CyberResearch ETS 2404 User manual

Serial Communications
USER’S MANUAL
VER. 2.0C • FEB 2008
No part of this manual may be reproduced without permission
ETS 2604
Ethernet to 4-Port RS-232/422/485
Interface Module, 230.4 kbps Data
Rate, with 4 DB-9M Connectors
®
CyberResearch®,Inc.
www.cyberresearch.com
25 Business Park Dr., Branford, CT 06405 USA
203-483-8815 (9am to 5pm EST) FAX: [phone]

CyberResearch® Serial Communications ETS 2604
CyberResearch, Inc. iii
25 Business Park Drive P: [phone]; F: [phone]
Branford, CT USA www.cyberresearch.com
©Copyright 2008
All Rights Reserved.
February 6th 2008
The information in this document is subject to change without prior notice
in order to improve reliability, design, and function and does not represent
a commitment on the part of CyberResearch, Inc.
In no event will CyberResearch, Inc. be liable for direct, indirect, special,
incidental, or consequential damages arising out of the use of or inability
to use the product or documentation, even if advised of the possibility of
such damages.
This document contains proprietary information protected by copyright.
All rights are reserved. No part of this manual may be reproduced by any
mechanical, electronic, or other means in any form without prior written
permission of CyberResearch, Inc.
Trademarks
“CyberResearch,” and “ETS 2604,” are trademarks of CyberResearch, Inc.
Other product names mentioned herein are used for identification
purposes only and may be trademarks and/or registered trademarks of
their respective companies.
• NOTICE •
CyberResearch, Inc. does not authorize any CyberResearch product for
use in life support systems, medical equipment, and/or medical devices
without the written approval of the President of CyberResearch, Inc. Life
support devices and systems are devices or systems which are intended
for surgical implantation into the body, or to support or sustain life and
whose failure to perform can be reasonably expected to result in injury.
Other medical equipment includes devices used for monitoring, data
acquisition, modification, or notification purposes in relation to life
support, life sustaining, or vital statistic recording. CyberResearch
products are not designed with the components required, are not subject
to the testing required, and are not submitted to the certification required
to ensure a level of reliability appropriate for the treatment and diagnosis of
humans.

ETS 2604 CyberResearch® Serial Communications
iv ©Copyright 2008CyberResearch, Inc.
ETS 2604
Revision # Description Date of Issue
1.0h Initial Release July 2002
2.0C Revision Feb 7th 2008

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Contents
INTRODUCTION.....................................................................................................................1
OVERVIEW .........................................................................................................................................1
WHAT’S INCLUDED............................................................................................................................1
FACTORY DEFAULT SETTINGS ...........................................................................................................1
CARD SETUP .........................................................................................................................2
ELECTRICAL INTERFACE SELECTION..................................................................................................2
RS-485 ENABLE MODES ....................................................................................................................2
SOFTWARE INSTALLATION...................................................................................................3
WINDOWS ..........................................................................................................................................3
LOCATING THE DEVICE AND ASSIGNING AN IP ADDRESS...................................................................3
SOFTWARE DEVICE CONFIGURATION .................................................................................4
WINDOWS ..........................................................................................................................................4
WEB CONFIGURATION .......................................................................................................................4
SECURITY...........................................................................................................................................4
PORT DEFAULTS.................................................................................................................................4
CUSTOM BAUD RATES .......................................................................................................................4
RS MODES .........................................................................................................................................4
RESETTING TO FACTORY DEFAULTS ..................................................................................................4
USING THE DEVICE WITH SUPPLIED SOFTWARE .................................................................5
TACTICAL SOFTWARE ........................................................................................................................5
Configuration .................................................................................................................................5
Usage Details..................................................................................................................................5
Configuration .................................................................................................................................6
Usage Details..................................................................................................................................6
RAW DATA SOCKETS .........................................................................................................................7
Configuration .................................................................................................................................7
Usage Details..................................................................................................................................7
TECHNICAL DESCRIPTION....................................................................................................8
CONNECTOR PIN ASSIGNMENTS.........................................................................................................8
RS-422/485 (DB 9 Male)...............................................................................................................8
RS-232 (DB-9 Male)......................................................................................................................8
SPECIFICATIONS ...................................................................................................................9
ENVIRONMENTAL SPECIFICATIONS ....................................................................................................9
MANUFACTURING ..............................................................................................................................9
MEAN TIME BETWEEN FAILURES (MTBF).........................................................................................9
APPENDIX A-TROUBLESHOOTING....................................................................................10
APPENDIX B-HOW TO GET ASSISTANCE .........................................................................11
APPENDIX C-ELECTRICAL INTERFACE............................................................................12
RS-232 ............................................................................................................................................12
RS-422 ............................................................................................................................................12
RS-485 ............................................................................................................................................12

