Omega OMG-CONV-485-ISO User Manual

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User’s Guide

OMG-CONV-485-ISO

Isolated RS-232 to RS-422/485 Converter

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It is the policy of OMEGA to comply with all worldwide safety and EMC/EMI regulations that
apply. OMEGA is constantly pursuing certification of its products to the European New Approach
Directives. OMEGA will add the CE mark to every appropriate device upon certification.

The information contained in this document is believed to be correct, but OMEGA Engineering, Inc. 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, patient-connected applications.

Contents

INTRODUCTION..........................................................................................................1

OVERVIEW...................................................................................................................1

WHAT ’S INCLUDED....................................................................................................1

FACTORY DEFAULT SETTINGS................................................................................1

CARD SETUP ................................................................................................................2

RS-485 E NABLE MODES.............................................................................................2

LINE TERMINATION AND RECEIVER ECHO OFF/ON.............................................4

OPERATING SYSTEM INSTALLATION (FOR WINDOWS USERS)..............6

RS-422/RS-485 Four Wire Master/RS-485 Two Wire Auto Mode

Installation ............................................................................................................ 6

RS-485 Two-Wire and RS-485 Four -Wire Slave Installation......................7

Other Operating Systems....................................................................................8

SYSTEM INSTALLATION.........................................................................................9

TECHNICAL DESCRIPTION...................................................................................10

CONNECTOR PIN ASSIGNMENTS............................................................................10

RS-232 Signals (DB-25 Female).....................................................................10

RS-422/485 Pin Assignments (6 Position Screw Terminal).......................10

SPECIFICATIONS......................................................................................................11

ENVIRONMENTAL SPECIFICATIONS......................................................................11

MANUFACTURING....................................................................................................11

MEAN TIME BETWEEN FAILURES (MTBF).........................................................11

PHYSICAL DIMENSIONS...........................................................................................11

APPENDIX A - TROUBLE SHOOTING..................................................................12

APPENDIX B - HOW TO GET ASSISTANCE......................................................13

APPENDIX C - ELECTRI CAL INTERFACE..........................................................14

RS-232.........................................................................................................................14

RS-422.........................................................................................................................14

RS-485.........................................................................................................................15

APPENDIX D - GROUND LOOP PHENOMENON..............................................16

WHAT IS G ROUND LOOP?.......................................................................................16

CABLING RECOMMENDATIONS..............................................................................16

APPENDIX E - ASYNCHRONOUS COMMUNICATIONS.................................17

APPENDIX F - SILK-SCREEN .................................................................................18

APPENDIX G - COMPLIANCE NOTICES.............................................................19

FEDERAL COMMUNICATIONS COMMISSION STATEMENT ...............................19

EMC DIRECTIVE STATEMENT ..............................................................................19

Figures

Figure 1- Dip switch SW1, RS-422 ....................................................................3

Figure 2 – Dip switch SW1, RS-485 ‘Auto’ Enable.........................................3

Figure 3 – Dip switch SW1, RS-485 ‘RTS’ Enable.........................................3

Figure 4 – Dip switch SW1, RS-485 ‘Data’ Enable.........................................3

Figure 5 – Dip switch SW2, Line Termination and Echo Off/On.................4

Figure 6 – RS-422 with Termination and Receiver Echo...............................5

Figure 7 – RS-485 2-wire with Termination and Receiver No Echo............5

Figure 8 – RS-485 2-wire no Termination and Receiver No Echo................5

Figure 9 – OMG-CONV-485-ISOI cabling example.....................................16

Figure 9 - Asynchronous Communications Bit Diagram.............................17

Introduction

Introduction

Overview

The OMG-CONV-485-ISO is a single channel RS-232 to RS-422/485
asynchronous converter. It provides one field selectable RS-422/485 serial port
supporting asynchronous data rates up to 115.2K bps. The OMG-CONV-485-
ISO is self-powered allowing it to correctly drive multi -drop RS-485 networks.

Configure the port as RS-422 for long distance device connections up to 4000ft.
where noise immunity and high data integrity are essential. Select RS-485 and
capture data from multiple peripherals in an RS-485 multi-drop network. Up to 31
RS-485 devices can be connected to automate your data collection.

In RS-422 and RS-485 Auto modes, the card works seamlessly with the standard
operating system serial driver. Two additional methods are provided to control
the communication line in RS-485 mode. The “Data Enable” mode uses the
transmitted data to enable the transmitter and requires no software drivers. This
mode has limitations that will be discussed in more detail later. The “RTS Enable”
mode uses the UART modem control signal RTS to control the communication
line and requires the serial driver.

What’s Included

The OMG-CONV-485-ISO is shipped with the following items. If any of these
items is missing or damaged, contact the supplier.

