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RS422/485 Information
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.)
Chapter 4 Page 44
PCI VELOCITY / OPTO ISO RS422/485
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