
HART®
A Bidirectional communication protocol that provides data access
between intelligent field instruments and host systems.
I
Interface
Noun; The measurement of the level of one liquid when that liquid is
below another liquid.
Interface
Adj.; The Software Graphical User Interface (GUI) that allows the user
to access software protocols (HART, DDA, MODBUS).
Intrinsic safety
‘Intrinsically safe’ - Type of protection based on the restriction of
electrical energy within apparatus of interconnecting wiring exposed to
potentially explosive atmosphere to a level below that which can cause
ignition by either sparking or heating effects.

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DDA Interface Manual
M
Mass
The property of a body that causes it to have weight in a gravitational
field, calculated by density at the reference temperature multiplied by
the volume correction factor (Density * VCF).
MODBUS
A serial communications protocol published by Modicon in 1979 for use
with its programmable logic controllers (PLCs). It has become a de fac-
to standard communications protocol in industry, and is now the most
commonly available means of connecting industrial electronic devices.
N
NEMA Type 4X
A product Enclosure intended for indoor or outdoor use primarily to
provide a degree of protection against corrosion, windblown dust and
rain, splashing water, and hose-directed water; and to be undamaged by
the formation of ice on the enclosure. They are not intended to provide
protection against conditions such as internal condensation or internal
icing.
NPT
U.S. standard defining tapered pipe threads used to join pipes and
fittings.
NSVP
‘Net Standard Volume of the Product’ – The temperature corrected
volume for the product liquid in the tank, requires the transmitter to
be ordered with temperature measurement capabilities. The NSVP is
calculated by multiplying the volume of the product liquid by a volume
correction factor based on temperature (GOVP * VCF).
R
Reference Temperature
The temperature at which the density measurement is given, the allow-
able values are 32 °F to 150 °F (0 °C to 66 °C).
S
Specific Gravity
The density ratio of a liquid to the density of water at the same condi-
tions.
Sphere Radius
The internal radius of the sphere that contains the liquid, the value is
used to calculate the volume along with the Sphere Offset.
Sphere Offset
An offset value that accounts for additional volume in a sphere from
non-uniform sphere geometry, the value is used to calculate the volume
along with the Sphere Radius.
Strap Table
A table of measurement correlating the height of a vessel to the volume
that is contained at that height. The transmitter can contain up to 100
points.
T
TEC
‘Thermal Expansion Coefficient’ - a value correlating the change in tem-
perature for an object with the change in its volume. Allowable values
are 270.0 to 930.0. TEC units are in 10 E-6/Deg F.
Temperature Correction Method
One of five product correction methods used to correct the product
volume in the tank due to changes in temperature from 60 °F including
(6A, 6B, 6C, 6C Mod, and Custom Table.
V
Volume Calculation Mode
One of two methods use to calculate volume measurements from level
measurements, including Sphere and Strap Table.
VCF
‘Volume Correction Factor’ – A table of measurements correlating
temperature points with correction factors for the liquids expansion/
contraction. The transmitter can contain up to 50 points.
W
Working Capacity
The maximum volume of liquid that the user desires for their vessel to
hold, typically 80% of the vessels maximum volume before overfill.

