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PCI 1 Port RS422/485
ii.) USE BELDEN TYPE 9729 etc. see above.
iii.) 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
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: -
i.) RS485 One Talker Many Listeners (HALF DUPLEX)
ii.) RS485 Many Talkers Many Listeners (HALF DUPLEX.)
iii.) 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
bus
contention
Chapter 3 Page 57
Software Setup
caused
by multiple
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