| ??? 11/14/06 18:14 Modified: 11/14/06 18:17 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#127981 - Some points Responding to: ???'s previous message |
1. You should have the signal ground of RS485 bus connected to each RS485 transciever in your application! So, your cable should not only contain the two RS485 pairs, but also the GND of RS485 bus.
Opto isolation might be needed, when connecting the GND of RS485 will create unsane ground loops. This can be the case with "safety class I" equipement, which has GND connected to protective earth. 2. Do you use twisted pairs for the RS485 wires?? Simple 4 core cable isn't enough. What you need is two twisted pairs!! 3. There's no need to connect the shield of RS485 cable to GND (signal ground). Connect it to protective earth, like PROFIBUS does. A connection between GND and shield isn't needed at all, because the shield doesn't carry any ground return currents. The shield is part of enclosure, not part of signals and must not be confused with the shield of a coax cable or other cables carrying unsymmetrical signals. 4. I highly recommend the use of slew rate limited RS485 transcievers, like LT1785 for instance, in your case. Kai |
