Internet PLC Forum

General => Technical support => Topic started by: DYL on January 17, 2005, 09:27:09 AM

Title: RS485 network
Post by: DYL on January 17, 2005, 09:27:09 AM

I am relatively new with the Super PLCs. I have a question about the RS485 network.

In the PLC User Manual, resistors are needed to terminate the RS485 network.

If I only have two PLCs, and I want the two PLCs to communicate with each other,
1. are the two PLCs' RS485 considered to be a RS485 network?
2. Do I need to add the Resistors (120ohms, 560ohms, and 560ohms)?
3. Can the 5 VDC reference voltage be connected to the RS485 network as shown on page 3-4 in the PLC User Manual?

Thanks.
Daniel

Title: Re:RS485 network
Post by: support on January 17, 2005, 10:08:25 AM

Two PLCs can form an RS485 network.

Biasing resistor and terminating resistors are needed when communicating over long distance (more than 100 ft). For short distance, PLC to PLC communcation you may be able to do without these resistors.

PLC 5V analog reference should not be used for this purpose. That may have adverse effect on your analog circuitry which should have quiet 5V reference voltage.