ASSURED LPCI-COM422-8 PCI Card User Manual
- August 9, 2024
- ASSURED
Table of Contents
ASSURED LPCI-COM422-8 PCI Card
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. Printed in the USA. Copyright
1995, 2006 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 THE COMPUTER POWER OFF BEFORE INSTALLING A CARD. CONNECTING AND
DISCONNECTING CABLES, OR INSTALLING CARDS INTO A SYSTEM WITH THE COMPUTER OR
FIELD POWER ON MAY CAUSE DAMAGE TO THE I/O CARD AND WILL VOID ALL WARRANTIES,
IMPLIED OR EXPRESSED.
Warranty
Before 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 that 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 the 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, the liability of ACCES is limited to replacing,
repairing, or issuing credit (at ACCES discretion) for any products that are
proven to be defective during the warranty period. In no case is ACCES liable
for consequential or special damage arriving from the 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’s 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 such equipment furnished or sold by
ACCES.
Introduction
The card was designed for effective multipoint transmission in the RS422 (EIS422) protocol. The card is 6.60 inches long and may be installed in an available 5V or 3.3V PCI expansion slot. The card features eight independent, asynchronous RS422 serial ports and type 16550 buffered UARTs. The card meets Universal PCI and MD2 Low Profile PCI Bus Specifications.
RS422 Balanced Mode Operation
The card supports RS422 communications and uses differential balanced drivers
for long-range and noise immunity. It also can add load resistors to terminate
the communications lines. RS422 communications require that a transmitter
supply a bias voltage to ensure a known “zero” state. Also, receiver inputs at
each end of the network should be terminated to eliminate “ringing”. The card
supports biasing by default and supports termination by jumpers on the card.
If your application requires the transmitter to be unbiased, please contact
the factory.
COM Port Compatibility
Type 16550 UARTs are used as the Asynchronous Communication Element (ACE).
These include 16-byte transmit/receive buffers to protect against lost data in
multitasking operating systems while maintaining 100% compatibility with the
original IBM serial port. The system assigns the address(es). A crystal
oscillator is located on the card. This oscillator permits precise selection
of baud rates up to 115,200 or, by changing a jumper, up to 921,600 with the
standard crystal oscillator. The driver/receiver used, the SN75176B, 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 a thermal
shutdown.
Communication Mode
The card supports Full-Duplex and Half-Duplex communications with a 4-wire
cable connection. Half-Duplex allows traffic to travel in both directions, but
only one way at a time.
Baud Rate Ranges
The card has the capability for two baud rate ranges and you can select which
you wish to use on a port-by-port basis. One range is up to 115,200 baud
applications and the other is up to 921,600 baud. Refer to Table 5-1, Baud
Rate Divisor Values in chapter 5 of the manual.
Specifications
Communications Interface
-
I/O Connection:
- 50-pin SCSI D-Connector
-
Serial Ports:
- Eight cable-terminated shielded male D-sub 9-pin with standard IBM AT Style connectors compatible with RS422 specifications
-
Character length:
- 5, 6, 7, or 8 bits
-
Parity:
- Even, odd, or none
-
Stop Interval:
- 1, 1.5, or 2 bits
-
Serial Data Rates:
- rates, up to 921,600, is achieved by jumper selection on the card. Type 16550 buffered UART Continuously mappable within 0000 to
-
Address: Continuously mappable within 0000 to FFFF (hex) range of PCI bus addresses
-
Receiver Input Sensitivity: +200 mV, differential input
-
Common Mode Rejection: +12V to -7V
-
Transmitter Output Drive Capability: 60 mA, with thermal shutdown
Environmental
- Operating Temperature Range : 0°C to +60°C
- Storage temperature Range: -50°C to +120°C
- Humidity: 5% to 95%, non-condensing
- Power Required: +5VDC at 200 mA typical
- Size: Low Profile Version: 6.6” long (167.64mm) by 2.21” tall (56.17mm) seated height.
Installation
A printed Quick-Start Guide (QSG) is packed with the card 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 card is on CD and must be installed onto your hard disk before use. To do this, perform the following steps as appropriate for your operating system.
Configure Card Options via Jumper Selection
Before installing the card into your computer, carefully read Chapter 3:
Option Selection of this manual, then configure the card according to your
requirements and protocol (RS-232, RS-422, RS-485, 4-wire 485, etc.). Our
Windows based setup program can be used in conjunction with Chapter 3 to
assist in configuring jumpers on the card, as well as provide additional
descriptions for usage of the various card options (such as termination, bias,
baud rate range, RS-232, RS-422, RS-485, etc.).