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DDA Interface Manual
2. Introduction
2.1 Purpose and use of this manual
The content of this technical documentation and of its various
annexes is intended to provide information on mounting, installation
and commissioning by qualified service personnel according to
IEC 60079-14 and/or factory trained service technicians and local
regulations.
2.2 Used symbols and warnings
Warnings are intended for your personal safety and for avoidance
of damage to the described product or connected devices. In this
documentation, safety information and warnings to avoid dangers that
might affect the life and health of personnel or cause material damage
are highlighted by the preceding pictogram, which is defined below.
3. Safety instructions
3.1 Intended use
The purpose of this document is to provide detailed information on
the protocol interface. All safety related information is in the product
specific operation manual. Consult the operation manual before
connecting to the level transmitter.
4. Quick start-up guide
4.1 Before you begin
Note:
You must use a RS-485 converter with “Send Data Control” and the
Set-up Software to ensure proper operation.
Example:
B & B Electronics 485BAT3 (815-433-5100 www.bb-elec.com).
FTDI USB-RS485-WE-1800-BT (www.ftdichip.com)
4.2 Quick start-up procedure
1. Connect +24 Vdc to terminals.
2. Connect data lines to terminals.
3. Connect the PC (or other device) to data lines.
(If you are using a PC, use a RS-232 to RS-485 converter. See
Note above for more information.)
4. Turn on power to the transmitter.
5. Start the Dashboard. Select Com Port and Address. Factory
default address is ‘192 ’ for DDA.
Before starting the operation of the equipment read this documentation
thoroughly and follow the safety information.
Symbol Meaning
NOTICE This symbol is used to point to situations
that may lead to material damage and/or personal
injury.
6. Change the address to one that is suitable for the installation
network.
7. Verify proper operation of product and or interface floats and
temperature.
8. Turn off power to the transmitter.
9. Remove data lines.
10. Install the transmitter into the vessel (see Operation Manual).
11. Reconnect power and data lines.
12. Calibrate current tank level using setup software (optional).
13. Verify communications with the host system.
Default communication parameters
DDA: 4800 BAUD 8, E, 1
5. Display Menu
All liquid level transmitters are shipped with a Stylus (Kenco Part #
79857) to be used for manipulating the display. For single and dual
cavity housings, the Stylus is designed to allow for programming of the
unit without removing the housing. When using the Stylus make sure
to align the Stylus with the shape outline around the buttons in the same
orientation. Failure to correctly align the Stylus can cause the display to
not function properly.
Note:
Do not use any device other than the Stylus to operate the display.
Note:
Improper use of the Stylus can cause the display to not function
properly.
5.1 Operation Modes
The level transmitter runs in one of the following modes of operation.
You can use these modes to calibrate and set up various operating
parameters.
5.1.1 Run Mode
Run mode is the primary mode of operation. This mode will perform
measurements, display data, and respond to DDA commands.
5.1.2 Program Mode
Program mode is the primary mode for commissioning and trouble-
shooting the level transmitter. The full menu and available functions are
shown in section 5.3 Menu Structure. To enter program mode use the
Stylus and press the Enter Key as shown in section 5.2 Display Diagram.
Program Mode is protected by a password to keep unwarranted chan-
ges from occurring. The factory default password is 27513. When in
program mode, remote communications are not functional. An automa-
tic timeout feature is provided so that the transmitter does not remain
inadvertently in program mode. The timeout is set for 1 minute before
prompted for additional time. Total timeout is 2 minutes.

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DDA Interface Manual
Note:
Whenever program mode is exited from the display the unit will reset
itself to insure all changes have been accepted. The reset will take
approximately 5 seconds before the level transmitter is able to respond
to commands.
Note:
In program mode, the transmitter will not respond to incoming DDA
commands. A busy error will be sent to the controller to notify the
unit is in program mode. This function will prevent a user at a remote
terminal from programming the unit while a user is accessing program
mode from the display.
5.2 Display Diagram
UP Arrow – Used to move cursor on screen up and to increment num-
ber.
DOWN Arrow – Used to move cursor on screen down and to decrease
number.
SCROLL Arrow – Used to move cursor on screen to the right, cursor will
cycle back.
ENTER Key – Used to Enter Program Mode, select Highlighted Item, and

EXIT Key – Hidden key in the middle of the display that is used to exit
menu at any time.
MEASURED VARIABLE – The process variable that is selected to display.
The display will automatically scroll between selected variables.
MEASUREMENT – The numerical value for the MEASURED VARIABLE
shown on the display.
UNITs – Unit of measurement for the MEASURED VARIABLE shown on
the display.
TEMPERATURE – Average temperature for the product in the tank. Only
shown if the level transmitter was purchased with temperature.
NOTIFICATIONS – Four squares with letters. Top left square will always
be present indicating a D for DDA mode or a M for Modbus mode. Top
right square, A, will only show when there is an alarm. Toggle the UP
Arrow key to view alarms. Bottom right square, F, will only show when
there is a fault. Toggle the DOWN Arrow key to view error codes. Bot-
tom left square, P, will only show when the units is being programmed
remotely.
5.3 Menu Structure
• Data From Device
Display
Units
`Length Units
`Temp Units
Address
Signal Strength
`Prod Trig Lvl
`Int Trig Lvl
• Calibrate
Product Level
`Current Level
`Offset
Interface Level
`Current Level
`Offset
• Factory
Settings
`Gradient
`Serial Number
`HW Revision
`SW Revision
`SARA Blanking
`Magnet Blanking
`Gain
`Min Trig Level
Temp Setup

Auto Threshold
Reset to Factory
6. Alarms
The DDA output is equipped with several alarms that will be shown on
the display. Use the stylus to tap the Up arrow to view alarms. The
DDA output is set to fail high, greater than the order length, if there is an
issue and the level value output cannot be trusted.
Fig. 1: DDA Display

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DDA Interface Manual
101 Missing
Magnet


