User manual

Chapter 1: Introduction
This serial communications board is designed for use in PC/104 compatible computers. Two isolated serial data
ports are provided on the board. Model COM-2S is simply a non-isolated version of the ICOM-2S.
Multipoint Opto-isolated Communications
The board allows 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. The on-board DC-DC converters provide
isolated power for the line driver circuits.
A crystal oscillator is located on the board. This oscillator permits precise selection of baud rates from 50 to
115,200. Baud rates up to 460,800 baud may be provided as a factory option. The Programming section of this
manual contains a table to use when selecting baud rate.
The output transceivers used, type 75176B, are capable of driving extremely long communication lines at high
baud rates. They can drive up to ±60mA on balanced lines and receive inputs as low as ±200mV differential
signal. Opto-isolators on the board 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.
You can select a base address anywhere within the I/O address range 000 to 3E0 hex.
Communication Modes
This model supports a variety of 2-wire and 4-wire cable connections. 2 wire or Half-Duplex allows traffic to
travel in both directions, but only one direction at a time. In 4 wire or Full-Duplex mode data travels in both
directions at the same time.
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 board supports these options with jumpers on the board. See Chapter 3, Option Selection
for more details.
Transceiver Control
RS485 communication requires the transmitter driver to be enabled and disabled as needed, to allow all boards
to share the communications line. The board has automatic driver control. When the board is not transmitting,
the receiver is enabled and the transmitter driver is disabled. Under automatic control, when data is to be
transmitted, the receiver is disabled and the driver is enabled. The board automatically adjusts its timing to the
baud rate of the data.
Manual 104-ICOM-2S, 104-COM-2S 5