CD Software Installation
The following instructions assume the CD-ROM drive is drive “D”. Please
substitute the appropriate drive letter for your system as necessary.
DOS
- Place the CD into your CD-ROM drive.
- Type to change the active drive to the CD-ROM drive.
- Type to run the install program.
- Follow the on-screen prompts to install the software for this board.
WINDOWS
1. Place the CD into your CD-ROM drive.
2. The system should automatically run the install program. If the install
program 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.
LINUX
- Please refer to linux.htm on the CD-ROM for information on installing under Linux.
Note: COM boards can be installed in virtually any operating system. We
do support installation in earlier versions of Windows and are very likely to
support future versions as well.
Caution! * ESDA single static discharge can damage your card and cause
premature failure! Please follow all reasonable precautions to prevent a
static discharge such as grounding yourself by touching any grounded surface
before touching the card.
Hardware Installation
- Make sure to set switches and jumpers from either the Option Selection section of this manual or from the suggestions of SETUP.EXE.
- Do not install the card into the computer until the software has been fully installed.
- Turn OFF computer power AND unplug AC power from the system.
- Remove the computer cover.
- Carefully install the card in an available 5V or 3.3V PCI expansion slot (you may need to remove a backplate first).
- Inspect for proper fit of the card and tighten screws. Make sure that the card mounting bracket is properly screwed into place and that there is a positive chassis ground.
- Install an I/O cable onto the card’s bracket-mounted connector.
- Replace the computer cover and turn ON the computer. Enter the CMOS setup program of your system and verify that the PCI plug-and-play option is set appropriately for your system. Systems running Windows 95/98/2000/XP/2003 (or any other PNP compliant operating system) should set the CMOS option to OS. Systems running under DOS, Windows NT, Windows 3.1, or any other non-PNP-compliant operating system should set the PNP CMOS option to BIOS or Motherboard. Save the option and continue booting the system.
- Most computers should auto-detect the card (depending on the operating system) and automatically finish installing the drivers.
- Run PCIfind.exe to complete installing the card into the registry (for Windows only) and to determine the assigned resources.
- Run one of the provided sample programs that was copied to the newly created card directory (from the CD) to test and validate your installation.
The base address assigned by BIOS or the operating system can change each time new hardware is installed into or removed from the computer. Please recheck PCIFind or Device Manager if the hardware configuration is changed. Software you write can automatically determine the base address of the card using a variety of methods depending on the operating system. In DOS, the PCI\SOURCE directory shows the BIOS calls used to determine the address and IRQ assigned to installed PCI devices. In Windows, the Windows sample programs demonstrate querying the registry entries (created by PCIFind and NTIOPCI.SYS during boot- up) to determine this same information.
Interrupts
Please note that, in Windows NT, changes must be made to the system registry
to support IRQ sharing. The following is excerpted from “Controlling Multiport
Serial I/O Cards” provided by Microsoft in the MSDN library,
documentid:mk:@ivt:nt40res/D15/S55FC.HTM,
also available in the WindowsNT Resource Kit. The Microsoft serial driver can
be used to control many dumb multiport serial cards. Dumb indicates that the
control includes no on-board processor. Each port of a multiport card has a
separate subkey under the HKLM\CurrentControlSet\Services\Serial\Parameters
subkey in the registry. In each of these subkeys, you must add values for
DosDevices, Interrupt, InterruptStatus, PortAddress, and PortIndex because
these are not detected by the Hardware Recognizer. (For descriptions and
ranges for these values, see Regentry.hlp, the Registry help file on the
WindowsNT Workstation Resource Kit CD.)
