MODBUS Multidropping


Thread Starter


I have an application where 30 Modbus RTU slave devices of similar make, but unique addresses are multidropped on a single RS485 comm port. Response times are not an issue since we have to gather information at sampling intervals of 1 second for each device.

I want to use Cimplicity from GE Fanuc to develop the HMI for the same. I have purchased the MODBUS driver for Cimplicity. How can I configure the driver so that each device can be polled at the required interval?

I am not able to find a runtime facility to switch the address to poll in Cimplicity

Please help!!

Rafael Jacomino

The selection of SCADA and HMI packages available perhaps extend beyond what one person can hope to grasp much less dominate. However, in my ICONICS, RS-View, Wonderware, and Monitor Pro 'adventures' I have only done device-specific register scanning/polling through VB and OPC/ActiveX tools--specially for a 30 device installation. Much cheaper, deterministic, and simpler for your application would be a PLC with SIMULTANEOUS ModBus master/slave capabilities.

For the cost one mid-range PLC I/O module, you can use a Modicon Momentum 171CCS78000 PLC and its ModBus XMIT instruction. You can easily and reliably set sampling rates and register counts for each device. Subsequently, your GE SCADA will just poll this PLC in the register "areas" that you have designate (as part of the XMIT instruction) for each ModBus RS-485 device.

Consider it a controlled data concentrator which facilitates both your SCADA hardware (the Momentum has a SECOND ModBus port operating in RS-232 mode) and software (Cimplicity will only poll one device). This is so cost-effective you might make it a standard tool in your future ModBus projects! Let me know if it helps.

Rajesh Agravat

If cimplicity is not supporting the device polling u can poll it by using a timer and an integer defined in the scada itself or u can use it in the PLC also, so through your logic polling will be carried out

i have done so through VB with RS-View
My suggestion is also the same as the latest reply. We have done such a project using lookout,were we had connected 12 Energy Meters to a single comm.Port. We used timers and counters so that the driver will poll the meters in a loop (First meter and in the next second second meter like this).

If you have similar projects in hand,we will suggest you go for Lookout. It's really good.

And one caution. How many parameters you are going to read from each RTU? Because this will effect on the communication throughput and the updation time.

If you are going for 1 second polling,I think you will find more failures. Consider making this to 2/3 seconds,if the data is not that critical.

You can contact us for any clarification on Modbus, Lookout, MemoBus (generally all the communication needs)

Mail :[email protected]

Mark Vyshkin

We have also tried the original Cimplicity modbus driver to poll over 70 slaves and have got the same timing problems. Now we are using a Matrikon OPC server for modbus and the OPC client inside Cimplicity with quite good results. We have downloaded the modbus OPC server from Matrikon ( for free. The OPC client is
included in the main Cimplicity package.

Mark Vyshkin
Project Manager
Biotechnical Center
Riga, Latvia
[email protected]

Ing. Barrera

We already make an app with 90 device Modbus RTU 485 with diferent ports but i recommend you a bridge to change Modbus RTU to Modbus TCP/IP with a bridge and making a master-slave arquitecture it works very well, we have PQM in a network and
you only need Cimplicity with Modbus TCPIP, if you need some help contact me or visit our Web site "":
The speed depends of how many information you demand of each device...
Good luck