Amplicon USB232U User manual

USB232U/485Ui
USB232U/485Ui
USB TO RS232 / RS485
SERIAL
COMMUNICATIONS
INTERFACE
W ITH ISOLATION
OPTION
This Instruction Manual is supplied with the USB232U/485Ui Adapter to provide the user with sufficient information to
utilise the purchased product in a proper and efficient manner. The information contained has been reviewed and is
believed to be accurate and reliable, however Amplicon Liveline Limited accepts no responsibility for any problems
caused by errors or omissions. Specifications and instructions are subject to change without notice.
USB232U/485Ui Instruction Manual Issue A2
© Amplicon Liveline Limited
Prepared by Helen Elcock
See www.amplicon.co.uk for updates

USB232U/485Ui
DECLARATION OF CONFORMITY
MEV LIMITED
SUITE 8, BAXALL BUSINESS CENTRE
ADSWOOD ROAD
STOCKPORT SK3 8LF UK
We declare that the product(s) described in th
is Instruction Manual
perform in conformity with the following standards or standardisation
documents:
Electro Magnetic Compatibility (EMC):
EMC Directive 89/336/EMC
CE Directive 93/68/EEC
David Cooper, MIEE
Director
MEV Limited

USB232U/485Ui
LIST OF CONTENTS
PARA SUBJECT PAGE
1INTRODUCTION ..................................................................................................................1
1.1 The Products Described in this Manual .................................................................................1
1.2 Features of the USB232U/485Ui Series of Data Communications Interfaces ........................1
1.3 General Description ..............................................................................................................2
1.3.1 Enhanced Features...............................................................................................................3
1.4 What the Package Contains..................................................................................................3
1.4.1 The Amplicon Warranty Covering the USB232U/485Ui interfaces.........................................3
1.4.2 Contacting Amplicon Liveline Limited for Technical Support or Service ................................3
1.4.3 Technical Support.................................................................................................................4
1.4.4 Repairs .................................................................................................................................4
2GETTING STARTED ............................................................................................................6
2.1 General Information..............................................................................................................6
2.2 Installing the Interface ..........................................................................................................6
2.2.1 Windows 98 Installation........................................................................................................6
2.2.2 Windows 2000/XP Installation...............................................................................................6
2.2.3 Linux Installation...................................................................................................................6
2.3 USB232U/485Ui Configuration Jumper Settings ...................................................................7
2.3.1 RS485 Full Duplex / Half Duplex Operation..........................................................................7
2.3.1.1 Jumper Selection For Full Duplex Operation ........................................................................7
2.3.1.2 Jumper Selection For Half Duplex Operation with Echo........................................................7
2.3.1.3 Jumper Selection For Half Duplex Operation without Echo...................................................7
2.3.2 Transmission Line Termination.............................................................................................8
2.3.2.1 Termination Resistors for RS422/485 Receivers...................................................................8
2.3.3 Jumper Factory Settings Summary.......................................................................................8
2.4 Application Software.............................................................................................................9
2.4.1 Installation Testing................................................................................................................9
3MAKING THE CONNECTIONS ..........................................................................................10
3.1 External Connections..........................................................................................................10
3.1.1 USB Connector...................................................................................................................10
3.1.2 RS232 9-way Port...............................................................................................................10
3.1.3 RS422/485 9-way Port........................................................................................................11
3.2 Connector Wiring for RS422/485 Full/Half Duplex ..............................................................11
3.3 The Ground Connections....................................................................................................11
3.3.1 USB232U Ground Connections ..........................................................................................11
3.3.2 USB485Ui Ground Connections..........................................................................................12
3.4 Choice of Cable..................................................................................................................12
4USING THE SERIAL COMMUNICATIONS DEVICES.........................................................13
4.1 Why Isolate? ......................................................................................................................13
4.2 Application Notes................................................................................................................14
4.2.1 Applicable Standards..........................................................................................................14
4.2.1.1 RS Standards .....................................................................................................................14
4.2.2 RS232 Application Notes....................................................................................................14
4.2.2.1 RS232 Electrical Levels......................................................................................................15
4.2.2.2 RS232 Data Rates and Cable lengths.................................................................................15
4.2.2.3 RS232 Cable Adapters .......................................................................................................15
4.2.3 RS422/485 Application Notes .............................................................................................15
4.2.3.1 RS422/485 Parameters ......................................................................................................16
4.2.3.2 RS422/485 Signalling Sense ..............................................................................................16

