Modbus RTU and Remote Operator

[Vidmas30]

Hi,Everybody,

We have at the plant a modbus rtu network (RS485) of 3 PLC (V570), where one is master and other two are slaves. We planned to do another RS485 network for Remote Operator connection (we have RS485 to Ethernet converter), but organisation that laid cables misunderstood and now we have one Modbus RTU network. Question is: if we connect our RS485 to ethernet converter in that network will it be possible to connect to PLC's through Remote Operator? We have all 2 ports free on each PLC (one of them for Modbus). Thank you!

[Simon]

Hi,

Unfortunately the traffic from the Remote Operator will conflict with the Modbus communications.

*Maybe* you might get some kind of success if you set the Modbus poll rate to be as slow as possible, but even so, you would see a lot of retries on both Remote Operator and Modbus. This would not be robust, but may get you out of trouble.

A full solution would only be possible if you set up a separate network using the second port on each PLC, as you originally intended. If cabling is a major obstacle, the Unitronics work very well over wireless telemetry data radios. I would use the Radios for Modbus and use the existing cable for Remote Operator, since Modbus uses relatively short packets, with more control over the polling rate.

[Vidmas30]

Thank you, Simon!

Between PLC1 and PLC2 we have 4-wire shielded cable and between PLC2 and PLC3 3-wire shielded cable. Maybe we ask to replace 3-wire cable to 4-wire cable and use one pair for modbus and one pair for Remote Operator (the length between PLC's ~100m).

Another bothering thing is the speed of Remote Operator..When display is loaded - all is good, but going to another display takes a while (when there's more graphic 30s and more)What could we do to speed it up? We do use Remote Operator file (don't remember extension) for graphic and fonts.

[Simon]

It could be possible to use a 2-pair cable. The minimum standard would be twisted pairs, and It would work best if the pairs were also individually shielded. Use the lowest baud rate possible on the Modbus, and highest possible for Remote Operator.

Regarding the speed of Remote Operator, is this just when you are connected to a single PLC? What baud rate are you using? Do you have temination active on the first and last PLCs in the network?

If everything is working properly, you should get a fast response with just one PLC, similar to the speed when using the RS232 programming cable.

[Vidmas30]

Had one v570 PLC on the table, decided to try to connect to it through RS485-ETHERNET converter and as I said there were delays when loading displays. Tried on different baud rates till maximum allowed (115200) The same result..

[Simon]

This is just a maybe, but the RS485 to Ethernet converter may be responsible for the delay.

Is there an option to install CAN bus cable and network the PLCs with CAN? By connecting the first PLC on ethernet, it can also be the bridge between ethernet and the CAN network, and this is quite fast.

I hope this helps.