Rs-422 Serial Port Cables; Rs-485 Serial Port Cables - Brainboxes ES-357 Product Manual

b) The PC is communicating with several RS-485 devices that are each able to recognize and
respond to their own unique address. The RS-485 devices only drive their TXD lines when they are
responding to requests from the PC to send data. In effect, the RS-485 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 RS-485
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 RS-485 devices attached to the PC.
When wired as below, the PC can transmit data at any time and all the RS-485 devices #1 to #n
simultaneously receive it. Only one of the RS-485 devices may talk, i.e. transmit data, at any one
time. Each RS-485 device recognizes commands and data addressed to it; it only talks when the PC
commands it to do so. When the RS-485 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 RS-485 devices remain in the 'receive only' mode when they are not being addressed, they do
not transmit any data at all.

RS-422 Serial Port Cables

Use screened twisted pair Belden cable 9729 and 9829, L type 2493 and 2919 cable to make the RS-
422 connection. Unscreened Belden type 8795 may also be used in less noisy environments. The on
board resistor networks terminate the receiving end of the twisted pair cable in its characteristic
impedance.

RS-485 Serial Port Cables

For best noise immunity use twisted pair cables to make the RS-485 connection. In Half Duplex wiring
only 1 twisted cable pair is needed. Two twisted pair cables are needed for Full Duplex
communications. Use screened twisted pair Belden cable 9729 and 9829, UL type 2493 and 2919
cable to make the RS-485 connection. Terminate the twisted pair cable.
Version 2.9
ES Range Product Manual Page 23 of 66
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