PCI Quad 422/485 Card
Chapter 10 Optional RS422/485 Operation
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RS485 Many Talkers- Many Listeners, Full Duplex.
The RS485 many talkers, many listeners, Full Duplex system can be used when all the
RS485 devices have separate Transmit and Receive channels. There is NO multiplexing
of the TXD and RXD signals on the same device. This system is especially useful when
there is no flow control available on the PC, usually due to the use of a third party
communications program that prevents the use of the RTS signal as a "transmit enable"
control, via the TXD GATE jumper. It can be used in the following situations:-
a)
The PC is connected to only ONE RS485 device.
b)
The PC is communicating with several RS485 devices that are each able to
recognize and respond to their own unique address. The RS485 devices only drive
their TXD lines when they are responding to requests from the PC to send data. In
effect, the RS485 device’s address and the command it receives is used to control access
to the devices TXD channel.
This is a Full Duplex system. Two twisted pair cables are required. One twisted pair, is
the PC’s TXD channel, it carries the data sent from the PC’s TXD outputs to the RXD
inputs of each of the RS485 devices. The second twisted pair, is the Devices TXD
channel, it carries the data sent from each of the devices’ TXD outputs to the RXD
inputs of the PC.
The advantages of this system are great, since no new communications, software is
needed, and the PC can talk and listen at the same time. In effect, the handshaking is
performed by the intelligence of the RS485 devices attached to the PC.
When wired as below, the PC can transmit data at any time and all the RS485 devices
#1 to #n simultaneously receives it.
Only one of the RS485 devices may talk, i.e. transmit data, at any one time. Each RS485
device recognizes commands and data addressed to it, it only talks when the PC
commands it to do so. When the RS485 device receives the command to talk from the
PC, it gates its TXD drivers on, sends the data down the device TXD channel, and
disables its TXD drivers. The other RS485 devices remain in the receive only mode
when they are not being addressed, they do not transmit any data at all.
RS485 Jumpers, Full Duplex.
Both jumpers left unconnected (not shorted).
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