Chapter 1: FUNCTIONAL DESCRIPTION
These Serial Interface Boards contains eight or four independent ports and provide effective RS-485 and RS-
422 multipoint communication. Each channel may be configured to either mode. Jumpers on the board permit
the choice of configuration, including termination, for each individual channel.
The boards are designed in the PC/104 format.
It’s dimensions are approximately 3.775 inches X 3.550 inches. All signal connections are made through a 50
pin connector, mounted on edge of the board.
RS-485 BALANCED MODE OPERATION
The board supports RS-485 modes that use differential balanced drivers for increased range and noise
immunity. The RS-485 specification defines a maximum of 32 devices on a single line. The number of devices
served on a single line can be expanded by use of “repeaters”.
The board also has the capability to add load resistors to terminate the communications lines. RS-485
communications requires that one transmitter supply a bias voltage to ensure a known “zero” state when all
transmitters are off. Also, receiver inputs at each end of the network should be terminated to eliminate “ringing”.
These boards support biasing by default and supports termination by jumpers on the board.
If your application
requires the transmitter to be un-biased, please contact the factory.
The driver/receiver used, type 75176B, 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.
COMM PORT COMPATIBILITY
Type 16550 UART’s are used as the Asynchronous Communication Element (ACE) . These include a 16-byte
transmit/receive buffer to protect against lost data in multitasking operating systems, while maintaining 100%
compatibility with the original IBM serial port. However, the ports are not restricted to the standard COM port
addresses.
Continuous address selection is available anywhere within the I/O address range 100 to 3F8 hex, and our
FINDBASE program will scan I/O Bus memory-mapped addresses in your computer for available addresses
that can be used without conflicting with other computer resources. This allows a port to be used as one of the
four “standard” COM ports (COM1 to COM4), or to coexist alongside them, in any combination.
A crystal oscillator is located on the board. This oscillator permits precise selection of baud rate from 300 to
921,600 with the standard crystal oscillator. The standard crystal oscillator is used to generate two clock rates.
One is the standard 1.8432 MHz clock. If higher baud rates are required, a 14.7456MHz rate can be selected
by jumper.
COMMUNICATION MODES
The board supports Half-Duplex communications with a 2-wire cable connection. Half-Duplex allows traffic to
travel in both directions, but only one way at a time. RS-485 communications commonly use the half-duplex
mode since they share only a single pair of wires.
Manual 104-COM-8S
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