Rs485 Operation; The Three Wiring Schemes Given Described Below Are - Brainboxes CC-357 User Manual

PCI VelocityRS232/422/485
Note:
i.) Receiver ends terminated in characteristic impedance
ONBOARD resistor networks.
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 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:

-
i.) RS485 One Talker Many Listeners (HALF DUPLEX)
ii.) RS485 Many Talkers Many Listeners (HALF DUPLEX.)
iii.) RS485 Many Talkers Many Listeners (FULL DUPLEX.)
Additional RS422/485 Operation
Chapter 5 Page 74