VFD ethernet to Micrologic 1400 Ethernet

Has anyone used messaging on a Micrologix 1400 to control a drive through the Ethernet port? If so do you have any suggestions?

I wish that everything about which I said "This is a bad idea" didn't become the most popular method.

As Ken says, it's a bad idea.

You should only attempt to read parameters and, at the most, edit the speed command, but never use ethernet from a SLC or Micrologix to start and stop a machine. These processors do not have Ethernet/IP, and rely on messaging to communicate with drives. This is not very reliable. If you post the type of VFD and interface card you are dealing with someone may be able to help with the details.

Paul

Assuming that the drive could be configured as a Modbus slave, would you consider Modbus to be an improvement over ethernet while still using messaging to read/write drive parameters?

You could wire PLC outputs to drive inputs to reliably start/stop the drive and use messaging for reading back status and parameters.

Exactly right. I am setting up a ML1400 where PLC outputs are wired to all drive inputs and all drive outputs are wired to PLC inputs. This part works great. I was hoping to change drive parameters on the fly (velocity and position) using Modbus. From the standpoint of reliability, I can't see where this will be a problem - but I haven't tried it yet....

I think one of the objections is that,even if the messaging goes through, the time for completion of the message is not guaranteed. So it is best not to use the communication linke for preciose time critical changes.

I was being glib in my first post, so I'd like to explain.

Controlling an AC drive on a communications network from a ControlLogix may, on paper, look the same as controlling it from a MicroLogix 1100/1400. The differences are very important.

The ControlLogix uses cyclic messaging that is performed by a dedicated network module with reserved resources that allow the control program to remain in constant and reliable connection with the drive.

MicroLogix uses non-cyclic messaging that is performed by the main CPU. The reliability of the connection with the drive depends on factors that are out of the control of the MicroLogix firmware; factors like retries and the reaction of the controller to communication failure are entirely up to the user program. The Ethernet port and CPU don't dedicate resources to this communication, so it can be affected by functions like messaging to other controllers, online monitoring with RSLogix, or the traffic from HMI computers or operator interfaces.

This isn't to say it can't be done, or can't be done with a good degree of reliability. There are sample programs in the RA "Connected Components" CD and website that do precisely this. I wrote a reliable and well-tested SLC-5/05 version to run PowerFlex 40 drives and filed it on the MrPLC.com Downloads section.

I've become a little jaded over the years with the ability of many users to write clear and reliable communications code, and I don't have the time to prevent them from mangling the examples I write. This becomes especially true when the user is a novice, though I've seen atrocious comms code from seasoned engineers.

My standard recommendation is that the MicroLogix should use discrete control for the drive's Start and Stop (at the very least the Stop) signals, and that Reference, Feedback, and Parameter Access can all be done perfectly happily across the network.