Access 104i-COM-8SM User manual

10623 Roselle Street, San Diego, CA 92121 [phone] www.accesio.com
PCI-104 SERIAL
COMMUNICATION BOARDS
104i-COM-8SM
104i-COM-4SM
104i-COM-8S
104i-COM-4S
USER MANUAL
FILE: m104i-COM-8SM.A1o

Manual 104i-COM-8SM
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Notice
The information in this document is provided for reference only. ACCES does not assume any liability arising
out of the application or use of the information or products described herein. This document may contain or
reference information and products protected by copyrights or patents and does not convey any license under
the patent rights of ACCES, nor the rights of others.
IBM PC, PC/XT, and PC/AT are registered trademarks of the International Business Machines Corporation.
Copyright by ACCES I/O Products, Inc. 10623 Roselle Street, San Diego, CA 92121. All rights reserved.
WARNING!!
ALWAYS CONNECT AND DISCONNECT YOUR FIELD CABLING WITH
THE COMPUTER POWER OFF. ALWAYS TURN COMPUTER POWER
OFF BEFORE INSTALLING A BOARD, CONNECTING AND
DISCONNECTING CABLES, OR INSTALLING BOARDS INTO A SYSTEM
WITH THE COMPUTER OR FIELD POWER ON MAY CAUSE DAMAGE TO
THE I/O BOARD AND WILL VOID ALL WARRANTIES, IMPLIED OR
EXPRESSED.

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Warranty
Prior to shipment, ACCES equipment is thoroughly inspected and tested to applicable specifications.
However, should equipment failure occur, ACCES assures its customers that prompt service and
support will be available. All equipment originally manufactured by ACCES which is found to be
defective will be repaired or replaced subject to the following considerations.
Terms and Conditions
If a unit is suspected of failure, contact ACCES' Customer Service department. Be prepared to give
the unit model number, serial number, and a description of the failure symptom(s). We may suggest
some simple tests to confirm the failure. We will assign a Return Material Authorization (RMA)
number which must appear on the outer label of the return package. All units/components should be
properly packed for handling and returned with freight prepaid to the ACCES designated Service
Center, and will be returned to the customer's/user's site freight prepaid and invoiced.
Coverage
First Three Years: Returned unit/part will be repaired and/or replaced at ACCES option with no
charge for labor or parts not excluded by warranty. Warranty commences with equipment shipment.
Following Years: Throughout your equipment's lifetime, ACCES stands ready to provide on-site or in-
plant service at reasonable rates similar to those of other manufacturers in the industry.
Equipment Not Manufactured by ACCES
Equipment provided but not manufactured by ACCES is warranted and will be repaired according to
the terms and conditions of the respective equipment manufacturer's warranty.
General
Under this Warranty, liability of ACCES is limited to replacing, repairing or issuing credit (at ACCES
discretion) for any products which are proved to be defective during the warranty period. In no case is
ACCES liable for consequential or special damage arriving from use or misuse of our product. The
customer is responsible for all charges caused by modifications or additions to ACCES equipment not
approved in writing by ACCES or, if in ACCES opinion the equipment has been subjected to
abnormal use. "Abnormal use" for purposes of this warranty is defined as any use to which the
equipment is exposed other than that use specified or intended as evidenced by purchase or sales
representation. Other than the above, no other warranty, expressed or implied, shall apply to any and
all such equipment furnished or sold by ACCES.

Manual 104i-COM-8SM
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Table of Contents
Chapter 1: INTRODUCTION ..................................................................................................5
Factory Options .................................................................................................................5
Figure 1-1: Block Diagram...............................................................................................6
Ordering Guide...................................................................................................................7
Model Options....................................................................................................................7
Special Order......................................................................................................................7
Included with your board ..................................................................................................7
Optional Accessories ........................................................................................................8
Chapter 2: INSTALLATION....................................................................................................9
CD Installation....................................................................................................................9
Windows.............................................................................................................................9
Installing the Hardware......................................................................................................9
To Install the Card..............................................................................................................9
Chapter 3: OPTION SELECTION.........................................................................................10
Figure 3-1: Simplified Dynamic Termination Schematic................................................10
Line Terminations............................................................................................................10
Figure 3-2: Option Selection Map..................................................................................11
Jumper Function..............................................................................................................12
RS-485 Balanced Mode Operation..................................................................................12
Chapter 4: ADDRESS SELECTION.....................................................................................13
Chapter 5: PROGRAMMING................................................................................................14
Chapter 6: CONNECTOR PIN ASSIGNMENTS...................................................................15
Table 6-1: P1 Connector Pin Assignment (8-port and 4-port boards)............................15
Table 6-2: P2 Connector Pin Assignment (8-port board only)........................................16
Table 6-3: DB9M Accessory Cable RS-232 Pin Assignment.........................................17
Table 6-4: DB9M Accessory Cable RS-422 Pin Assignment.........................................17
Table 6-5: DB9M Accessory Cable RS-485 Pin Assignment.........................................17
Port to Port Cable Wiring ................................................................................................18
Table 6-6: Port to Port Cable Wiring..............................................................................18
Chapter 7: SPECIFICATION................................................................................................19
Appendix A: APPLICATION CONSIDERATIONS...............................................................20
Table A-1: RS-422 Specification Summary ...................................................................21
Figure A-1: Typical RS-485 Two-Wire Multidrop Network.............................................22
Customer Comments ..........................................................................................................23

