I think there is a problem with GE motor protection relays or their manuals re a RS-485 loop.

We have approximately 100 devices in different locations (no more than 20 per SCADA drop). We have asked GE for info and they indicate that they have not used a RS 485 loop (only a RS232 loop). Maybe this is only an Ontario solution. I need to know of any application that uses a RS485 two wire loop with these devices that works and what problems or cautions I should observe.

We have talked to GE (locally) and they have indicated that the wiring diagrams in the manuals for RS 485 connections are not valid. They have also indicated that the 1nF capacitor is not needed for termination only a 120 resistor.

I would appreciate any info others have for this type of loop and terminations.

Thanks

Dennis [email protected]

 

I have used the GE Modbus card (can't think of the number) that has an RS485 port on it to communicate with a Multilin SR489 generator protection relay successfully. The biggest problem was getting the placement of words etc in the ladder programme right. Once right, it worked fine and has been working without a problem for some 3 years. The PLC I used was a 90-30.
On the same project I used the same card to communicate Modbus RTU RS485 to a generator controller. From this unit I was able to pull data out of a total of 4 generator controllers from one device via the generator controller LAN. Once again, getting the ladder right was the major problem. This also has been working successfully for some 3 years.
There are some programming examples on the GE-Fanuc website. I found them by doing a search.
With respect to the ceramic cap in series with the resistor, I would suggest that you use this method for your terminator as it definately helps with noise on the line. My project was high voltage generation and I was having a few "misread" problems until the insertion of the cap. Using it is standard practice with me now.
Oh, by the way, the card I used was a GE co-processor card that was modified by a third party supplier, can't think of their name but thay make cards for Hitachi PLCs and others as well. I think the card was a CMM and I bought it from my local GE distributor.

 

We have set up a two-wire RS-485 multi-drop network with seven Multilin 737's and two Multilin 489's without any difficulty.

We didn't use anything special for our network cable and terminated only with 120 ohm resistors. The only wiring diagram I could find was a little fuzzy but it appears that the RS485 connections were: B2 (+) and B3 (-) (for the 489).

What type of troubles are you having or are you just checking to see if it will work?


Rhonda Netzel
Sequoia Engineering
North Vancouver, BC Canada