ETS 2604 CyberResearch® Serial Communications
vi ©Copyright 2008CyberResearch, Inc.
APPENDIX D-ASYNCHRONOUS COMMUNICATIONS .........................................................13
APPENDIX E–BACK PANEL...............................................................................................14
APPENDIX F-COMPLIANCE NOTICES ...............................................................................15
FEDERAL COMMUNICATIONS COMMISSION STATEMENT..................................................................15
EMC DIRECTIVE STATEMENT..........................................................................................................15

CyberResearch® Serial Communications ETS 2604
CyberResearch, Inc. 1
25 Business Park Drive P: [phone]; F: [phone]
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Introduction
Overview
The CyberResearch ETS 2604 is a four-port RS-232/422/485 Ethernet based serial I/O adapter supporting data rates
up to 230.4 K bps.
The RS-232 compatibility allows for connection to devices utilizing the RS-232 electrical interface, such as
modems, data-entry terminals, and plotters.
RS-422 provides excellent communications for long distance device connections up to 4000ft., where noise
immunity and high data integrity are essential.
RS-485 is optimized for ‘Multi-Drop’ or ‘Party-line’ operations selecting data from multiple peripherals (as many as
31 devices can be connected on an RS-485 bus).
What’s Included
The ETS 2604 is shipped with the following items. If any of these items is missing or damaged, contact
CyberResearch, Inc.
•ETS 2604 Ethernet Terminal Adapter
•CyberResearch Software
•5 volt power adapter
•Ethernet cable, straight through (yellow)
•Ethernet cable, cross over (blue)
Factory Default Settings
The ETS 2604 factory hardware default settings for all ports are as follows:
Set up for RS422 or RS485.
T = 120 ohm termination between RX+ an d RX -
PU Pull up on RX +
PU Pull up on RX -
1 2 3 4 5 6 7
ON OFF 8
EN
T
PD
PU
EN
T
PD
PU
RS-422/485

ETS 2604 CyberResearch® Serial Communications
2©Copyright 2008 CyberResearch, Inc.
Card Setup
Electrical Interface Selection
Each port on the ETS 2604 has the ability to be used in either RS-232 or RS-422/485. This is selectable using the
software setup utility and correctly configuring DIP-switches S1-S4.
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. One of the unique features of the ETS 2604 is the ability to be RS-485 compatible without the
need for special software or drivers. This ability means that the user can effectively use the ETS 2604 in an RS-485
application with the standard CyberResearch Software supplied drivers.
Typically, each end of the RS-485 bus must have a line-terminating resistor (RS-422 terminates the receive end
only). A 120-ohm resistor (silk-screen ‘T’) is across each RS-422/485 input in addition to a 1K-ohm pull-up (silk-
screen ‘PU’) and a 1K-ohm pull down (silk-screen ‘PD’) combination that biases the receiver inputs. Only the ends
of an RS-485 network should have the 120-Ohm terminating resistor. The presence of the 120-Ohm termination
resistor across the input data pins (RX+/RX-) as well as the pull-up and pull-down resistors may be removed or
inserted with DIP-switches S1-S4. To add the termination/bias resistors select the ‘On’ position, to remove it select
the ‘Off’ position.
1 2 3 4 5 6 7
ON OFF 8
EN
T
PD
PU
EN
T
PD
PU
RS-232
1 2 3 4 5 6 7
ON OFF 8
EN
T
PD
PU
EN
T
PD
PU
RS-422/485
1 2 3 4 5 6 7
ON OFF 8
EN
T
PD
PU
EN
T
PD
PU
RS-422/485 No termination

