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PCI VelocityRS232/422/485
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 in Figure 5-6 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
Additional RS422/485 Operation
Chapter 5 Page 77
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