USB232U/485Ui
4.2.3.3 RS422/485 Multi-drop Applications.....................................................................................16
4.2.3.4 RS422/485 Bus Termination...............................................................................................17
4.2.3.5 RS422/485 Data Rates and Cable lengths..........................................................................18
4.2.3.6 Cabling of RS422/485 Bus..................................................................................................18
4.2.4 USB Application Notes .......................................................................................................18
4.2.4.1 Data Rates and Flow Control for USB to RS232 .................................................................18
4.3 Testing and Troubleshooting...............................................................................................19
4.3.1 Testing with the Application Software .................................................................................19
4.3.2 The Loop Back Connections ...............................................................................................20
4.3.3 Loop-back Testing Using A Terminal Emulator ...................................................................20
5PROGRAMMING................................................................................................................22
5.1 Programming with Windows 98, 2000 and XP Device drivers. ............................................22
5.2 Half Duplex Transmission Turnaround................................................................................22
APPENDIX A - TECHNICAL SPECIFICATIONS................................................................................. I
A.1 HARDWARE SPECIFICATION..............................................................................................i
A.1.1 USB.......................................................................................................................................i
A.1.1.1 Series B connector ................................................................................................................i
A.1.2 USB232U RS232 Port ..........................................................................................................ii
A.1.2.1 9-way Connector...................................................................................................................ii
A.1.2.2 Data Rate .............................................................................................................................ii
A.1.2.3 I/O Protection .......................................................................................................................ii
A.1.2.4 ESD Protection.....................................................................................................................ii
A.1.3 USB485Ui RS422/485 Port.................................................................................................. iii
A.1.3.1 9-way Connector..................................................................................................................iii
A.1.3.2 Data Rate ............................................................................................................................ iii
A.1.3.3 Rx I/P Termination ..............................................................................................................iii
A.1.3.4 Tx O/P Protection................................................................................................................ iii
A.1.3.5 I/O Isolation......................................................................................................................... iii
A.1.3.6 ESD Protection.................................................................................................................... iii
A.1.3.7 Modes .................................................................................................................................iii
A.1.3.8 Transmit Turnaround ........................................................................................................... iii
A.1.4 UART ..................................................................................................................................iv
A.1.4.1 Baud Rates..........................................................................................................................iv
A.1.4.2 Data Bits..............................................................................................................................iv
A.1.4.3 Stop Bits..............................................................................................................................iv
A.1.4.4 Parity...................................................................................................................................iv
A.1.5 PC INTERFACE ..................................................................................................................iv
A.1.5.1 Compatibility .......................................................................................................................iv
A.1.5.2 Host resources.....................................................................................................................iv
A.1.5.3 USB Frame Rate .................................................................................................................iv
A.1.5.4 USB packet size ..................................................................................................................iv
A.2 SOFTWARE........................................................................................................................iv
A.3 ENVIRONMENTAL CONDITIONS........................................................................................v
A.3.1 Specific conditions................................................................................................................v
A.3.1.1 Dimensions...........................................................................................................................v
A.3.1.2 Temperature Range..............................................................................................................v
A.3.1.3 Power Requirements ............................................................................................................v
A.3.1.4 Dissipation............................................................................................................................v

USB232U/485Ui
LIST OF FIGURES
Figure 2.1 Jumper Settings For Full Duplex Operation........................................................................7
Figure 2.2 Jumper settings for Half Duplex Operation with Echo.........................................................7
Figure 2.3 Jumper settings for Half Duplex Operation without Echo....................................................7
Figure 2.4 Jumper Settings for Termination Resistor Selection ...........................................................8
Figure 2.5 Factory Jumper Setting Summary......................................................................................8
Figure 3.1 USB Connector Termination Assignment .........................................................................10
Figure 3.2 RS232 9-way Port Pin Designations.................................................................................11
Figure 3.3 RS422/485 Amplicon Standard Port Pin Designations......................................................11
Figure 4.1 Communications Standards Summary .............................................................................16
Figure 4.2 RS422 Connected in Broadcast Mode..............................................................................17
Figure 4.3 RS485 Connected in Multi-Drop, Half Duplex Mode .........................................................17
Figure 4.4 RS232 Loop-Back Test Connections ................................................................................20
Figure 4.5 RS422/485 Loop-Back Test Connections .........................................................................20