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Software Installation
Software installation depends on your operating system, so see the appropriate section below.
Windows
If you choose to use the ETS 2604 device as a virtual serial port, you will need to install Tactical Software's serial
port redirector software Serial/IP. A 30-day trial version is included on the cd. For installation help or to download
the latest version of Serial/IP, consult www.tactical-sw.com. For software configuration, see the section "Using the
Device with supplied Software" below. In addition to the Serial/IP program, a utility to configure the ETS 2604
device from Windows is provided.
Locating the Device and Assigning an IP Address
Before the ETS 2604 can be used, it must be assigned an IP address valid to your network. When first powered up,
the device will attempt to automatically find an IP address using DHCP. If the device has link but cannot locate a
DHCP server, the link LED on the box will flash, indicating that the device does not have valid network settings. In
that case, you will need to assign it a static address using the configuration utility provided. The configuration utility
is also useful for locating devices to determine the IP address. The method depends on your operating system, so see
the appropriate section below.
Tip: If your device has obtained DHCP (the link light is solid) and you do not wish to put it on the local network,
you can use your DHCP server software to find its IP using the MAC address located on the device's label. Once
you have the IP, configure the device via the web (see the section Web Configuration).

ETS 2604 CyberResearch® Serial Communications
4©Copyright 2008 CyberResearch, Inc.
Software Device Configuration
Windows
Windows device configuration is performed through the “SeaLink Configuration” utility provided with the software
installation. When started, the utility will display all devices on the local network (the utility cannot see devices on
the other side of a router). If your device is not listed, verify that it is connected to the network (see the section
Troubleshooting). To assign static IP addresses, select your device in the list, then uncheck the box that says
“Enable DHCP” and change the “IP”, “Gateway”, and “Network Mask” fields to values valid on your network.
Then click “Apply Changes”. The device will reboot and temporarily disappear from view, so you may need to click
“Search Again” to relocate the device.
Web Configuration
Once the device has been assigned an IP address reachable from your computer, you can configure it through the
onboard web server. Open a web browser such as Internet Explorer or Netscape and type “http://x.x.x.x” into the
URL field, where “x.x.x.x” is the device’s IP address. The web interface provides a summary page that lists various
details about the device, a port settings page that allows you to configure aspects of each individual serial port, and
an administration page that lets you change the device’s network and password settings.
Security
The password you can set from the Administration page currently applies to the Web Configuration only. The web-
based settings have been identified as a key area where security is important, since users do not need to install
software to modify settings.
Port Defaults
The Port Settings page allows you to set defaults to which each serial port will be set when first opened. If you are
using a virtual serial port protocol such as SeaLink or Tactical Software, programs may change these settings
dynamically after opening the ports. If you want programs to always use the defaults you set, check the box labeled
"Always use defaults". This can be useful especially when specifying a custom baud rate that is not available from
your operating system (see Custom Baud Rates).
Custom Baud Rates
From the Port Settings Page, you can enter a baud rate of any value from 1 to 921600. While it may not be possible
to hit the exact desired baud rate, the Uart will attempt to set it as closely as possible and will be within an
acceptable 2% range. After you click Submit, the page will refresh and display the baud rate actually being used.
RS Modes
The Port Settings page allows you to specify the RS mode for a given port. This may not be the best way to
configure RS modes in all situations (see the section SeaLink Protocol::Configuration). However, if you are using a
protocol that does not allow you to configure the RS mode, you must set the RS mode here.
In all cases, the RS mode must be set in software as well as hardware. See the section Electrical Interface Selection
for details on configuring the hardware.
Resetting to Factory Defaults
If you have forgotten your password or otherwise wish to reset the device to the factory defaults, manually browse
to "http://x.x.x.x/debug.htm" (where x.x.x.x is the device's IP address) and click the button marked "Reset to Factory
Defaults". You will be prompted for a username and password. For the username, enter "debug". In the password
field, type the MAC address of the unit, located either on the summary page or on the label on the device. The MAC
address should be entered without formatting such as dashes or spaces.