Manual 104i-COM-8SM
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Chapter 1: INTRODUCTION
This communications card interfaces to the CPU through a 32bit PCI bus. It is a PCI-104
card, with no ISA bus connector.
!High Performance Octal PCI UART
!16550 Compatible Register Set
!Up to 921.6kbps Serial Data Rate in all modes
!Global Interrupt Source Register
!Data Transfer in Byte, Word, and Double-Word
!64-Byte Transmit and Receive FIFOs per each of eight UARTs
!Transmit and Receive FIFO Level Counters
!Programmable Transmit and Receive FIFO Trigger Level
Factory Options
!Extended Temperature Operation of -40˚ to +85˚C
!RoHS Compliance
The 104i-COM-8SM and 104i-COM-4SM (PCI-104) multi-port multi-protocol serial
communication cards deliver eight or four high-speed serial communication ports for use
in a wide variety of applications. The cards were developed for use by system
integrators and manufacturers in the design of industrial and commercial systems.
Based on the XR17D158, the boards have eight enhanced 16550 UARTs, each with a
set of modem signals (CTS, RTS, RI, DTR, DSR, CD) in RS-232 mode. Each UART has
both a 64 byte transmit and a 64 byte receive FIFO.
The provided Windows drivers are 100% compatible with the normal Microsoft-provided
Serial Application Programming Interface. This means every program you've ever used
with a standard serial port in Windows will also work with these ports, no problem. In
addition, we provide several utility and sample programs to help you write your own
code, if you're not using something off-the-shelf.
This serial Interface board contains eight independent ports and provides RS-232, RS-
422 and RS-485 multipoint communication. The -4SM board contains four such ports.
Each channel may be configured to any protocol using jumpers on the board, including
termination.
In RS-232, RS-422 or RS-485 modes, the board can achieve speeds up to 921.6kbps per
port. Use a high-quality cable for best results and to achieve higher baud rates.

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Figure 1-1: Block Diagram

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Ordering Guide
104i-COM-8SM PCI-104 Eight Port RS-232/422/485 Serial Communication Board
104i-COM-4SM PCI-104 Four Port RS-232/422/485 Serial Communication Board
104i-COM-8S PCI-104 Eight Port RS-422/485 Serial Communication Board
104i-COM-4S PCI-104 Four Port RS-422/485 Serial Communication Board
Model Options
-T Extended temperature operation of -40˚C to +85˚C
-RoHS This product is available in a RoHS compliant version. Please call for specific
pricing then be sure to add this suffix to the model number on any hard-copy
or verbal purchase orders.
Special Order
Contact factory with your special requirement. Examples of special orders would be
conformal coating, latching I/O headers, custom baud rates, etc.
Included with your board
The following components are included with your shipment, depending on options
ordered. Please take the time now to ensure that no items are damaged or missing.
PCI/104 Board
Software Master CD
Printed Quick-Start-Guide

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Optional Accessories
PCI-P104-ADAP
Develop & test software and hardware
in a desktop PCI slot
C104-40F-D9M
40 pin ribbon cable with female header
to four DB9M connectors, 12" long
C104-40F/D37F
40 pin ribbon cable with female header
to DB37F connector, 9” long
C104-D37M/D9M
This 3' round-wire external serial
breakout cable provides four male DB9
connectors from a male DB37
connector. Useful for system cabling
and can be mated with C104-40F/D37F
for various PC/104 serial requirements.
CAB40F-X
Ribbon cable assembly with 40-pin
female header connectors
(X = length in feet)
STB-40
Screw terminal board, typically ships
with standoffs but can also mount on
DIN-SNAP
DIN-SNAP-6
Six inch length of SNAP-TRACK with
two clips, for mounting one STB-40
screw terminal board on a DIN rail
DIN-SNAP
One foot length of SNAP-TRACK with
four clips, for mounting up to two STB-
40 screw terminal boards on a DIN rail
104-HDW-KIT(X)
Hardware kit for ACCES PC/104
boards; includes 4 screws, nuts and
standoffs. Specify metric (M) or
standard (S) hardware kit.

