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Manual 00650-519-1
Chapter 1: Introduction
The card is five inches long and should be installed in either long or short slots of IBM PC/XT/AT
or compatible computers. If installed in an eight-bit slot, the high interrupts (IRQ 10-15) will not
be available to the RS422I-PA.
Multipoint Opto-isolated Communications
The RS422I-PA serial communications card was designed for multipoint transmission on long
communication lines in noisy environments using RS422 or RS485 differential line drivers. The
data lines are opto-isolated from the computer and from each other to assure communication when
large common mode noise are superimposed. An on-board DC-DC converter provides isolated
power for the line driver circuits.
A crystal oscillator is located on the card. This oscillator permits precise selection of baud rate
from 50 to 115,200 in the 1X mode. In the 4X mode, baud rates up to 460,800 baud may be
selected. The Programming section of this manual contains a table to use when selecting baud rate.
The output transceiver used, type 75176B, is capable of driving extremely long communication
lines at high baud rates. It can drive up to ±60mA on balanced lines and receive inputs as low as
±200mV differential signal. Opto-isolators on the card provide protection to maximum 500 V. In
case of communication conflict, the transceivers feature thermal shutdown.
COM Port Compatibility
Type ST16C550 UARTs are used as the Asynchronous Communication Element (ACE) which
include a 16-byte transmit/receive buffer to protect against lost data in multitasking operating
systems, while maintaining 100 percent compatibility with the original IBM serial port.
RS422I-PA card is not restricted to the standard DOS addresses of COM1 - COM4. You can
select a base address anywhere within the I/O address range 000 to 3F8 hex and the FINDBASE
program will help you choose from available addresses.
Line Bias and Termination
For increased noise immunity, the communication lines may be loaded at the receiver and biased at
the transmitter. RS485 communications requires that one transmitter supply a bias voltage to
ensure a known "zero" state when all transmitters are off, and the last receiver input at each end of
the network be terminated to prevent "ringing". The RS422I-PA supports these options with
jumpers on the card. See Chapter 3, Option Selection for more details.