USB232U/485Ui
Page 1
1 INTRODUCTION
The USB232 Universal (USB232U) and USB485 Universal isolated (USB485Ui) serial
communications interfaces are a range of USB peripherals that provide legacy RS232 or
RS485 serial communication interfaces to computers with USB support.
1.1 The Products Described in this Manual
There are two variants of USB232U/485Ui. Each supports a single serial communications
interface that appears to operating system as a standard serial port.
USB232U A non-isolated RS232 interface.
USB485Ui An isolated RS485 interface.
1.2 Features of the USB232U/485Ui Series of Data Communications Interfaces
Each of the USB232U/485Ui series of interfaces is designed to meet stringent performance
requirements and ease of use while maintaining compatibility with standard serial
communications products.
•USB 1.1 specification compliant
•Windows 98, 2000, XP and Linux compatible
•Data rates selectable up to 1 M Baud (RS232) and 3 M Baud (RS485)
•FIFO modes supporting a 384 character receive buffer and 128 byte transmit buffers
•Three point isolated serial communications on isolated versions
•Full Duplex, Half Duplex, Multi-drop applications
•RS485 transmitter can be enabled automatically in Half Duplex mode
•Full specification signal levels
•Implementation of all RS232 primary control lines
•Low power consumption

USB232U/485Ui
Page 2
1.3 General Description
The USB232U and USB485Ui are advanced RS232/RS485 serial communications interfaces
suitable for any IBM PC or compatible computer that supports USB 1.1.
All models are supported by third party Windows 98, 2000, XP and Linux drivers. Microsoft
Windows drivers are distributed on the CD that accompanies this product. The Linux drivers
are available as standard as part of the 2.4.0 kernel and above.
These drivers are a superset of the Microsoft serial drivers and allow the devices to operate as
standard serial ports. These drivers also allow the advanced features of the devices to be fully
utilised by the application software.
There are two models available in the range.
USB232U provides a 9-pin RS232 port
USB485Ui provides an isolated 9-pin RS422/485 port, jumper configurable for half-
duplex.
Each interface consists of a small unit which takes up to one USB unit load, 100mA, from the
PC USB interface. It connects to the USB interface via a standard USB interface cable and
one serial port designed to meet one of the following standards:
RS232 full duplex serial port furnished with a 9-pin D-type male connector conforming to IBM
practice. The port is fully configured with two data, six control I/O lines and one signal return.
RS485 full duplex, half-duplex, multi-drop operation or RS422 full duplex without RTS-CTS
control lines. Depending on the application, RS485 or RS422 operation is selected by making
the appropriate connections. Each RS422 / 485 port is furnished with a 9-pin D-type male
connector.
The RS485 ports have a link selectable “Half Duplex” mode that causes the transmit buffer to
be enabled automatically, by the UART, whenever data is waiting to be sent from the UART
Transmit buffer.
The USB485Ui is designed to isolate the serial communications from the computer power. The
design is such that it will sustain common mode voltages up to 75Vdc, which is outside of the
low voltage directive (73/23/EEC). However in fault conditions they are protected to sustain
common mode voltages up to peaks of 1 kVdc. It provides isolation of the data and can be
used in serial communications systems where high integrity data transmission and reception
are required in an electrically severe environment.

USB232U/485Ui
Page 3
1.3.1 Enhanced Features
The USB232U supports data rates up to 920k Baud. The USB485Ui supports data rates up to
3M Baud. There is a 384 byte receive FIFO buffer and 128 byte transmit FIFO buffer
associated with each port. This allows the processor to service the ports in a timely manner
over the USB bus.
If you have an RS485 Half Duplex or Multi-drop application. There is a link selectable Half-
Duplex mode. In this mode, the RS485 transmit buffer is enabled automatically by the UART,
whenever data is waiting to be sent in the UART Transmit buffer. This feature makes efficient
multi-drop communication simple to implement by removing the burden of the Transmit buffer
enable control from the software.
1.4 What the Package Contains
The package as delivered from Amplicon Liveline Ltd contains:
1. The USB serial interface as ordered, in a protective bag. The model will be one of the
following, and is identified by the type number printed on the PCB.
USB232U USB to RS232 Serial Communications Controller
USB485Ui USB to RS422/485 Serial Communications Controller with I/O signal
isolation
2. The included distribution software and this manual on CD
3. Any additional accessories (mating connectors, software etc.) may be packed separately.
1.4.1 The Amplicon Warranty Covering the USB232U/485Ui interfaces
This product is covered by the warranty as detailed in the Terms and Conditions stated in the
current domestic or international Amplicon Liveline Ltd catalogue and at our website at
http://www.amplicon.co.uk.
1.4.2 Contacting Amplicon Liveline Limited for Technical Support or Service
The USB Series of interfaces are designed and manufactured by Amplicon Liveline Ltd and
maintenance is available throughout the supported life of the product.

