| ??? 11/14/06 13:20 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#127952 - Are the devices plugged in in an outlet with groun Responding to: ???'s previous message |
There are 10 instruments in a Control Panel, Mounted close to one another. These are connected by using 4 core shielded cable for RS485 (2-wire mode). The cable length between 2 instruments is more than 6 inches (done that purposely...I read it somewhere that it is required to be more that 6 inches).
I wonder where, I know of no such. These Instruments do NOT have a Signal Ground or Earth Terminal. So the sheild is shorted between the incoming and outgoing cable, but NOT Grounded. Which is THEORITICALLY OK. Are the devices plugged in in an outlet with ground? Have you grounded the PC? And yes it is terminated using a 100E resistor at the end of the cable. is that BOTH ends? The Cable comes out of this panel and enters another Panel that has 4 numbers of AC Drives having RS485 Port each. Similarly the cable runs into this panel and connects to each of the AC Drive. The cable comes out of this panel and comes to the PC about 5-6 meters away and gets connected to the RS232-RS485 Convertor and then connects to the PC Serial port. The Convertor has 100E terminating resistor and Also a Convertor Ground Terminal (But this is NOT Earth Terminal) The Shield that is Continuous (electrically, but with physical break/shorting at each instrument to maintain continuous shield) from one end to the other and Connected to the GND Terminal of Convertor. sounds right With this system The communication with Instruments and AC Drive works fine. But when the AC Drive starts, the communication fails and starts giving random characters instead of data. If I connect the cable to the Control Panel containing the Instruments only then it works fine even if the AC Drive is in the RUN Mode. But if the cable is connected to the Control Panel containing the AC Drive, then the problem is seen again when AC Drive is in RUN Mode. NOISE! You need to ground these AC drives. that may solve it. However with all these floating grounds that may leave you up the creek anyhow. What I would consider is to split the RS 485 in two one for the instruments and one for the AC drives. To test this - what happends if you disconnect the instrunents and run the AC drives only? If you have the problem after the AC drives are grounded and the instruments disconnected you will need to hunt where the noise comes from using standard motor noise killers. One last thought: do you have 3 resistor termination at either end, if not try it (it is in 10 ways) Erik |
