Serial Communication between OMRON PLC & other devices

[Jojo712 0]

Hello All, I'm still working on our project which involves serial communciation between OMRON PLC, Schneider ATV312 inverters and OMRON E5CN Temperature Controllers. The OMRON PLC and the devices are communicating already although I have to add 5 more units of inverters in to the loop. My biggest concern is the speed of communication between the OMRON PLC and the devices. At present, it takes about 5-7 seconds (sometimes can reach up to 10 seconds) for the current cycle to complete. The cycle starts from changing the value (SP of ATV 312 or E5CN) in the HMI/SCADA, sending the value value to PLC, PLC sending the value to the device, PLC read the present value and then finally PLC sent the present (changed) value to the HMI/SCADA. What is the optimum/minimum time required between prtocol macro commands? That is to send the write command to a device and then send again a read command to the device to check if the change is successful or to send read command from one device to another. Is there an existing good practice or standard flow chart to improve or optimize communication between PLC and other devices connected through ModBus serial?

[BobB 58]

Put in more SCU cards and break the comms up. Lets face it, serial is slow no matter what. I never try to communicate with more than 5 devices from one port.

[Mendon Systems 22]

Modbus and Devicenet are inherently slow serial protocols because of the overhead involved. As Bob suggests, the best thing to do is break you communications up into as many separate loops as possible and run the communications at the highest possible baud rate.

[Jojo712 0]

Hello Bob/Dick, Thanks for the information. I will try this approach on the next project as I can't put additional cards in the PLC anymore. Cheers.