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Using the Device with supplied Software
Accessing the serial ports through software can depend on your situation. There are two basic methods. The first
method implements a virtual serial port on your computer, so you can access the remote port just like a normal
onboard serial port. This is probably the best method for most situations, and allows for the most control. To use
virtual serial ports in Windows, see the section Tactical Software. The second method may be useful for software
developers and allows you to send/receive data through a serial port using TCP sockets. For more information, see
the section Raw Data Sockets.
Tactical Software
Tactical Software (www.tactical-sw.com) implements a serial port “redirector” called Serial/IP, which allows you to
create a virtual serial port on your Windows machine. All accesses to the virtual port are redirected to the remote
port through the IP address you set, and you can access the virtual port with any program designed to use serial
ports. For detailed instructions on using Tactical Software’s Serial/IP program, consult the help packaged with that
software. What follows is an overview of getting the software to work with the ETS 2604.
Configuration
To install a SeaLink serial port, configure a new Tactical Software virtual port and point it to the device’s IP
address. Note that the port number here refers to the TCP port and not the serial port on the device; however, the
TCP port number to use depends on which serial port you want. The base TCP port for RFC 2217, the protocol
which Tactical Software uses, is 4680. That corresponds to the first serial port on the device. Just increment the
TCP port to get to the next serial port. For example, the second serial port would be at TCP port 4681, the third at
4682, and so on. You must also select “Telnet” from the list of 3 protocols for each port to work correctly. If you
want the software to try and reconnect in case of accidental network loss, click the appropriate checkbox. Note that
the username and password field should be left blank even if you have a password set in the Web Configuration (see
the section Web Configuration::Security for more details).
You must also configure the RS mode before use if your device supports multiple electrical interfaces. Use the Web
Configuration page to do this (see the section Web Configuration). Browse to the Port Settings page and select the
appropriate RS mode for each port, and then click the Submit button at the bottom of the page. Be sure to configure
the hardware to match the desired setting (see section Electrical Interface Selection).
Usage Details
Since Tactical Software virtualizes a serial port, any program that works with serial ports is able to access a remote
port just like an onboard port. Just use the program to open the COMx number you specified when installing the
port. Software developers can use the standard serial port API. See the Microsoft help on Communications
Resources for more information.

ETS 2604 CyberResearch® Serial Communications
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Configuration
Before you can access a serial port, you must “install” it to the client computer. Launch the “SeaLink
Configuration” console utility by typing “/usr/sbin/sealinkcfg”. If your device is on the local network, you should
see it listed. Type ‘i’ followed by the number of the hub in the list (probably “i 1”) to install it. If your device is not
listed, type 'n' and follow the prompts to tell the software about your device.
You must also configure the RS mode before use if your device supports multiple electrical interfaces. To do this,
open the “SeaLink Configuration” utility by typing “/usr/sbin/sealinkcfg” and set each port’s mode using the ‘p’
command. Be sure to configure the hardware to match the desired setting (see section Electrical Interface Selection).
Warning: Be careful when setting the RS mode, because the mode may conflict with the one set in the Web
Configuration page. If the two conflict and the “Always use defaults” checkbox is not checked, the mode may be set
incorrectly for a brief period or, worse, just set incorrectly. To be safe, leave the Web Configuration page’s RS
mode set to “No Default”.
When finished configuring ports, type ‘q’ to quit. The SeaLink driver must be restarted (or started for the first time),
so type either “/sbin/sealink restart” or “/sbin/sealink start”. If one gives you an error, try the other one.
Usage Details
Once a device has been installed, it can be accessed through the character devices “/dev/ttySL[0-n]”, where ‘n’ is
the number of ports on your device – 1. For example, “echo hello > /dev/ttySL0” will send the ASCII characters
“hello” to the first serial port on your device, while “echo hello > /dev/ttySL2” will do the same for the third serial
port. Additional installed ports will be assigned to the next available range, so if you have two four-port devices
installed they will be assigned to “/dev/ttySL[0-3]” and “/dev/ttySL[4-7]” respectively.
The /dev/ttySL devices are designed to virtualize a serial port, so programs can access them just like a /dev/ttyS port
using the termios functions (see the man page for termios). Note that unlike a normal serial port, SeaLink serial
ports can only be opened by one program at a time. This is done to ensure that the correct client receives the correct
data/signals.

