| ??? 11/14/06 19:40 Read: times |
#127989 - Some points >> Kai Klaas Responding to: ???'s previous message |
Hello Prof. Kai,
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. These Instruments are imported from Taiwan. These instruments have just 2 terminals RxTx+ and RxTx-. They do NOT have a terminal for connecting the "Signal Ground of RS485" to the RS485 Transciever. I will try by passing the "Signal Ground of RS485" through one of the wires of the RS485 cable. 2. Do you use twisted pairs for the RS485 wires?? Simple 4 core cable isn't enough. What you need is two twisted pairs!! I have used a 4-core shilded cable, But I will also try with the Twisted pairs of cable. I will try the STP-8 core cable that is used for networking computers. 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. I will try your method and the methods suggested by other members and I will definitely post the result of my visit. 4. I highly recommend the use of slew rate limited RS485 transcievers, like LT1785 for instance, in your case. I am afraid this cannot be done, because these products are designed and manufactured in Taiwan, and we cannot tamper the instrument, but yes I had a look inside the instrument (once) and they use the 75176 for RS485 and yes with Optoisolators. Many thanks again, From Kiran V Sutar |