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Chapter 2: INSTALLATION
A printed Quick-Start Guide (QSG) is packed with the board for your convenience. If you’ve
already performed the steps from the QSG, you may find this chapter to be redundant and
may skip forward to begin developing your application.
The software provided with this PC/104 Board is on CD and must be installed onto your
hard disk prior to use. To do this, follow the steps below as appropriate for your operating
system. Substitute the CD-ROM drive letter where you see d: in the examples below.
CD Installation
The following instructions assume the CD-ROM drive is drive “D”. Please substitute the
appropriate drive letter for your system as necessary.
Windows
1. Place the CD into your CD-ROM drive.
2. The system should automatically run the install program. If it does not run promptly,
click START | RUN and type , click OK or press .
3. Follow the on-screen prompts to install the software for this board.
Installing the Hardware
Before installing the board, please run setup.exe. The SETUP Program can be used to
assist in configuring the switches and jumpers on the board. The setup program does not
set the options on the board, these must be set manually by the user.
To Install the Card
1. Install jumpers for selected options and position the slide switches to select the
position in the stack.
2. Remove power from the PC/104 stack.
3. Assemble standoff hardware (optionally available) to stack and secure the boards.
4. Carefully plug the board onto the PCI/104 connector on the CPU or onto the stack,
ensuring proper alignment of the pins before completely seating the connectors.
5. Install I/O cables onto the board’s I/O connectors and proceed to secure the stack
together or repeat steps 3-5 until all boards are installed.
6. Verify all connections in the PCI/104 stack and secure then power up the system.
7. With the CD you received with your shipment in the drive, install the software for the
card, following the printed QuickstartGuide.pdf you also received.
a. In Windows, each COM port is assigned a number, which will correlate to a
COM letter (COMA/COM5, COMH/COM12).
8. Run one of the provided sample programs appropriate for your operating system
that was installed from the CD to test and validate your installation.
If a 104i-COM-8SM channel is configured for RS-485 the Properties dialog box must be
used to enable auto-RTS. You can find this dialog box in the Device Manager program for
the ports involved.
To gain a better understanding of the XR17D158 register set, refer to the chip datasheet
located in the ChipSpecs directory on the ACCES I/O Software Master CD.

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Chapter 3: OPTION SELECTION
To help you locate the jumpers described in this section, refer to the OPTION SELECTION
MAP in Figure 3-2, as well as running the SETUP.EXE utility program installed from the
Software Master CD with the software package for this board.
Figure 3-1: Simplified Dynamic Termination Schematic
Line Terminations
A transmission line should be terminated at the receiving end in its characteristic impedance.
Installing a jumper at the location labeled TX TERM or RX TERM applies a 120Ω load in
series with a small capacitor across the transmit/receive input/output for RS-485 operation or
input for RS-422 operation respectively.
In RS-485 operations where there are multiple terminals, only the RS-485 ports at each end
of the network should have terminating resistors as described above. Also, for RS-485
operation, there must be a bias on the TX+/RX+ and TX-/RX- lines. If the board is not to
provide that bias, contact the factory technical support.

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Figure 3-2: Option Selection Map