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Raw Data Sockets
Raw Data Sockets are probably the easiest way for a software developer to control a serial port, but provide the least
control from a software standpoint. You can simply open a socket to the device on the Raw Data TCP port
corresponding to the serial port you want and stream data back and forth. The data will not be inspected or modified
in any way. This method provides the least control because serial port functions such as baud rate and modem
control signals cannot be monitored or changed programmatically. However, Raw Data is platform-independent and
requires no software installation. If your purposes do not require dynamic serial port control, Raw Data may be the
most efficient choice for you.
Configuration
Since Raw Data does not allow for programmatic changing of serial port controls, the serial ports must be
configured before use via the onboard web server (see the section Web Configuration). From the Port Settings page,
configure the ports to the desired settings and click Submit at the bottom of the page. The serial ports will
automatically be set to these defaults when opened. When setting the RS mode, be sure to configure the hardware to
match the desired setting (see section Electrical Interface Selection).
Usage Details
To access a serial port using Raw Data mode, open a TCP socket to the device’s IP address. The TCP port to use
depends on the serial port you want. The Raw Data base is 4760, which corresponds to the first serial port on the
device. Just increment the TCP port to get to the next serial port. For example, the second serial port would be at
TCP port 4761, the third at 4762, and so on.
Only one client can open each serial port at a time. All further socket requests to a TCP port will be denied.

ETS 2604 CyberResearch® Serial Communications
8©Copyright 2008 CyberResearch, Inc.
Technical Description
The CyberResearch ETS 2604 provides a PCI interface adapter with 4 RS-232/422/485 asynchronous serial ports
for industrial automation and control applications.
Connector Pin Assignments
RS-422/485 (DB 9 Male)
Signal Name Pin # Mode
GND Ground 5
TX + Transmit Data Positive 4 Output
TX- Transmit Data Negative 3 Output
RTS+ Request To Send Positive 6 Output
RTS- Request To Send Negative 7 Output
RX+ Receive Data Positive 1 Input
RX- Receive Data Negative 2 Input
CTS+ Clear To Send Positive 9 Input
CTS- Clear To Send Negative 8 Input
RS-232 (DB-9 Male)
Signal Name Pin # Mode
GND Ground 5
TD Transmit Data 3 Output
RTS Request To Send 7 Output
DTR Data Terminal Ready 4 Output
RD Receive Data 2 Input
CTS Clear To Send 8 Input
DSR Data Set Ready 6 Input
DCD Data Carrier Detect 1 Input
RI Ring Indicator 9 Input
Technical Note: Please terminate any control signals that are not going to be used. The most common way to do this
is connect RTS to CTS and RI. Also, connect DCD to DTR and DSR. Terminating these pins, if not used, will help
ensure you get the best performance from your adapter.