• OMG-CONV-485-ISO Serial I/O Adapter

• Software

• Power Supply

Factory Default Settings

The OMG-CONV-485-ISO factory default settings are as follows:

Port # Electrical Specification

Port 1 RS-422

To install the OMG-CONV-485-ISO using factory default settings, refer to
Installation.
For your reference, record installed OMG-CONV-485-ISO settings below:

Port # Electrical Specification

Port 1

OMG-CONV-485-ISO Page 1

Card Setup

Card Setup

RS-485 Enable Modes

RS-485 is ideal for multi-drop or network environments. RS-485 requires a tri-state
driver that will allow the electrical presence of the driver to be removed from the
line. The driver is in a tri-state or high impedance condition when this occurs.
Only one driver may be active at a time and the other driver(s) must be tri-stated.
The output modem control signal Request To Send (RTS) is typically used to
control the state of the driver. Some communication software packages refer to
RS-485 as RTS enable or RTS block mode transfer.

Dip switch SW1 is used to control the RS-485 mode functions for the driver
circuit. The selections are:

‘RS-422 Mode’ (Always). This is the factory default mode that ensures the
transmitter is always enabled. This mode is intended for Full Duplex applications
such as RS-422 or RS-485 4-wire Master mode where tri-state control of the
transmitter is not required.

‘RS-485 Auto Mode’. This mode uses an onboard MicroChip PIC
microcontroller to sense the baud rate and control the transmitter enable signal
based on the detected baud rate. The logic assumes a minimum baud rate of
1200bps and a word length of 10 bits (1 start bit, 8 data bits, and 1 stop bit). For
baud rates of 38.4K or less, line turnaround should not exceed 4 bit times, 57.6K
should not exceed 10 bit times, and 115.2K should not exceed 20 bit times after
successfully autobauding.

‘RS-485 RTS Enable Mode’. The ‘RTS’ enable mode uses the ‘RTS’ modem
control signal to enable the RS-485 interface and provides backward
compatibility with existing software products. The serial driver should be used
and configured for ‘RTS’ control providing a robust RS-485 solution.

‘RS-485 Data Enable Mode’. The ‘Data’ enable mode uses the transmit data
stream to enable/disable the transmitter. When the data bit is High (logic 1), the
transmitter will be tri-stated and relies on the line biasing resistors to place the
line in the proper state. For this reason, line termination cannot be used thus
reducing the data rate and maximum cable length. This mode should only be used
in applications that prevent the use of the RS-485 Auto Mode or the serial driver.

OMG-CONV-485-ISO Page 2

Card Setup

Communication Mode Examples (Dip switch SW1)

ON

OFF

1
2

Figure 1- Dip switch SW1, RS-422

OFF

ON

1
2

Figure 2 – Dip switch SW1, RS-485 ‘Auto’ Enable

OFF

ON

1
2

Figure 3 – Dip switch SW1, RS-485 ‘RTS’ Enable

OFF

ON

1
2

Figure 4 – Dip switch SW1, RS-485 ‘Data’ Enable

OMG-CONV-485-ISO Page 3

Card Setup

Line Termination and Receiver Echo Off/On

Typically, each end of the RS-485 bus must have a line-terminating resistor
(RS-422 terminates at the receive end only). A 120-ohm resistor is across each
RS-422/485 input in addition to a 1K-ohm pull-up/pull-down combination that
biases the receiver inputs. Dip switch SW2 allows customization of this interface
to specific requirements. Each dip switch position corresponds to a specific
portion of the interface. If multiple OMG-CONV -485-ISO adapters are configured
in an RS-485 network, only the adapters on each end should have dip switch
SW2 position 1 on. One adapter should have dip switch SW2 positions 2 and 3
on. Refer to the table below for each position’s operation.

Dip switch SW2 position 6 is used to control the RS-485 enable/disable functions
for the receiver circuit and determines the state of the RS-422/485 driver. The RS­485 ‘Echo’ is the result of connecting the receiver inputs to the transmitter
outputs. Every time a character is transmitted, it is also received. This can be
beneficial if the software can handle echoing (i.e. using received characters to
throttle the transmitter) or it can confuse the system if the software does not. To
enable the ‘No Echo’ mode dip switch SW2 position 6 should be on.

SW

2

1 Adds or removes the 120-ohm termination
2 Adds or removes the 1K-ohm pull-up resistor in the RS-422/RS-485

receiver circuit (Receive data only)

3 Adds or removes the 1K-ohm pull-down resistor in the RS-422/RS-485

receiver circuit (Receive data only)
4 Connects TX+ to RX+ for RS-485 two-wire operation
5 Connects TX- to RX- for RS-485 two-wire operation

6 Receiver Echo Off/On (On = Echo Off)

Function

ON

OFF

1
2
3

4
5
6

Figure 5 – Dip switch SW2, Line Termination and Echo Off/On

OMG-CONV-485-ISO Page 4