Chapter 1: Introduction
This Serial Communications Card was designed for use in PCI-Bus computers and provides effective
communication in either RS422 (EIA422) or RS485 (EIA485) over long communication lines. The card
is 4.80 inches long (122 mm) and may be installed in any 5-volt PCI slot in IBM 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
In RS422 mode, the card uses differential (or balanced) line drivers for noise immunity and to increase
the maximum distance to 4000 feet. The RS485 mode improves on RS422 with switchable transceivers
and the ability to support multiple devices on a single “party line”. The number of devices served on a
single line can be expanded by using “repeaters”.
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 user has the option to terminate the transmitter
and/or the receiver lines.
RS485 communications requires that a transmitter supply a bias voltage to ensure a known “zero” state
when no device 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 16550 UART is used as the Asynchronous Communication Element (ACE). It includes 16-byte
transmit/receive FIFO buffers to protect against lost data in multitasking operating systems, while
maintaining 100 percent compatibility with the original IBM serial port. PCI bus architecture allows
addresses between 0000 and FFF8 hex to be assigned to the cards.
The crystal oscillator on the card permits precise selection of baud rates up to 115,200 or, by changing
a jumper, up to 460,800 baud with the standard crystal oscillator. Baud rate is program selected and
rates available are listed in a table in the Programming section of this manual.
The driver/receiver used, the 75ALS176, 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 Modes
The cards support Simplex, Half-Duplex, and Full-Duplex communications in a variety of two and four-
wire cable connections. Simplex is the simplest form of communications with transmission occurring
only in one direction. Half-Duplex allows traffic to travel in both directions, but only one way at a time. In
Full-Duplex operation, data travels in both directions at the same time. Most RS485 communications
use the Half-Duplex mode because only a single pair of wires need to be used and installation costs are
dramatically reduced.
Manual PCI-COM-1S5