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Specifications
Environmental Specifications
Specification Operating Storage
Temperature
Range 0º to 50º C
(32º to 122º F) -20º to 70º C (-4º to 158º F)
Humidity Range 10 to 90% R.H.
Non-Condensing 10 to 90% R.H. Non-Condensing
Manufacturing
•All Printed Circuit boards are built to U. L. 94V0 rating and are 100% electrically tested. These printed circuit
boards are solder mask over bare copper or solder mask over tin nickel.
Mean Time Between Failures (MTBF)
Greater than 150,000 hours. (Calculated)

ETS 2604 CyberResearch® Serial Communications
10 ©Copyright 2008 CyberResearch, Inc.
Appendix A - Troubleshooting
CyberResearch Software is supplied with the ETS 2604 and will be used in the troubleshooting procedures. Using
this software and following these simple steps can eliminate most common problems, without the need to call
Technical Support.
1. Verify that the Power LED is lit and remains on. Caution: Use only the supplied AC/DC adapter. Any
other adapter may cause damage to the ETS 2604. If a different power solution must be made, please
contact CyberResearch, Inc.
2. Verify that the Link LED remains on to determine that the 10/100 Base-T connections and the polarity are
correct. If the Link LED is lit, you have a valid Link condition.
3. Verify that the Ethernet LED remains on to determine that the 10/100 Base-T connections and the polarity
are correct. If the Ethernet LED is lit, you have a working Ethernet connection
4. Verify that the correct cable is being used (i.e. ‘Straight-thru’ for switch/hub connection and ‘cross over’
for direct connection to NIC), and that all cables are connected securely.
5. Verify that the Ethernet hub and any other network devices between the server and the ETS 2604 are
powered up and operating.
6. To isolate the unit from the network, use a ‘crossover’ Ethernet cable to connect the unit directly to the
NIC card in the server.
7. Reboot the server.
8. Ensure that server software is correctly loaded.
9. Remove and reload the server software.
10. Verify that the Ethernet hub and any other network devices between the server and the ETS 2604 are
powered up and operating.
Always use the CyberResearch diagnostic software when troubleshooting a problem. This will eliminate any
software issues from the equation. If you are unable to resolve the problem, contact Technical Support.

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Appendix B - How To Get Assistance
Please refer to

ETS 2604 CyberResearch® Serial Communications
12 ©Copyright 2008 CyberResearch, Inc.
Appendix A - Troubleshooting prior to calling Technical Support.
1. Read this manual thoroughly before attempting to install the adapter in your system.
2. When calling for technical assistance, please have your user manual and current adapter settings. If
possible, please have the adapter installed in a computer ready to run diagnostics.
3. http://www.cyberresearch.com provides the latest software updates and newest manuals for all our
products.
4. Technical support is available Monday thru Friday from 8:00 a.m. to 5:00 p.m. eastern time. Technical
support can be reached at [phone].

