Hardware manual

RS422/485 Information PCI VELOCITY / OPTO ISO RS422/485
Chapter 4 Page 46
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 4-6 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 TXD
channel, and disables its TXD drivers. The other RS485 devices