For example, if you have an eight-port card configured to use address 0xFC00 with an interrupt of 05, the values in the Registry are:
Serial##### Subkey: | Serial##### Subkey: |
---|---|
PortAddress = REG_DWORD 0xFC00 | PortAddress = REG_DWORD 0xFC20 |
Interrupt = REG_WORD 5 | Interrupt = REG_DWORD 5 |
DosDevices = REG_SZ COM5 | DosDevices = REG_SZ COM9 |
InterruptStatus = REG_DWORD 0xFC40 | InterruptStatus = REG_DWORD 0xFC40 |
PortIndex = REG_DWORD 1 | PortIndex – REG_DWORD 5 |
Serial##### Subkey: | Serial##### Subkey: |
PortAddress = REG_DWORD 0xFC00 | PortAddress = REG_DWORD 0xFC20 |
Interrupt = REG_WORD 5 | Interrupt = REG_DWORD 5 |
DosDevices = REG_SZ COM5 | DosDevices = REG_SZ COM9 |
InterruptStatus = REG_DWORD 0xFC40 | InterruptStatus = REG_DWORD 0xFC40 |
PortIndex = REG_DWORD 1 | PortIndex – REG_DWORD 5 |
Serial##### Subkey: | Serial##### Subkey: |
PortAddress = REG_DWORD 0xFC08 | PortAddress = REG_DWORD 0xFC28 |
Interrupt = REG_DWORD 5 | Interrupt = REG_DWORD 5 |
DosDevices = REG_SZ COM6 | DosDevices = REG_SZ COM10 |
InterruptStatus = REG_DWORD 0xFC40 | InterruptStatus = REG_DWORD 0xFC40 |
PortIndex = REG_DWORD 2 | PortIndex = REG_DWORD 6 |
Serial##### Subkey: | Serial##### Subkey: |
PortAddress =_DWORD 0xFC10 | PortAddress = REG_DWORD 0xFC30 |
Interrupt = REG_DWORD 5 | Interrupt = REG_DWORD 5 |
DosDevices = REG_SZ COM7 | DosDevices = REG_SZ COM11 |
InterruptStatus = REG_DWORD 0xFC40 | InterruptStatus = REG_DWORD 0xFC40 |
PortIndex – REG_DWORD 3 | PortIndex = REG_DWORD 7 |
Serial##### Subkey: | Serial##### Subkey: |
PortAddress = REG_DWORD 0xFC18 | PortAddress = REG_DWORD 0xFC38 |
Interrupt = REG_DWORD 5 | Interrupt = REG_DWORD 5 |
Dos Devices = REG_SZ COM8 | DosDevices = REG_SZ COM12 |
InterruptStatus = REG_DWORD 0xFC40 | InterruptStatus = REG_DWORD 0xFC40 |
PortIndex = REG_DWORD4 | PortIndex = REG_DWORD8 |
As this example shows, the shared IRQ status Register is located at the Base address + 0x40.
Option Selection
To help you locate the jumpers described in this section, refer to the Option Selection Map at the end of this section. The operation of the serial communications section is determined by jumper installation as described in the following paragraphs.
Terminations
A transmission line should be terminated at the receiving end in its
characteristic impedance. Installing a jumper at the locations labeled LOAD
INPUT apply a 120Ω load across the receive input for RS422 operation.
Data Cable Wiring
SignalPin Connection
- Ain- 1
- Aout+ 2
- Aout- 3
- 100 Ω to Ground 5
- Ain+ 9
Baud Rate Ranges
The jumper labeled X8CLK is provided to select baud rates in either of two
ranges. When not installed, the baud rate range is up to 115,200 baud. When
installed in the X8CLK, the baud rate range is 200 to 921,600 baud.
Address Selection
The card uses one address space. COM A, COM B, COM C, COM D, COM E, COM F, COM G, and COM H each occupy eight consecutive register locations. The interrupt register which indicates which port or ports caused the interrupt is located at base address + 64. 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. To determine the base address that has been assigned, run the PCIFind.EXE, or PCINT utility program provided. This utility will display a list of all of the cards detected on the PCI bus, the addresses assigned to each function on each of the cards, and the respective IRQs (if any) allotted. Alternatively, some operating systems (Plug and Play Windows) can be queried to determine which resources were assigned. In these operating systems, you can use either PCIFind or the Device Manager utility from the System Applet of the control panel. The card is installed in the Data Acquisition class of the Device Manager list. Selecting the card, clicking Properties, and then selecting the Resources Tab will display a list of the resources allocated to the card. The PCI bus supports 64K of I/O space, so your card’s addresses may be located anywhere in the 0000 to FFFF hex range. Vendor ID code is 494F (ASCII for “I/O”) Device ID code is 1068
Programming
Sample Programs
There are sample programs provided with the card in C, Pascal, QuickBASIC, and
several Windows languages. DOS samples are located in the DOS directory and
Windows samples are located in the WIN32 directory.
Windows Programming
The card installs into Windows as COM ports. Thus the Windows standard API
functions can be used. In particular:
- CreateFile() and CloseHandle() for opening and closing a port.
- SetupComm(), SetCommTimeouts(), GetCommState(), and SetCommState() to set and change a port’s settings.
- ReadFile() and WriteFile() for accessing a port.
See the documentation for your chosen language for details. Under DOS, the process is very different. The remainder of this chapter describes DOS programming.