• Verify Float(s) are not in inactive zone.
• Verify Auto Threshold is enabled.
• Cycle power to sensor. If proper operation
is not restored, Contact Factory.
102 Internal
Fault 1
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
103 Internal
Fault 2
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
104 Internal
Fault 3
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
105 Lobe Fault 1 • Verify Auto Threshold is enabled.
• Cycle power to sensor.
• If proper operation is not restored, Contact
Factory.
106 Lobe Fault 2 • Verify Auto Threshold is enabled.
• Cycle power to sensor.
• If proper operation is not restored,
Contact Factory.
107 Delta Fault Contact Factory to discuss application.
108 Internal
Fault 4
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
109 Peak Fault • Verify Auto Threshold is enabled.
• Cycle power to sensor.
• If proper operation is not restored,
Contact Factory.
110 Hardware
Fault 1
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
111 Power Fault • Cycle power to sensor.
• Verify Power Supply rating.
• Verify wiring.
• If proper operation is not restored,
Contact Factory.
112 Hardware
Fault 2
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
113 Hardware
Fault 3
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
114 Hardware
Fault 4
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
115 Timing
Fault 1
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
116 Timing
Fault 2
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
117 Timing
Fault 3
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
118 DAC Fault 1 Cycle power to sensor. If proper operation
is not restored, Contact Factory.
119 DAC Fault 2 Cycle power to sensor. If proper operation
is not restored, Contact Factory.
120 DAC Fault 3 Cycle power to sensor. If proper operation
is not restored, Contact Factory.
Fault
Code
Description Corrective Action
Fault
Code
Description Corrective Action
121 DAC Fault 4 Cycle power to sensor. If proper operation
is not restored, Contact Factory.
122 SPI Fault 1 Cycle power to sensor. If proper operation
is not restored, Contact Factory.
123 SPI Fault 2 Cycle power to sensor. If proper operation
is not restored, Contact Factory.
124 Setpoint
Fault
The analog setpoints are too close. Mini-
mum distance is 150 mm (6 in.) for analog
and 290 mm (11.5 in.) for SIL. Adjust
programmed setpoints as needed. (Analog
only) If proper operation is not restored,
Contact Factory.
125 Loop 1 Out
of Range
Verify that magnets are positioned within
expected measuring range. Adjust program-
med setpoints as needed. (Analog only) If
proper operation is not restored, Contact
Factory.
126 Loop 2 Out
of Range
Verify that magnets are positioned within
expected measuring range. Adjust program-
med setpoints as needed. (Analog only) If
proper operation is not restored, Contact
Factory.
127 EEPROM
Fault 1
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
128 EEPROM
Fault 2
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
129 Flash Failure Cycle power to sensor. If proper operation
is not restored, Contact Factory.
130 Internal
Error
Cycle power to sensor. If proper operation
is not restored, Contact Factory.
7. Error Codes (Faults)
8. DDA Interface
8.1 Data line termination and biasing
Termination and biasing of RS-485 data lines are as follows:
Biasing
Each transmitter uses a Fail-safe, Slew Rate-Limited RS-458/RS-422
Transceiver. No additional biasing resistors should be present on the
connecting devices (PLC, DCS, PC, Converter).
Termination
Each transmitter uses a Fail-safe, Slew Rate-Limited RS-
458/RS-422 Transceiver. No additional termination resistors are
necessary in the connecting devices (PLC, DCS, PC, Converter).
8.2 Communication parameters
The 2-wire differential communication interface and all data transmissi-
ons must be at half duplex. Only one device (either the master or a
single transmitter) can transmit data at any given time. BAUD rate
limitations are listed below.
DDA: 4800 BAUD 8, N, 1
(Reference) Monitor: DDA RTU Variable BAUD Rate 8, E, 1