USB232U/485Ui
Page 4
1.4.3 Technical Support
Should the device appear defective, please check the information in this manual and any 'Help'
or 'READ.ME' files appropriate to the program in use to ensure that the product is being
correctly applied.
If an application problem persists, please request Technical Support in one of the following
ways:
Web: www.amplicon.co.uk
Email: [e-mail]
Telephone: UK 0906 293 0293
International [phone]
Calls cost 25p per min from a BT landline. Calls from other services may vary
Fax: UK 01273 570 215
International [phone]
It will assist the support engineer if you have the following information available when you call:
Your Amplicon account number or postcode
Date of purchase
The Operating System you are running under
The specification of your computer
The nature of your problem and the results of any tests you have
conducted.
You may also be asked if the operating system prompted you for a disk when you installed the
device.
1.4.4 Repairs
If the unit requires repair then please return the goods enclosing a repair order detailing the
nature of the fault. If the device is still under warranty, there will be no repair charge.
For traceability when processing returned goods, a Returned Materials Authorisation (RMA)
procedure is in operation. Before returning the goods, please request an individual RMA
number by contacting Amplicon Customer Services as detailed above. Please mark the RMA
number on the outside of the packaging to ensure that the package is accepted by the Goods
Inwards Department.
Give the reason for the return and, if the goods are still under warranty, the original invoice
number and date. Repair turnaround time is normally five working days but the Service
Engineers will always try to co-operate if there is a particular problem of time pressure.
Address repairs to:
Customer Services Department
AMPLICON LIVELINE LIMITED
Centenary Industrial Estate

USB232U/485Ui
Page 5
Brighton, East Sussex
BN2 4AW
England

USB232U/485Ui
Page 6
2 GETTING STARTED
2.1 General Information
All of the USB232U/485Ui series are Plug and Play and come complete with all the software to
install and operate the device in any USB version 1.1 compliant host PC running under
Windows or Linux and allow execution of normal serial communications programs.
When installing one or more USB232U/485Ui Series Data Communications interfaces, ensure
that the host computer has sufficient capacity. The USB 1.1 standard has support for up to 127
physical devices attached to the bus. However the bandwidth and performance of the port will
be affected by bus loading, especially if high bandwidth devices are used as well.
The computer must run under one of the following operating systems: Windows 98, Windows
ME, Windows 2000, Windows XP or Linux.
2.2 Installing the Interface
Connect a USB Type B cable between the USB232U/485Ui serial communications device and
the host computer.
When the device is physically connected to the PC the Windows 98, 2000 or XP operating
system will discover new hardware. From then on respond to the questions asked. When
prompted for a disk use the disk supplied. Exact installation instructions are operating system
dependent.
2.2.1 Windows 98 Installation
Please refer to “Installing FT8U232/245 devices under Windows 98” (AN232-03.pdf) which can
be found on the associated CD.
2.2.2 Windows 2000/XP Installation
Please refer to “Installing FT8U232/245 devices under Windows 2000” (AN232-05.pdf) which
can be found on the associated CD.
2.2.3 Linux Installation
Linux Drivers for FT8U232BM and FT8U100BX devices have been added to Linux Kernel
2.4.0 and above. The devices will be detected automatically and will typically be configured as
/dev/ttyUSBn devices.