Initialization
Initializing the chip requires knowledge of the UART’s register set. The first
step is to set the baud rate divisor. You do this by first setting the DLAB
(Divisor Latch Access Bit) high. This bit is Bit 7 at Base Address +3. In C
code, the call would be: outportb(BASEADDR +3,0×80); You then load the divisor
into Base Address +0 (low byte) and Base Address +1 (high byte). The following
equation defines the relationship between baud rate and divisor: desired baud
rate = (UART clock frequency) / (32 * divisor) On the card, the UART clock
frequency is 1.8432 MHz. On the next page is a table for the popular divisor
frequencies.
Baud Rate| Divisor x1| Divisor x8| Max
Diff. Cable **Length***
---|---|---|---
921600| –| 1| 250 ft
460800| –| 2| 550 ft
230400| –| 4| 1400 ft
153600| –| 6| 2500 ft
115200| 1| 8| 3000 ft
57600| 2| 16| 4000 ft
38400| 3| 24| 4000 ft
28800| 4| 32| 4000 ft
19200| 6| 48| 4000 ft
14400| 8| 64| 4000 ft
9600| 12| 96 – Most Common| 4000 ft
4800| 24| 192| 4000 ft
2400| 48| 384| 4000 ft
1200| 96| 768| 4000 ft
These are theoretical maximums for typical conditions and good quality cables based on the EIA 485 and EIA 422 standard for balanced differential drivers.
Table 5-1: Baud Rate Divisor Values
In C, the code to set the chip to 9600 baud is:
- outportb(BASEADDR, 0x0C);
- outportb(BASEADDR +1,0);
The second initializing step is to set the Line Control Register at Base Address +3. This register defines word length, stop bits, parity, and the DLAB. Bits 0 and 1 control word length and allow word lengths from 5 to 8 bits. Bit settings are extracted by subtracting 5 from the desired word length. Bit 2 determines the number of stop bits. There can be either one or two-stop bits. If Bit 2 is set to 0, there will be one stop bit. If Bit 2 is set to 1, there will be two stop bits. Bits 3 through 6 control parity and break enable. They are not commonly used for communications and should be set to zeroes. Bit 7 is the DLAB discussed earlier. It must be set to zero after the divisor is loaded or else there will be no communications. The C command to set the UART for an 8-bit word, no parity, and one stop bit is: outportb(BASEADDR +3, 0x03)
Reception
Reception can be handled in two ways: polling and interrupt-driven. When
polling, reception is accomplished by constantly reading the Line Status
Register at Base Address +5. Bit 0 of this register is set high whenever data
are ready to be read from the chip. A simple polling loop must continuously
check this bit and read in data as it becomes available. The following code
fragment implements a polling loop and uses a value of 13, (ASCII Carriage
Return) as an end-of-transmission marker:
Interrupt-driven communications should be used whenever possible and is required for high data rates. Writing an interrupt-driven receiver is not much more complex than writing a polled receiver but care should be taken when installing or removing your interrupt handler to avoid writing the wrong interrupt, disabling the wrong interrupt, or turning interrupts off for too long a period. The handler would first read the Interrupt Identification Register at Base Address +2. If the interrupt is for Received Data Available, the handler then reads the data. If no interrupt is pending, control exits the routine. A sample handler, written in C, is as follows:
Transmission
To transmit a string of data, the transmitter must first check Bit 5 of the
Line Status Register at Base Address +5. That bit is the transmitter-holding-
register-empty flag. If it is high, the transmitter has sent the data. The
process of checking the bit until it goes high followed by a write is repeated
until no data remains.
The following C code fragment demonstrates this process:
Connector Pin Assignments
Input/Output Connections
The card uses a 50-pin SCSI D-connector to interface to a spider cable. The
spider cable has eight individual DB9M connectors provided with it. To ensure
that there is minimum susceptibility to EMI and minimum radiation it is
important that the card mounting bracket be properly screwed into place and
that there be a positive chassis ground. Also, proper EMI cabling techniques
(cable connect to chassis ground at the aperture, shielded twisted-pair
wiring, etc) must be used for the input/output wiring.
