Modbus circuit failures when adding more than one slave

[Perrinoly]

We are commissioning a tank level radar project and we are having some intermittent modbus failures (no slave response) when we poll more than one device on the loop. We can talk perfectly to each individual device if only polling that one device but when we try to talk to all 5 devices on the loop with PLC (quantum 584 through a converter) master polling, the first device in line (daisy chain) has no issues at all but the rest show intermittent failures (lack of slave response about 25% of the time, this varies slightly for each device). The line terminations have been respected and the grounding configuration appears to be OK. Any help at alll would be appreciated.

 

[LionelHutz]

Each end device had a terminating resistor (120 ohm)as well as the RS485/232 converter.

Well, this red flag sticks out. Try a single terminating resistor only at each end of the daisy chained wire. If the run is short (say only a few 100 feet) you could also try no terminating resistors at all.

Before you go further, the very first step is determining if each device is isolated or not. Read through this note until you understand the common/grounding connections as described.

http://www.robustdc.com/pdf/an003.pdf

 

[Perrinoly]

Yes danw2 there are JBs on the walkway to each tank. 1 2 pair cable goes to the tank side monitor. One pair to the tank and one pair back (this second pair gets terminated in the box and from here goes on to the next JB.

Thanks for the follow up, I had no time today but hopefully tomorrow to troubleshoot. I've got some more ammo now, I'll follow up with results (if any!)