Modbus Repeater

[Rich_Lannaghan]

Can anybody suggest a reliable Modbus repeater, do Schneider make one ?

We are going to end up with over the 32 devices on a modbus chain (iEM3000 range) and can't get round it any other way, would a repeater be the best option ?

[Hal_Etheridge · Accepted solution]

If the number of the devices is just over 32 and the RS485 wire length is not near the maximum I would try it as is.

Most RS485 transceivers produced in the last 20 years are actually 1/4 (or better) loading of the standard so putting a few more on the loop should be fine.  This is assuming the RS485 is properly terminated etc of course.

Remember that just because electrically it may work does not mean that the software can get the data out of the device efficiently.  The more devices you have on the loop means ever slower polling intervals.  With 32+ devices on a loop you are looking at a fastest update period possible of at least 10-15 seconds and likely very significantly larger depending on what is being requested.

This configuration is NOT recommended if there is any other possible solution.  Please consider splitting this large loop up.  It will help prevent performance problems from occurring in the future which will likely be more expensive to fix then than making the change now.

[Rich_Lannaghan]

Hal, hi

Yep, it's just over, in fact 35 devices on a very short chain <20M, from experience i think it will work but we need to allow for the cost of the hardware just in case.

Any suggestions for a repeater that we know works ? We are using a PM5560 as the gateway talking to 35 x iEM3155's just energy and power readings coming back to others software not under our supply (not PME either unfortunately)

Cheers

[michael_franz]

TEAM:

There are several great comments regarding devices, cable lengths with RS485.  Rules are rules but sometimes can be broken.   As devices are added to a daisy-chain the data loading on the string can cause a slow down in data acquisition. 

When simple meters without onboard logging are used this is typically not a big deal.  However if there are several software packages will be accessing devices on a single RS485 string it is important to take this into consideration.

This can happen if BMS/BAS, SCADA or Generator packages are being used besides PME.  This can cause unexpected drop outs if too many requests are being sent along this data path.

If more advanced meter with onboard logs and waveform data are involved this can also take a lot of the band width on a serial string.

Thanks,

Michael Franz