RS 485 communication question

Farid

Member
F
Join Date
Jul 2004
Location
UAE
Posts
79
hi all,

my plc has rs485 port two wires, and I want to communicate with a driver that has rs485 but four wires!

can I make these two devices communicate??

thanks in advance
 
no you would not have the handshaking 2 wire is half duplex however you need to check one thing there are converters that will simulate the handshaking such as the ADAM 4520 part number ADAM-4520-d2 Industrial communications products. You can sometimes get away with two wire on a 4 wire but only if the software does not require the handshaking
 
The only difference between RS485 and RS422(electrically they are the same) is that one of them ( I can never remember which) is two wire and the other is four wire. The four wire can receive and transmit at the same time (using a pair of wirs to transmit and the other pair to receive) The two wire transmits and recieves over the same two wires. Some devices are capable of both methods but you will need to check your driver. It may be that it has terminals for four wires and that you can select RS485 or RS422 mode.
 
consys said:
The only difference between RS485 and RS422(electrically they are the same) is that one of them ( I can never remember which) is two wire and the other is four wire. The four wire can receive and transmit at the same time (using a pair of wirs to transmit and the other pair to receive) The two wire transmits and recieves over the same two wires. Some devices are capable of both methods but you will need to check your driver. It may be that it has terminals for four wires and that you can select RS485 or RS422 mode.
Click to expand...

RS-485 can use 4 wires. Here is a simple comparison of the systems from BB Electronics...
Q: What is the difference between RS-422 and RS-485?
A: RS-422 devices don't tri-state the transmitter, but RS-485 devices do. With RS-422, two units cannot connect to one receiver, because when one device tries to "talk" the other device is trying to hold the lines in one state, the other trying to toggle them high/low. RS-422 is always full duplex (if the device is not listen only), RS-485 can be full (4-wire mode) or half duplex (2-wire mode). RS-422 devices can have more signals for handshaking (RTS/CTS & DTR/DSR). RS-485 never has more than 2 signals, receive and transmit.
Click to expand...

 
Some devices, have 4 wires, but run half-duplex 2 wire RS-485, dependimg on wiring and internal configuration:

Note how the TX+ and RX+ are jumpered together, as are the TX- and RX- for half duplex operation.

RS-485_half_or_full_duplex.JPG
 

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