USB232U/485Ui
Page 7
2.3 USB232U/485Ui Configuration Jumper Settings
Most configuration settings on the USB485Ui are performed by the plug and play installation
program. However the USB485Ui has some jumpers and these need to be set before
connecting the unit to the host. The functions of the jumpers on the USB485Ui control whether
the unit operates in Full Duplex or Half Duplex mode and whether or not the RS485 port is
terminated.
2.3.1 RS485 Full Duplex / Half Duplex Operation
The RS485 port on a USB485Ui can be used in either full duplex or half duplex.
In full duplex operation, the transmitter can be left in its enabled state at all times. In half
duplex receive operation, the transmitter must be disabled by switching to its high impedance
state. This allows reception of data over the same serial lines.
2.3.1.1 Jumper Selection For Full Duplex Operation
Link Function
JP2 2-3
JP3 2-3
Full Duplex (4 wire) operation
JP4 1-2 Receiver enabled when device is awake
JP5 2-3 Transmitter enabled when device awake
Figure 2.1 Jumper Settings For Full Duplex Operation
2.3.1.2 Jumper Selection For Half Duplex Operation with Echo
Link Function
JP2 1-2
JP3 1-2
2 wire operation.
TxA is connected to RxA’ and TxB is connected to RxB’
JP4 1-2 LOCAL ECHO ON
Receiver enabled when device is transmitting
JP5 1-2 Transmitter only enabled when device is transmitting
Figure 2.2 Jumper settings for Half Duplex Operation with Echo
2.3.1.3 Jumper Selection For Half Duplex Operation without Echo
Link Function
JP2 1-2
JP3 1-2
2 wire operation.
TxA is connected to RxA’ and TxB is connected to RxB’
JP4 2-3 LOCAL ECHO OFF.
Receiver disabled when device is transmitting.
JP5 1-2 Transmitter only enabled when device is transmitting
Figure 2.3 Jumper settings for Half Duplex Operation without Echo

USB232U/485Ui
Page 8
2.3.2 Transmission Line Termination
2.3.2.1 Termination Resistors for RS422/485 Receivers
Operation of RS422/485 systems requires that the data lines be terminated at the final
receiver. The simplest termination is by a line-to-line resistor, and provision is made to connect
this 120Ωterminator using jumper, JP1, which is factory fitted.
Link Function
JP1 1-2 Data line terminated
JP1 2-3 Data line NOT terminated
Figure 2.4 Jumper Settings for Termination Resistor Selection
Further notes on RS422 and RS485 operation are given later in Chapter 4, 'Using the Serial
Communications Devices'
2.3.3 Jumper Factory Settings Summary
The USB485Ui interface can be reset to its factory defaults by re-configuring the device
according to the following tables.
Link Function
JP1 1-2 Data Line Terminated
JP2 2-3
JP3 2-3
Full Duplex (4 wire) operation
JP4 1-2 Receiver enabled when device is awake
JP5 1-2 Transmitter only enabled when device is transmitting
Figure 2.5 Factory Jumper Setting Summary

USB232U/485Ui
Page 9
2.4 Application Software
The ‘HyperTerminal’ program supplied with Windows is compatible with the USB232U/485Ui
interfaces and is useful for testing and operation on COM1 to COM4 channels.
The software supplied with this device supports operation with Windows 98, ME, 2000, and XP.
The device will operate in conjunction with all commercially available communications and
data acquisition packages that support Windows 98, 2000 and XP serial ports.
Install the required application software in accordance with the vendor’s instructions, assigning
communications to the required COM port.
2.4.1 Installation Testing
Many applications packages include a self-test feature, and when the hardware and software
have been satisfactorily installed, any such test should be performed to check correct
operation.

USB232U/485Ui
Page 10
3 MAKING THE CONNECTIONS
This chapter describes the signal and control connections that the user must make between the
USB232U/485Ui interfaces and any external serial communications devices.
3.1 External Connections
3.1.1 USB Connector
Connection to the USB port is made using a standard type B cable.
Contact Number Signal Name
1 VBUS
2 D-
3 D+
4 GND
Shell Shield Drain Wire
Figure 3.1 USB Connector Termination Assignment
3.1.2 RS232 9-way Port
Connections to the RS232 are made via 9-way D-type connectors on the interface. The on-
board connectors are male and the mating sockets are available as accessories. Provision is
made for securing by screw jacks.
The 9-way D-type connector pin-out conforms to the industry standard and will support
commercially available cables and adapters.
5
9
4
8
3
7
2
6
1
Ring Indicator (RI)
Clear To Send (CTS)
Request To Send (RTS)
Data Set Ready (DSR)
Reference Signal Ground (SG)
Data Terminal Ready (DTR)
Transmitted Data (TXD)
Data Carrier Detect (DCD)
Received Data (RXD)
PC Chassis Ground

