Manual Number: 00650-146-1
Page 8
PCI-COM485/8 Manual
Chapter 2: Functional Description
The PCI-COM485/8 Serial Interface Card was designed for effective multipoint transmission in
RS-485 (EIA485) protocol. The card is 7.83 inches long and may be installed in 5-volt PCI-bus
slots of IBM PC or compatible computers. The card features eight independent, asynchronous RS-
485 serial ports, type 16550 buffered UARTS, and, for Windows compatibility, automatic control
to transparently enable/disable the transmission drivers.
RS-485 Balanced Mode Operation
The PCI-COM485/8 supports RS-485 communications and uses differential balanced drivers for
long range and noise immunity. RS-485 operation involves switchable transceivers and the ability
to support multiple devices on a single “party line”. 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”.
PCI-COM485/8 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”. The PCI-COM485/8 supports biasing by default and supports
termination by jumpers on the card. If your application requires the transmitter to be un-biased,
please contact the factory.
Comm Port Compatibility
Type 16550 UART’s are used as the Asynchronous Communication Element (ACE). These in-
clude a 16-byte transmit/receive buffer to protect against lost data in multitasking operating sys-
tems, while maintaining 100% compatibility with the original IBM serial port. The system assigns
the address(es).
A crystal oscillator is located on the card. This oscillator permits precise selection of baud rate 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
PCI-COM485/8 supports Half-Duplex communications with a 2-wire cable connection. Half-Du-
plex 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.
Baud Rate Ranges
The card has capability for two baud rate ranges and you can select which you wish to use on a
port-by-port basis. One range is up to 115,200 baud applications and the other is up to 460,800
baud.