Pin | RS-422 Signals | Pin | RS-422 Signals |
---|---|---|---|
1 | GND Ground | 26 | GND Ground |
2 | Aout+ | 27 | Eout+ |
3 | Aout- | 28 | Eout- |
4 | Ground through 100 Ohm R | 29 | Ground through 100 Ohm R |
5 | Ain+ | 30 | Ein+ |
6 | Ain- | 31 | Ein- |
7 | GND Ground | 32 | GND Ground |
8 | Bout+ | 33 | Fout+ |
9 | Bout- | 34 | Fout- |
10 | Ground through 100 Ohm R | 35 | Ground through 100 Ohm R |
11 | Bin+ | 36 | Fin+ |
12 | Bout- | 37 | Fout- |
13 | GND Ground | 38 | GND Ground |
14 | Cout+ | 39 | Gout+ |
15 | Cout- | 40 | Gout- |
16 | Ground through 100 Ohm R | 41 | Ground through 100 Ohm R |
17 | Cin+ | 42 | Gin+ |
18 | Cin- | 43 | Gin- |
19 | GND Ground | 44 | GND Ground |
20 | Dout+ | 45 | Hout+ |
21 | Dout- | 46 | Hout- |
22 | Ground through 100 Ohm R | 47 | Ground through 100 Ohm R |
23 | Din+ | 48 | Hin+ |
24 | Din- | 49 | Hin- |
25 | GND Ground | 50 | GND Ground |
Table 6-1: Connection Pin Assignments
Signal | Connector Pin |
---|---|
Ain- | Pin 1 |
Aout+ | Pin 2 |
Aout- | Pin 3 |
100 Ohm to Ground | Pin 5 |
Ain+ | Pin 9 |
Table 6-2: Data Cable Wiring
Application Considerations
Introduction
Working with RS422 and RS485 devices is not much different from working with
standard RS232 serial devices and these two standards overcome deficiencies in
the RS232 standard. First, the cable length between two RS232 devices must be
short; less than 50 feet at 9600 baud. Second, many RS232 errors are the
result of noise-induced on the cables. The RS422 standard permits cable
lengths up to 5000 feet and, because it operates in the differential mode, it
is more immune to induced noise. Connections between two RS422 devices (with
CTS ignored) should be as follows:
Device #1 | Device #2 |
---|---|
Signal | Pin No. |
Gnd | 7 |
TX+ | 24 |
TX– | 25 |
RX+ | 12 |
RX– | 13 |
Table A-1: Connections Between Two RS422 Devices
A third deficiency of RS232 is that more than two devices cannot share the same cable. This is also true for RS422 but RS485 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 all receive.
Balanced Differential Signals
The reason that RS422 and RS485 devices can drive longer lines with more noise
immunity than RS232 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. RS485 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. A maximum operating voltage range is from
+6V to -6V allows 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.
Parameter | Conditions | Min. | Max. |
---|---|---|---|
Driver Output Voltage (unloaded) | 4V | 6V | |
-4V | -6V | ||
Driver Output Voltage (loaded) | LD and LDGND | 2V | |
jumpers in | -2V | ||
Driver Output Resistance | 50Ω | ||
Driver Output Short-Circuit Current | +150 mA | ||
Driver Output Rise Time | 10% unit interval | ||
Receiver Sensitivity | +200 mV | ||
Receiver Common Mode Voltage Range | +7V | ||
Receiver Input Resistance | 4KΩ |
Table A-2: RS422 Specification Summary
To prevent signal reflections in the cable and to improve noise rejection in
both the RS422 and RS485 mode, the receiver end of the cable should be
terminated with a resistance equal to the characteristic impedance of the
cable. (An exception to this is the case where the line is driven by an RS422
driver that is never “tristated” or disconnected from the line. In this case,
the driver provides a low internal impedance that terminates the line at that
end.)
Note
You do not have to add a terminator resistor to your cables when you use the
card. Termination resistors for the RX+ and RX- lines are provided on the card
and are placed in the circuit when you install the LD and LDGND jumpers. (See
the Option Selection section of this manual.)
RS485 Data Transmission
The RS485 Standard allows a balanced transmission line to be shared in a
party-line mode. As many as 32 driver/receiver pairs can share a two-wire
party line network. Many characteristics of the drivers and receivers are the
same as in the RS422 Standard. One difference is that the common mode voltage
limit is extended and is +12V to -7V. Since any driver can be disconnected (or
tristated) from the line, it must withstand this common mode voltage range
while in the tristate condition. The following illustration shows a typical
multidrop or party line network. Note that the transmission line is terminated
on both ends of the line but not at drop points in the middle of the line.
RS485 Four-Wire Multidrop Network
An RS485 network can also be connected in a four-wire mode. In a four-wire
network, it’s necessary that one node be a master node and all others be
slaves. The network is connected so that the master communicates to all slaves
and all slaves communicate only with the master. This has advantages in
equipment that uses mixed protocol communications. Since the slave nodes never
listen to another slave’s response to the master, a slave node cannot reply
incorrectly.
Customer Comments
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References
- Industrial Embedded Computers and Displays - Assured Systems
- Industrial Embedded Computers and Displays - Assured Systems
- Industrial Embedded Computers and Displays - Assured Systems
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