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Most PCI bus signals are common to all four cards in the PCI stack. However, there are four
unique signal groups, one for each card. The slide switches select which signal group goes
to each card. The card in the stack closest to the CPU board must get signal group 0.
Only four PCI boards are allowed in a PC/104-Plus stack, each board must get a specific
set of signals. These signals are selected with two slide switches, labeled SEL-1 and SEL-
2, which form a binary value to control the mux (SEL-1 is the least significant bit and SEL-2
is the most significant bit). If this card is furthest from the CPU, slide both switches to the
OFF STATE. This will select the signal with the longest trace on the CPU board (signal
group 3). If this card is closest to the CPU, slide both switches to the ON STATE. This will
select the signal with the shortest trace on the CPU board (signal group 0). Place the SEL-
1 switch to ON and SEL-2 to OFF to select signal group 2, place the SEL-1 switch to OFF
and SEL-2 to OFF to select signal group 1.
Jumper Function
(refer to option selection map)
Each communication channel (COM A through COM H) has four configuration jumpers.
Four modes are possible: RS-232, RS-422 (also 4 wire RS-485 Master), 2 wire RS-485,
and 4 wire RS-485 Slave. The communication modes are implemented with an RS-232
transceiver, a differential signal half-duplex transceiver (auto-RS-485 mode Tx/Rx and RS-
422 Tx), and a differential signal receiver (RS-422 Rx).
The first jumper enables the RS-232 transceiver.
The second jumper connects the auto-RTS signal from the UART to the transmit-enable pin
on the half-duplex differential transceiver or it connects either a permanent ON (for RS-422)
or a permanent OFF signal (for RS-232) to the transmit-enable pin.
The third jumper enables the RS-422 receiver and sends a permanent ON signal (for RS-
422 Tx) to the transmit-enable pin on the half-duplex differential transceiver.
The fourth jumper either disables the half-duplex differential transceiver's receiver (not used
for RS-422, 4 wire RS-485, or RS-232) or connects the auto-RTS signal to the receiver's
enable pin (2 wire RS-485).
RS-485 Balanced Mode Operation
The board supports RS-485 modes that use differential bus transceivers for increased range
and noise immunity. The RS-485 specification defines a maximum of 32 devices on a single
line. The number of devices served on a single line can be expanded by use of “repeaters”.
The board also has the capability to add load resistors to terminate the communications
lines. RS-485 communications requires that one transceiver supply a bias voltage to ensure
a known state (an “idle one”) when all transmitters are off. Also, receiver inputs at each end
of the network should be terminated to eliminate “ringing”. The board supports biasing by
default and supports termination by jumpers on the board. If your application requires the
transmitter to be un-biased, please contact the factory.
The driver/receiver used, type 75176B, is capable of driving extremely long communication
lines at high baud rates. It can drive up to ±60 mA on balanced lines and receive inputs as
low as 200 mV differential signal superimposed on common mode noise of +12 V or -7 V. In
case of communication conflict, the driver/receivers feature thermal shutdown.

Manual 104i-COM-8SM
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Chapter 4: ADDRESS SELECTION
The system BIOS or operating system will assign the address. The card occupies 4K bytes
of memory space.
PCI architecture is Plug-and-Play. This means that the BIOS or Operating System
determines the resources assigned to PCI cards rather than you selecting those resources
with switches or jumpers. As a result, you cannot set or change the card's base address.
You can only determine what the system has assigned.
The Vendor ID for the card is 494F. (ASCII for "IO")
The Device ID for the card is 10EA.

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Chapter 5: PROGRAMMING
Sample Programs
There are sample programs with source-code provided with the card in a variety of common
languages. DOS samples are located in the DOS directory and Windows samples are
located in the WIN32 directory.
Windows COM Utility Program
WinRISC is a COM utility program provided on CD with the installation package for this
card that is very useful when working with any serial ports and serial devices. If you haven’t
used this program yet, do yourself a favor and run this program to test your COM ports.
Windows Programming
The card installs into Windows as COM ports so standard API functions can be used. See
the documentation for your chosen language for details.

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Chapter 6: CONNECTOR PIN ASSIGNMENTS
Pin
Ch
RS-485 Functions
Ch
RS-422 Functions
Ch
RS-232 Functions
1
A
CD
2
A
DSR
3
A
TX/RX+
A
TX+
A
RX
4
A
TX/RX-
A
TX-
A
RTS
5
A
TX
6
A
RX-
A
CTS
7
A
RX+
A
DTR
8
A
RI
9
A
Ground
A
Ground
A
Ground
10
11
B
CD
12
B
DSR
13
B
TX/RX+
B
TX+
B
RX
14
B
TX/RX-
B
TX-
B
RTS
15
B
TX
16
B
RX-
B
CTS
17
B
RX+
B
DTR
18
B
RI
19
B
Ground
B
Ground
B
Ground
20
21
C
CD
22
C
DSR
23
C
TX/RX+
C
TX+
C
RX
24
C
TX/RX-
C
TX-
C
RTS
25
C
TX
26
C
RX-
C
CTS
27
C
RX+
C
DTR
28
C
RI
29
C
Ground
C
Ground
C
Ground
30
31
D
CD
32
D
DSR
33
D
TX/RX+
D
TX+
D
RX
34
D
TX/RX-
D
TX-
D
RTS
35
D
TX
36
D
RX-
D
CTS
37
D
RX+
D
DTR
38
D
RI
39
D
Ground
D
Ground
D
Ground
40
Table 6-1: P1 Connector Pin Assignment (8-port and 4-port boards)