USB232U/485Ui
Page 11
Figure 3.2 RS232 9-way Port Pin Designations
3.1.3 RS422/485 9-way Port
Connections to the RS485 are made via 9-way D-type connectors on the interface. The on-
board connectors are male and the mating sockets are available as accessories. Provision is
made for securing by screw jacks. The 9-way D-type connector pin-out compatible with the
default pin-out of the Amplicon PCI248H(i)/PCI249H(i) series boards.
Figure 3.3 RS422/485 Amplicon Standard Port Pin Designations
3.2 Connector Wiring for RS422/485 Full/Half Duplex
In the full duplex position, the serial RS422/485 data signals are transmitted on the wire pair
Tx-A (pin 4) and Tx-B (pin 5) and the incoming signals are received on Rx-A' (pin 8) and Rx-B'
(pin 9). In half duplex RS485 operation, the data signals are both transmitted and received on
the two pairs externally wired in parallel.
3.3 The Ground Connections
Each connector is equipped with two or more different ground connections and care must be
taken in the use of these grounds and the cable design to ensure that the EMC requirements
are met, and that isolation integrity is maintained in the case of the devices with isolated
input/output.
The connector pin-out is such that commercially available cables and adapters are normally
usable without modification, but the user must be aware of the requirements to maintain
isolation when the devices at each end are not at nominal earth potential. The choice of
proper, screened cables is also important for the system to maintain Electro-Magnetic
Compatibility.
3.3.1 USB232U Ground Connections
In full duplex operation, pins 4 and 5 provide the balanced
Transmitter output, pins 8 and 9 provide the balanced Receiver input
In half duplex operation, pins 4 and 5 are linked to pins 8 and 9 via
jumpers JP2 and JP3 to provide transceiver input and output
5
9
4
8
3
7
2
6
1
Received Data
-
B'
Received Data
-
A'
Transmitted Data
-
B
Transmitted Data
-
A
Reference Ground
PC Chassis Ground
(non-isolated version)

USB232U/485Ui
Page 12
Reference Signal Ground (pin 5 of 9-way) is internally connected to the PC USB signal
ground and should be externally connected to the Signal Ground pin of the RS232
device at the other end of the link.
The Connector Shell is internally connected to the PC USB chassis ground. Protective
ground should be externally connected to the protective ground pin of the RS232 device
at the other end of the link. To maintain shield integrity, metal connector hoods should
be used, preferably at both ends. These hoods make contact with the connector shell
and the cable screen so maintaining an overall shield.
3.3.2 USB485Ui Ground Connections
Reference Ground is internally connected to the isolated signal ground that must be
connected to the signal ground of the remote RS422/485 device. Any voltage appearing
between this reference ground and the host PC USB ground is common mode and must
not exceed the isolation voltage.
Connector Shell is not connected. To maintain shield integrity, a metal connector hood
should be used which must be connected to the cable screen at the remote device.
3.4 Choice of Cable
As speeds and distances of communication increase the choice of cable over which the
communication is to take place becomes critical.
Amplicon recommend Belden Cable, part no. 8132 for use in long distance high speed
RS422 & RS485 networks.
This is multi-shielded twisted pair cable. It has a capacitance of 20pF/m and
characteristic impedance of 120 ohms.
In order to maintain compliance with the EMC directive, 89/336/EEC, it is strongly
recommended that the final system integrator uses good quality screened cables for
external connections. It is up to the final system integrator to ensure that compliance
with the Directive is maintained. Please contact our sales staff for further information.