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DDA Interface Manual
9. Hardware and software environments
The DDA digital transmitter operates in a networked, intrinsically safe
RS-485 DDA software environment. This environment supports up to
8 multi-dropped transmitters on one communication line. The network
requires a 4-wire bus to provide both power and communications to
each of the transmitters located in the hazardous area. The transmitters
are connected in multi-point configuration (see Figure 2).
The RS-485 network operates in a master/slave mode where the master
(host computer or similar type network controller) interrogates each
slave (DDA transmitter) for a specific type of data. Each slave has a
. . .
2
2
+24 Vdc
power
supply
RS-485
Examples:
PC, PLC, DCS
I.S.B.*
Approved intrinsic
safety barrier
(3 required)
General application
4-wire bus
Intrinsically
safe area
Maximum stub
length = 200 feet
Up to 8
transmitters
(DDA interface)
unique switch programmable hardware address that is issued by the
host computer to activate a particular transmitter. In addition, the DDA
hardware supports a command decoder that supports up to 128 different
commands. The host computer interrogates a transmitter for data by
sending an address byte, followed immediately by a command byte. The
addressed transmitter will ‘wake up’, identify itself by transmitting an
echo of its own local address followed by the received command, and
then perform the requested action. After the requested action has been
completed, the data (if any) will be transmitted back to the host computer
on the RS-485 network. Refer to Section 10 for ‘DDA Command decoder
examples’ and for more information.
10. DDA command decoder examples
10.1 Serial data transmission format
Example 1:
0X X X X X X X X P1
D1 bit Stop bit
Start bit Parity bit
D8 bit
After the DDA address decoder circuitry receives the 11-bit word, an
even parity check is performed across the 8-bit data field. If a parity error
is found, the word is ignored and the decoder circuitry resets for the next
transmission. If the parity check is good, the decoder circuitry checks for
a valid address byte. The address decoder circuitry uses the ‘D8’ bit to
distinguish the difference between address bytes and command bytes.
Address bytes are defined as having the most significant bit ‘D8’ set equal
to one. Valid address byte values include ‘C0’ hex to ‘FD’ hex (192 to
253 decimal). Address byte values from 80 hex to ‘BF’ hex are reserved
for future use, address byte values ‘FE’ and ‘FF’ hex are reserved for test
functions. (see Example 2).
Fig. 2: Typical Electrical Connections - Intrinsically Safe System

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DDA Interface Manual
10.2 Address byte
Example 2:
X X X X X X X 1
D8 bit = 1
(8-bit word - shown as D1 bit)
If the received address byte matches the local DDA address, the DDA
power supply circuitry is activated. If a valid address byte has been
found, the decoder circuitry checks to see if the next received word is a
command byte. Valid command byte values include ‘00’ hex to ‘7F’ hex
(0 to 127 decimal). In addition, all data byte values are restricted to be
within ‘00’ hex and ‘7F’ hex (see Example 3).
10.3 Command byte (and data bytes)
Example 3:
X X X X X X X 0
D8 bit = 1
(8-bit word - shown as D1 bit)
Again, an even parity check is performed on the command byte. If the
parity check is good, the eight bit data word is latched into a command
buffer. This buffer is read by the DDA software to determine which
command to execute. If the parity check fails, the command byte
is rejected and the old command (from the previous interrogation
sequence) will be left in the command buffer. The DDA hardware cannot
determine if the current command was possibly rejected. The host
computer must then verify if the correct command was received by
reading the echo of the address byte and command byte sent by the DDA
transmitter. This is the only guaranteed way to determine that both the
address and command bytes were received properly.
This method also insures proper verification, even if the parity check fails
to detect a multiple bit data error in either the address byte or command
byte. If the host computer determines that either the address byte or
command byte has been corrupted, it must wait the proper time-out
period and ignore the received message from the DDA transmitter that
was improperly interrogated. The time-out period is variable and is based
on the duration of the selected DDA command.
11. DDA/Host computer communication protocol
The DDA/Host computer communication protocol consists of two parts:
the interrogation sequence generated by the host computer and the
data response generated by the interrogated DDA transmitter. The host
interrogation sequence always consists of an address byte followed
immediately by a command byte (see Example 4).
11.1 DDA/Host communication
Example 4:
<address byte><command byte>
00 Hex to 7F Hex (0 to 127 decimal)
C0 Hex to FD Hex (192 to 253 decimal)
The maximum delay between the address byte and the command byte is
5 milliseconds. The DDA transmitter will not receive the new command
byte if this delay period is exceeded (and the old command byte will
be left in the command buffer). See previous section for additional
information on verification of the Address/Command bytes. An example
of an interrogation sequence to access a transmitter programmed for
address ‘F0’ hex (see Example 5).
The transmitter response consists of several components. After a
transmitter has been interrogated, the transmitter first responds by
transmitting its own local address and the command that was received
from the host computer. This re-transmission of the transmitter address
and received command serves two purposes. The first being a simple
identification that the correct transmitter received the correct command
and that it is currently active. The second purpose is to reset the DDA
Address/Command decoder circuitry for the next interrogation sequence.
11.2 Interrogation data sequence
Example 5:
<F0><0A>
Command 0A Hex (10 decimal)
Address F0 Hex (240 decimal)
Note:
If the DDA transmitter does not respond to the first interrogation by
the host, the Address/Command decoder will be left in an intermediate
state. If this occurs, the host will have to reinterrogate the respective
transmitter to reset the Address/Command decoder circuitry and then
interrogate the respective transmitter again to perform a new transmit-
ter measurement. This hardware feature must be considered when writ-
ing software communication drivers to access DDA transmitter data.

