rs-485 random failure

Hello,

I am pretty new to the rs-485 serial world. And I have a customer that has an RS-485 network that links a PLC with a VFD and several power meters. The PLC is a Unitronics V570 and the drive is a Lenze ESV. I did not write the plc program, but I do have access to the code. The issue is that always at a certain time in the start sequence of this machine, the modbus write status will faill to the drive. And this prevents the machine from starting. But if you exit the start sequence and start over, chances are it will work just fine. The PLC pulls in a contactor that powers up the drive, then in 10 seconds, the plc starts sending commands to the drive and it is somewhere in that interval where it usually errors.

Please help. Does this sound like a program problem. I set my timeout settings around 3 seconds and 3 retries and it still happened. Or does it sounds like a hardware issue or network installation problem? I think it has been doing it about since new, but former contractor didn't fix it.

It sounds like a program issue for two reasons.
- You can reproduce it (Happens when the drive initializes.)
- Once pass this step the communication does not error.

Do you have any errors indicated on the drive when communication fails? What is the error code from the Modbus RTU command?
These may narrow down the possibilities of where the error is occurring.

What are the commands that are being sent to the drive? Is the parameters checking the status of the drive before trying to write?

Just a few thoughts.
Regards,

i don't know that the vfd is throwing any errors itself. The error code of the Modbus function block itself is either Timeout or no Modbus comm. It switches between those two codes.

Usually this error comes up just after the plc has told the vfd to enter run mode. Then the plc looks for a return run signal over comm. That is what it never gets back and hangs up the start sequence.

if the drives are just powering up sounds to me like drive just needs more time for comms to come online

labeledas said:

if the drives are just powering up sounds to me like drive just needs more time for comms to come online

Click to expand...

I second that. Sounds that PLC sends modbus query but drive don't respond to modbus yet.

Make 10s wait time longer. Even better is to send modbus commands again from PLC if there is error on querys.

Lare said:

I second that. Sounds that PLC sends modbus query but drive don't respond to modbus yet.

Make 10s wait time longer. Even better is to send modbus commands again from PLC if there is error on querys.

Click to expand...

I like this idea. After the initial drive power-on, try to read the drive status at some repeating interval.
For debug, count the errors/intervals to see HOW long you MUST wait.
If this system has worked well in the past, maybe the drive has startup issues due to age.
Are the startup contactor's actual contacts still in good condition?

Out of curiosity, does this comm issue happen if the PLC turns OFF the drive contactor the immediately BACK ON?