Brainboxes PX-101 Product Manual page 30

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.
Note: The receiver end of MAIN line terminated in characteristic impedance, stubs off the main not
terminated.
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 cable to make the RS-485 connection.
Terminate the twisted pair cable.
PX Range Product Manual Version 2.3 © Copyright Brainboxes Limited 2013 Page 30 of 45