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Pin
Ch
RS-485 Functions
Ch
RS-422 Functions
Ch
RS-232 Functions
1
E
CD
2
E
DSR
3
E
TX/RX+
E
TX+
E
RX
4
E
TX/RX-
E
TX-
E
RTS
5
E
TX
6
E
RX-
E
CTS
7
E
RX+
E
DTR
8
E
RI
9
E
Ground
E
Ground
E
Ground
10
11
F
CD
12
F
DSR
13
F
TX/RX+
F
TX+
F
RX
14
F
TX/RX-
F
TX-
F
RTS
15
F
TX
16
F
RX-
F
CTS
17
F
RX+
F
DTR
18
F
RI
19
F
Ground
F
Ground
F
Ground
20
21
G
CD
22
G
DSR
23
G
TX/RX+
G
TX+
G
RX
24
G
TX/RX-
G
TX-
G
RTS
25
G
TX
26
G
RX-
G
CTS
27
G
RX+
G
DTR
28
G
RI
29
G
Ground
G
Ground
G
Ground
30
31
H
CD
32
H
HSR
33
H
TX/RX+
H
TX+
H
RX
34
H
TX/RX-
H
TX-
H
RTS
35
H
TX
36
H
RX-
H
CTS
37
H
RX+
H
HTR
38
H
RI
39
H
Ground
H
Ground
H
Ground
40
Table 6-2: P2 Connector Pin Assignment (8-port board only)

Manual 104i-COM-8SM
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Cable accessories are available to provide interface to the 40-Pin headers on this board,
connecting to a 40 pin screw terminal board, a DB37 for panel or enclosure mounting (an
external / system cable is optionally available to mate with that DB37 and breaks it out to
four DB9M’s) or to four DB9M connectors.
DB9 Male
DB-9 Male Pin for
each of Ch A-H
RS-232 Signals
(DTE Standard)
RS-232 Signal
Descriptions
Ch x - 1
DCD
Data Carrier Detected
Ch x - 2
RX
Receive Data
Ch x - 3
TX
Transmit Data
Ch x - 4
DTR
Data Terminal Ready
Ch x - 5
GND
Signal Ground
Ch x - 6
DSR
Data Set Ready
Ch x - 7
RTS
Request To Send
Ch x - 8
CTS
Clear to Send
Ch x - 9
RI
Ring Indicator
Table 6-3: DB9M Accessory Cable RS-232 Pin Assignment
DB-9 Male Pin for
each of Ch A-H
RS-422 Signals
RS-422 Signal
Descriptions
Ch x - 2
TX+
Transmit Data +
Ch x - 4
RX+
Receive Data +
Ch x - 5
GND
Signal Ground
Ch x - 7
TX-
Transmit Data -
Ch x - 8
RX-
Receive Data -
Table 6-4: DB9M Accessory Cable RS-422 Pin Assignment
DB-9 Male Pin for
each of Ch A-H
RS-485
Signals (2 Wire)
RS-485 Signal
Descriptions
Ch x - 2
TX/RX +
Transmit / Receive +
Ch x - 4
TX/RX -
Transmit / Receive -
Ch x - 5
GND
Signal Ground
Table 6-5: DB9M Accessory Cable RS-485 Pin Assignment

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Port to Port Cable Wiring
The following connections are used to communicate between two ports (usually on different
devices), depending on the communication mode:
Note that the user cable MUST include a ground to ground connection for RS-232,
RS-422 and RS-485 as high common mode between external devices will burn out
the transceiver IC’s.
Mode
Cable Pins
Port 1
to
Port 2
RS-232
RX
to
TX
TX
to
RX
Ground
to
Ground
RS-422 Full-Duplex1
RX+
to
TX+
RX-
to
TX-
TX+
to
RX+
TX-
to
RX-
Ground
to
Ground
RS-422 Simplex Receive Only
RX+
to
TX+
RX-
to
TX-
Ground
to
Ground
RS-422 Simplex Transmit Only
TX+
to
RX+
TX-
to
RX-
Ground
to
Ground
RS-485 Half-Duplex
TX+/RX+
to
TX+/RX+
TX-/RX-
to
TX-/RX-
Ground
to
Ground
1RS-485 4-Wire (not a TIA/EIA standard)
Port 1 designates the Master,
should be an RS-422 (TX not shared)
Port 2 designates the Slaves,
must be an RS-485 (TX shared)
Table 6-6: Port to Port Cable Wiring

