User manual
Chapter 1: Introduction
These cards provide effective asynchronous serial communications in RS422 and RS485 networks
on long communications lines in noisy environments. The cards feature independent serial ports and
use type 16550 buffered UARTs. The data lines are opto-isolated from the computer and from each
other to assure communication through large common mode noise.
Both the RS422 and RS485 cards are 12.283 inches (312 mm) long and may be installed in 5-volt
PCI-bus slots of IBM PC or compatible computers. For Windows compatibility, the cards feature
automatic control in the RS485 mode to transparently enable/disable the transmission drivers.
RS422 Balanced Mode Operation
Model 422 cards support RS422 communications and use differential balanced drivers for long cables
and high noise immunity. These cards also have the capability to add load resistors to terminate the
communications lines. RS422 communications requires that a transmitter supply a bias voltage to
ensure a known "zero" state. Also, receiver inputs at each end of the network should be terminated to
eliminate "ringing". These cards support biasing by default and support termination by jumpers on the
card. If your application requires the transmitter to be un-biased, please contact the factory.
RS485 Balanced Mode Operation
The 485 cards support RS485 communications and also use differential balanced drivers for long
range and high noise immunity. RS485 operation involves switchable transceivers and the ability to
support multiple devices on a single "party line". The RS485 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".
Both 485 models also have the capability to add load resistors to properly terminate the
communications lines. Receiver inputs at each end of the network should be terminated to eliminate
'ringing". Also, RS485 communications requires that one transmitter supply a bias voltage to ensure a
known "zero" state when all transmitters are off. These cards support biasing by default and support
termination by jumpers on the card. If your application requires the transmitter to be un-biased,
please contact the factory.
COM Port Compatibility
Type 16550 UARTs 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 percent compatibility with the original IBM serial port.
A crystal 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 a 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.
Manual PCI-ICM-422(485)/2(4)
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