USB232U/485Ui
Page 13
4 USING THE SERIAL COMMUNICATIONS DEVICES
4.1 Why Isolate?
The USB485Ui provides complete electrical isolation between any signals connected to a serial
port, signals connected to the other port and the circuits of the host computer. In a compact
office or laboratory environment such isolation may not be necessary, however in a wide-field
industrial situation, serial digital communication can be disrupted by many types of
contaminating signals, most of which can be eliminated by the proper use of isolation.
Both input receivers and output drivers make use of opto-couplers to provide a signal path
between external and internal circuits without any electrical connection being made. An opto-
coupler comprises a Light Emitting Diode (LED) which is turned on by the incoming signal, and
a photo-detector that receives the light and converts it into an electrical signal. The emitter and
detector are mounted in a package that provides a light path together with high voltage
insulation between the two sides. Power to the input/output circuit is provided by an isolated
DC-DC converter.
Interfering signals can come from a variety of sources and are often greater in amplitude than
the desired signal. Such an interfering signal is 'common mode' and is effectively the potential
difference between the zero-signal reference points of the signal source and receiver. By using
the USB485Ui, this potential difference appears across the isolation barrier and not in series
with the signal as would be the case if direct connection was employed.
The two ends of a link would normally have their grounds connected together through the
signal ground line, the protective ground line and/or the cable shield. Such a connection has an
impedance such that an interfering signal caused by a difference in ground potentials is likely
to be a few volts in amplitude, maybe peaking to tens of volts. However there may be
circumstances or fault conditions in which higher common mode voltages exist, and the circuits
of the USB485Ui are designed to hold off peak common mode voltages up to 250V.
RS422 and RS485 standards provide a better level of ground noise immunity than RS232, the
maximum common mode voltages being –7.0 V to +7.0 V and –7.0 V to +12.0 V respectively
for the two standards. However, any ground offset, noise, hum or other interference exceeding
these peak amplitudes will be detected as real signals and may corrupt the data or control
functions.
Breaking the common ground connection by the use of the USB485Ui, completely eliminates
the detection of these interfering signals as real signals with no loss or corruption of data.
In general, isolation is only required at one end of a data link.
Multi-drop or broadcast systems with many RS485 or RS422 nodes on a single circuit may
require all circuits to be isolated for error free serial data communication.
DANGER
High Voltage
WARNING
DANGER OF ELECTRIC SHOCK
When the USB485Ui isolated device is in use without the
supplied plastic case, high voltages may be exposed on the
connector and associated circuitry.

USB232U/485Ui
Page 14
4.2 Application Notes
As previously noted, many applications packages are designed to support data transfer over
the standard serial communications ports. When set up as specified and conventional multi-
way cables and connectors are in use, then the interface will be transparent to the user.
However, for special applications, particularly when use of the RS485 interface is required, the
following notes will be helpful.
4.2.1 Applicable Standards
4.2.1.1 RS Standards
The prefix 'RS' signifies Recommended Standard, and the three interface types, RS232,
RS422 and RS485, supported by the Amplicon range of devices are all well defined in the
appropriate standards publications. The functions and features of the data and control
interfaces presented by the USB232U/485Ui series generally conform to normal industry
practice employing a subset of the full standards.
The RS232 serial interface standard is now at revision E and its full title is EAI/TIA RS232E,
"Interface Between Data Terminal Equipment and Data Communication Equipment Employing
Serial Binary Data Interchange". Similar European standards CCITT V.24 and V.28 define an
interface which is operationally compatible with RS232.
The RS232 port of the USB232U are designated as DTE (Data Terminal Equipment) in
accordance with normal practice for a computer driven serial port. The current RS232 standard
specifies the 25 way male D-type connector for DTE. The full 25 lines are not required to
support the common functions of RS232, and IBM and other manufacturers frequently adopt a
9-way D-type connector that provides all the necessary functionality. Where this 9-way system
is used, manufacturers conform to interface standard EIA 574, with the pin-out configuration as
shown in Figure 3.2.
The RS422 interface is defined in the EIA/TIA standard RS422-B, "Electrical Characteristics of
Balanced Voltage Digital Interface Circuits". As stated, this standard defines the electrical
characteristics but does not specify a connector type or pin-out configuration. The USB485Ui
meets the RS422 standard specification with communication via a 9-way D-type connector
wired as shown in Figure 3.3 with the exception of RTS and CTS control lines.
The RS485 specification is also published as EIA standard EIA 530 pt 87 and is closely allied
to the RS422 standard. The USB485Ui meets the requirements laid down in the RS485
standard.
4.2.2 RS232 Application Notes
RS232 was approved as a standard interface in 1969. Before and since that time this interface
in its various revisions has been very popular despite its many shortcomings and usage outside
its intended role. Most small computers now have an RS232 interface as standard, or easily
added, and many peripherals and instruments support this serial interface, but not all in the
same way. Hence the application of an RS232 connection needs careful study of the
equipment manufacturers connections and methods before the devices are linked. However
RS232 lines are quite safe if wrongly connected, so the experimental use of a break-out box or
'hit and miss' methods of establishing communication are often employed.