Manual 104i-COM-8SM
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Chapter 7: SPECIFICATION
PCI Bus: Rev. 2.3
Memory Space: Requires 4k
16550 Compatible
Data Size: 5, 6, 7 or 8 bits
Stop bit length: 1, 1.5 or 2 bits
Parity: Odd, even, none, mark, or space
Break condition: On or off
Maximum Baud Rate: 921.6kbps (RS-232, RS-422 & 485 modes)
Full Duplex Operation
Eight 64 Byte Transmit FIFO, 64 Byte Receive FIFO (16C550 FIFO = 16 Bytes)
S/W programmable FIFO trigger levels (16C550 trigger levels are fixed)
12 standard registers for UART monitoring and control plus special registers
Loop-back mode
Scratch pad register
Transceiver I/O Characteristics
RS-232 Mode:
Receiver Input Resistance: 3K to 7K ohm
Receiver Input Sensitivity: ±3V
Receiver Input Range: ±15V
Driver Slew Rate: 24V/S minimum
Driver Load Impedance: 3K to 7K ohm
Driver Output Signal Level: Loaded ±5.4V typical
RS-422/485 Mode:
Receiver Input Sensitivity: ±200 mV differential input
Common Mode Range: +12V to -7V
Transmitter Output Drive: 60 mA with thermal shutdown
Input Impedance: 12 K ohm Min
Input Hysteresis: 50 mV typical
ESD: ±15KV Human Body Model
±15KV Air Discharge
±8KV Contact Discharge
Communications Interface:
Eight channels split between two 40 pin male headers
Meets or exceeds the IEEE RS-232 standard
Environmental
Operating Temperature: 0˚C to +70˚C
-40˚C to +85˚C (Extended temp version)
Humidity: 5% to 95%, non-condensing
Storage Temperature: -65 ˚C to +125 ˚C
Power Required: 5V @ 50mA quiescent, 150mA maximum (nothing connected
on I/O connectors)
Size: PC/104-Plus format, 3.5" x 3.75"

Manual 104i-COM-8SM
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Appendix A: APPLICATION CONSIDERATIONS
INTRODUCTION
Working with RS-485 devices is not much different from working with standard RS-232 serial
devices and this standard overcomes deficiencies in the RS-232 standard. First, the cable
length between two RS-232 devices must be short; less than 50 feet. Second, many RS-232
errors are the result of noise induced on the cables. The RS-485 standard permits cable
lengths up to 4000 feet and, because it operates in differential mode, it is more immune to
induced noise.
A third deficiency of RS-232 is that more than two devices cannot share the same cable.
This is also true for RS422 but RS-485 offers all the benefits of RS422 plus allows up to 32
devices to share the same twisted pairs. An exception to the foregoing is that multiple
RS422 devices can share a single cable if only one will talk and the others will always
receive.
BALANCED DIFFERENTIAL SIGNALS
The reason that RS422 and RS-485 devices can drive longer lines with more noise immunity
than RS-232 devices is that a balanced differential drive method is used. In a balanced
differential system, the voltage produced by the driver appears across a pair of wires. A
balanced line driver will produce a differential voltage from ±2 to ±6 volts across its output
terminals. A balanced line driver can also have an input “enable” signal that connects the
driver to its output terminals. If the “enable” signal is OFF, the driver is disconnected from
the transmission line. This disconnected or disabled condition is usually referred to as the
“tristate” condition and represents a high impedance. RS-485 drivers must have this control
capability. RS422 drivers may have this control but it is not always required.
A balanced differential line receiver senses the voltage state of the transmission line across
the two signal input lines. If the differential input voltage is greater than +200 mV, the
receiver will provide a specific logic state on its output. If the differential voltage input is less
than -200 mV, the receiver will provide the opposite logic state on its output. The maximum
operating voltage range is from +6V to -6V allowing for voltage attenuation that can occur on
long transmission cables.
A maximum common mode voltage rating of ±7V provides good noise immunity from
voltages induced on the twisted pair lines. The signal ground line connection is necessary in
order to keep the common mode voltage within that range. The circuit may operate without
the ground connection but may not be reliable.