MicroLogix 1100 & PF700 over Modbus

What happens if you set the Message Timeout in an MSG block to zero? Does it ignore the timeout and stay enabled?

Heres the situation: We are communicating over Modbus from a ML1100 to PF700 drives. If one drive is powered down then turned back on, the PLC will not re-establish communications. I found the message timeout set to 0 in the MSG block for that drive. Could this be the cause of the problem? The other drive will re-establish comms just fine and it's MSG timeout is set to 1. Unfortunately I am 3 hours from the site and did not find that discrepency until leaving. The tech at the site claimed he tried changing the timeout to 1, with the same results.

Any ideas?

My guess is that a value of 0 for the timeout means "infinite", but I'd have to check and test to be certain.

Whenever you are controlling a drive with MSG instructions on a non-IO network like DF1 or Modbus, you have to set a control timeout value in the drive after which it will fault if it doesn't receive a message from the master device. The drive also won't start if this value remains zero.

I very strongly recommend that when you are using a MicroLogix or similar controller to run a drive that the Start and Stop commands be hardwired. You can save on analog modules by making the reference and feedback networked, but it greatly simplifies the message logic if you don't worry about the command message timing.

Can you post the MSG management logic from your MicroLogix program?

The default timeout value for a Modbus RTU master message in a MicroLogix controller is 2 seconds.

If the value of zero is placed in the Timeout field, the message will never time out on its own. It can be cancelled by setting the .TO bit or using the DCOMM button on the controller to change the port mode. I verified this on a MicroLogix 1200 with FRN11.

If you post the messaging logic someone may be able to help more.