Need Help Troubleshooting Modbus (AD dl250-1 to 4 AD vfds (either GS1 or GS2))

the dl250-1 is controlling 4 vfds, running them as pairs. I'm being told that maintenance would see the 'run' light come on the drives, but that the frequency would stay at zero.
Last week when I was troubleshooting I watched a timer in the logic complete and upon completion the two drives that maintenance had told me earlier weren't coming on, both started right up and ramped up to speed.
The drives are either GS1 or GS2, I don't remember. They've been in service since as early as late 2007 (per stamp on software cd-rom) though maintenance thinks that they were installed in late 2008.
What would be a good way to troubleshoot this, and would it be a good idea to replace the cables going to the drives?

Russ said:

the dl250-1 is controlling 4 vfds, running them as pairs. I'm being told that maintenance would see the 'run' light come on the drives, but that the frequency would stay at zero.

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Russ,

Can you describe the method of control for these drives? Your original post mentions Modbus in the title and this statement.

Russ said:

and would it be a good idea to replace the cables going to the drives?

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This would imply that there is either serial or either net comms to the drives. Is there also an output (digital or possibly analog) to one or more of the drive inputs? Is there any inter connecting between the drives? Like a digital output of one drive to a digital input of another? Or a analog output of one drive to an analog input of another?

Concerning the "run" light and frequency issue. Is the display on the drive showing an "F" or an "H" before the number? When the "run" light comes on does the stop light flash?

Is there a pair of counters like these in the code?