Chapter 1: Introduction
This Serial Communications Card was designed for effective asynchronous transmission in either RS422
(EIA422) or RS485 protocols over long communications lines. The data lines are opto-isolated from the
computer and from each other to assure communication when large common-mode noise is
superimposed.
The card is 4.80 inches long (122 mm) and may be installed in 5-volt PCI-bus slots of IBM PC or
compatible computers. A type 16550 buffered UART is used and, for Windows compatibility, automatic
control is included to transparently enable/disable the transmission drivers.
Balanced Mode Operation and Load Termination
The card uses differential balanced drivers for long range and noise immunity. RS422 operation permits
multiple receivers on the communications lines and RS485 operation permits up to 32 transmitters and
receivers on the same set of data lines. Devices at the ends of these networks should be terminated to
avoid "ringing". The card gives you jumper positions to add load resistors to terminate the
communications lines.
Also, RS485 communications requires that a transmitter supply a bias voltage to ensure a known "zero"
state when none of the devices is transmitting. This card supports biasing by default. If your application
requires the transmitter to be un-biased, please contact the factory.
COM Port Compatibility
A type 16550 UART is used as the Asynchronous Communication Element (ACE). It includes a 16-byte
FIFO transmit/receive buffer to protect against lost data in multitasking operating systems, while
maintaining 100% compatibility with the original IBM serial port. The system assigns the I/O address.
A crystal-controlled 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 460,800 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 thermal shutdown.
Communication Mode
This card supports Full-Duplex and Half-Duplex communications with four-wire cable connection. Half-
Duplex allows traffic to travel in both directions, but only one way at a time. Many RS485 applications
commonly use the Half-Duplex mode because a single pair of wires can be shared.
Baud Rate Ranges
The card has capability for two baud rate ranges and you can select which you wish to use by a jumper
placement. One range is for up to 115,200 baud applications and the other is up to 460,800 baud
applications. Baud rate is program selected and rates available are listed in a table in the Programming
section of this manual.
Manual PCI-ICM-1S
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