AB Powerflex 700 Modbus Communications

Good day. We have a problem with the configuration of an Allen Bradley Powerflex 700 VFD drive. I am hoping that someone can help out with this.

We are controlling the drive via Modbus. We have a 20-XCOMM-DC module and a 20-COMM-H Modbus module installed. The VFD communicates with a nearby CompactLogix PLC.

The Modbus communications works fine and we can control the stsrt/stop and speed control of the VFD. The problem we are having is trying to override the Modbus control with a switch hardwired to one of the DI inputs. We want it so that when the switch is closed, the VFD will run regardless of what the Modbus is telling it to do and the speed then gets controlled by a local potentiometer connected to the analog (pot) input.

At present, the drive works as described above, but if the VFD is running from the local switch and the Modbus sends out a stop bit (momentary), the VFD will stop, even though the switch is telling it to run.

Has anyone run into this problem?

"STOP trumps everything else".

You cannot disable the STOP function of any interface to a PowerFlex drive.

This is just an Allen-Bradley operational principle: there should never be a case where you press the STOP button and the drive keeps running. A-B drives have run this way for as long as I can remember, back to the 1336 drives.

Any connected interface can issue a STOP command. STOP commands are accepted from interfaces that are disabled by the Logic Mask, and even accepted when another interface has taken Local Control.

The simple solution is: Never send a STOP command over the Modbus network if the Digital Input for "Local Reference" is true.

Thanks Ken!