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Appendix C - Electrical Interface
RS-232
Quite possibly the most widely used communication standard is RS-232. This implementation has been defined and
revised several times and is often referred to as RS-232-C/D/E or EIA/TIA-232-C/D/E. It is defined as “Interface
between Data Terminal Equipment and Data Circuit- Terminating Equipment Employing Serial Binary Data
Interchange”. The mechanical implementation of RS-232 is on a 25-pin D sub connector. The IBM PC computer
defined the RS-232 port on a 9 pin D sub connector and subsequently the EIA/TIA approved this implementation as
the EIA/TIA-574 standard. This standard has defined as the “9-Position Non-Synchronous Interface between Data
Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange”. Both
implementations are in wide spread use and will be referred to as RS-232 in this document. RS-232 is capable of
operating at data rates up to 20K bps / 50 ft. The absolute maximum data rate may vary due to line conditions and
cable lengths. RS-232 often operates at 38.4K bps over very short distances. The voltage levels defined by RS-232
range from -12 to +12 volts. RS-232 is a single ended or unbalanced interface, meaning that a single electrical
signal is compared to a common signal (ground) to determine binary logic states. A voltage of +12 volts (usually +3
to +10 volts) represents a binary 0 (space) and -12 volts (-3 to -10 volts) denote a binary 1 (mark). The RS-232 and
the EIA/TIA-574 specification define two types of interface circuits Data Terminal Equipment (DTE) and Data
Circuit-Terminating Equipment (DCE). The CyberResearch Adapter is a DTE interface.
RS-422
The RS-422 specification defines the electrical characteristics of balanced voltage digital interface circuits. RS-422
is a differential interface that defines voltage levels and driver/receiver electrical specifications. On a differential
interface, logic levels are defined by the difference in voltage between a pair of outputs or inputs. In contrast, a
single ended interface, for example RS-232, defines the logic levels as the difference in voltage between a single
signal and a common ground connection. Differential interfaces are typically more immune to noise or voltage
spikes that may occur on the communication lines. Differential interfaces also have greater drive capabilities that
allow for longer cable lengths. RS-422 is rated up to 10 Megabits per second and can have cabling 4000 feet long.
RS-422 also defines driver and receiver electrical characteristics that will allow 1 driver and up to 32 receivers on
the line at once. RS-422 signal levels range from 0 to +5 volts. RS-422 does not define a physical connector.
RS-485
RS-485 is backwardly compatible with RS-422; however, it is optimized for party line or multi-drop applications.
The output of the RS-422/485 driver is capable of being Active (enabled) or Tri-State (disabled). This capability
allows multiple ports to be connected in a multi-drop bus and selectively polled. RS-485 allows cable lengths up to
4000 feet and data rates up to 10 Megabits per second. The signal levels for RS-485 are the same as those defined
by RS-422. RS-485 has electrical characteristics that allow for 32 drivers and 32 receivers to be connected to one
line. This interface is ideal for multi-drop or network environments. RS-485 tri-state driver (not dual-state) will
allow the electrical presence of the driver to be removed from the line. Only one driver may be active at a time and
the other driver(s) must be tri-stated. RS-485 can be cabled in two ways, two wire and four wire mode. Two-wire
mode does not allow for full duplex communication, and requires that data be transferred in only one direction at a
time. For half-duplex operation, the two transmit pins should be connected to the two receive pins (Tx+ to Rx+ and
Tx- to Rx-). Four wire mode allows full duplex data transfers. RS-485 does not define a connector pin-out or a set
of modem control signals. RS-485 does not define a physical connector.

ETS 2604 CyberResearch® Serial Communications
14 ©Copyright 2008 CyberResearch, Inc.
Appendix D - Asynchronous Communications
Serial data communications implies that individual bits of a character are transmitted consecutively to a receiver that
assembles the bits back into a character. Data rate, error checking, handshaking, and character framing (start/stop
bits) are pre-defined and must correspond at both the transmitting and receiving ends.
Asynchronous communications is the standard means of serial data communication for PC compatibles and PS/2
computers. The original PC was equipped with a communication or COM: port that was designed around an 8250
Universal Asynchronous Receiver Transmitter (UART). This device allows asynchronous serial data to be
transferred through a simple and straightforward programming interface. A start bit, followed by a pre-defined
number of data bits (5, 6, 7, or 8) defines character boundaries for asynchronous communications. The end of the
character is defined by the transmission of a pre-defined number of stop bits (usually 1, 1.5 or 2). An extra bit used
for error detection is often appended before the stop bits.
Remain Idle or
next start bit
Odd, Even
or
Unused
STOP
P
BIT
5 to 8 Data Bits
Idle state of
line
1
0
1
1.5
2
Figure 1 - Asynchronous Communications Bit Diagram
This special bit is called the parity bit. Parity is a simple method of determining if a data bit has been lost or
corrupted during transmission. There are several methods for implementing a parity check to guard against data
corruption. Common methods are called (E)ven Parity or (O)dd Parity. Sometimes parity is not used to detect errors
on the data stream. This is refereed to as (N)o parity. Because each bit in asynchronous communications is sent
consecutively, it is easy to generalize asynchronous communications by stating that each character is wrapped
(framed) by pre-defined bits to mark the beginning and end of the serial transmission of the character. The data rate
and communication parameters for asynchronous communications have to be the same at both the transmitting and
receiving ends. The communication parameters are baud rate, parity, number of data bits per character, and stop bits
(i.e. 9600,N,8,1).