PCI Quad 422/485 Card
Chapter 10 Optional RS422/485 Operation
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Serial Port 1 To Other PC Cable.
SERIAL PORT 1 Side
Other PC SERIAL PORT Side.
9 Pin Female D Connector
9 Pin Female D Connector
Note:
Receiver ends terminated in characteristic impedance ONBOARD resistor networks.
USE BELDEN TYPE 9729 etc. see above.
RS485 Gating & Multiplex Jumpers as Factory Set.
RS485 Operation.
The RS485 standard is intended for up to 32 driver receiver pairs on the bus. The line
drivers used in the Serial Solutions RS422/485 card are designed to work correctly in
both RS422 and RS485 systems. The main difference therefore is in how the system is
implemented. Though the card uses a 9 pin D connector, in general, not all the lines are
used for RS485 systems. The RTS+/- and CTS+/- lines, though driven by the card, are
usually not connected. In two wire, Half-Duplex configurations the TXD+ line is
connected to the RXD+ whilst the TXD- line is connected to the RXD-, only one pair of
twisted wire cable is used in RS485 Half Duplex communications.
The hardware handshaking performed by the CTS+/- and RTS+/- lines in RS422
systems are handled by a software protocol in RS485 systems. In situations where more
than one device may transmit data on the shared data line, each cards RTS line is used
as a gating signal to enable the TXD driver only when that card needs to transmit data,
i.e. set TXD GATE or AUTO jumper. This mechanism prevents bus contention caused
by multiple transmitters holding the line in opposing states. Revision 3 and higher
versions of this card have a facility which automatically “gates” the RTS line, thus
enabling the transmitter independently of any software. This “Auto gating” is described
in more detail in the previous 485 half duplex section
The three wiring schemes given described below are: -
RS485 One Talker Many Listeners (HALF DUPLEX)
RS485 Many Talkers Many Listeners (HALF DUPLEX.)
RS485 Many Talkers Many Listeners (FULL DUPLEX.)
RS485 Cable.
For best noise immunity use twisted pair cables to make the RS485 connection. In Half
Duplex wiring only 1 twisted cable pair is needed. Two twisted pair cables are needed
for Full Duplex communications.
Use screened twisted pair Belden cable 9729 and 9829, UL type 2493 and 2919 or IBM
Part No 4716748 cable to make the RS485 connection. Terminate the twisted pair cable