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DDA Interface Manual
After the DDA transmitter has retransmitted its local address and
received command, it will perform the requested measurement as
defined by the received command. After the requested measurement has
been completed, the data for that measurement will be transmitted to
the host in a predefined format including certain control characters. The
DDA transmitted data format begins with a ‘start of text’ ‘STX’ character
(STX = 02 hex). The ‘STX’ character set is immediately followed by
the requested data and then terminated with an ‘end of text’ ‘ETX’
character set (ETX = 03 hex). Certain commands allow multiple data
fields to be transmitted within one transmitted data sequence. For these
data transmissions, each data field is separated by an ASCII colon ‘:’
character (: = 3A hex), (see Examples 6 and 7).
Single field data transmission
Example 6:
<STX><dddd.ddd><ETX>
Multiple field data transmission
Example 7:
<STX><dddd.ddd:dddd.ddd:dddd.ddd><ETX>
All transmitted data will consist of 7-bit ASCII characters limited to hex
values between ‘00’ hex and ‘7F’ hex (i.e. data bit D8 = 0).
After a DDA transmitter has completed a data transmission, the
host must wait 50 milliseconds before another interrogation can
be performed. This delay is required to enable the previously
interrogated transmitter to go into sleep mode and release the network
communication lines.
All DDA control commands support a checksum calculation function,
Data Error Detection (DED) that allows the host computer (master) to
check the integrity of the transmitted data. The actual checksum value
that is transmitted is the compliment (2’s compliment) of the calculated
value. The checksum scheme is based on a 16-bit summation of the hex
data within the transmitted block (including ‘STX’ and ‘ETX’ character
sets) without regard to overflow. The two byte result of the adding
process is then complimented and appended to the transmitted data
block.
This compliment process makes the final checksum comparison more
efficient in that the checksum result added to its compliment will always
result in a zero sum for uncorrupted data transmissions. Checksum
data (two hex bytes) can range from ‘0000’ hex to ‘FFFF’ hex. Since the
communication network only allows transmitted data values between
‘00’ and ‘7F’ hex, special processing is required on the hex checksum
value before it can be transmitted.
This two byte hex value must first be converted to numeric (decimal)
ASCII characters before transmission. For example, a checksum value
of ‘FFFF’ hex would be transmitted as ASCII 65535. The host computer
would then have to convert ASCII 65535 back to FFFF hex and perform
its own checksum calculation and comparison for the received data
from the DDA transmitter. An example is shown (see Example 8) of a
single field data transmission including checksum data and an sample
checksum calculation.
Example 8:
<STX><dddd.ddd><ETX><ccccc>
Append checksum value
Note:
The appended checksum value will always consist of five decimal (AS-
CII) characters ranging from 00000 to 65535. The checksum function
can be enabled or disabled.
Message transmitted from DDA transmitter (command 12 Hex):
<STX><265.322.109.456><ETX>64760
Hex character equivalent of transmitted data record including <STX>
and <ETX> characters:
02, 32, 36, 35, 2E, 33, 32, 32, 3A, 31, 30, 39, 2E, 34, 35, 36, 03
Two byte Hex summation of data: 0308 Hex
Two’s compliment: FCF8 Hex
Convert to decimal ASCII: 64760
To verify transmitted data from the DDA transmitter, perform the two byte
Hex summation over the data record (including ‘<STX>’ and ‘<ETX>’)
shown in (Example 8). The result in this example is 0308 Hex. Then
convert the decimal ASCII checksum value back to Hex (for example,
64760 to FCF8 Hex). Add the Hex summation value to the Hex checksum
value and the result will be zero (disregarding overflow) for uncorrupted
data. 0308 Hex + FCF8 Hex = 0000 Hex.
Note:
Cyclic Redundancy Check (CRC) error checking will be offered at a
later date. A command switch will be defined that will let the DDA data
be transmitted with CRC error checking instead of checksum error
checking. The checksum calculations will use the CRC-CCITT defined
polynomial with a 16-bit CRC result. This 16-bit CRC value will be
appended to each transmitted message. Since the communication net-
work only allows transmitted data values between 00 and 7F hex, spe-
cial processing is required on the 16-bit hex CRC value before it can be
transmitted. This 16-bit (two byte) hex value must first be converted to
numeric (decimal) ASCII characters before transmission. For example,
a checksum value of ‘FFFF’ hex would be transmitted as ASCII 65535.
Checksum Calculation