Created Date: September 03,2016

Created by: Frontier

I'm sure I'm missing something incredibly obvious, but I'm not seeing it, so it's time to reach out:

I have a P2-550 CPU talking to a GS1 VFD over RS485. I'm able to read registers from the VFD and load commands to it (e.g., set a frequency, change the run command bit from 0 to 1). I'm showing zero communication faults in my ladder, so I know they 're talking well. The problem is that even though I see a valid run command on the VFD (9.27), the VFD doesn't actually start, and the run LED on the display never turns on. My other settings are: 3.00: 4 and 4.00: 5. I 've also disabled any transmission fault handling on the VFD, just in case that was interfering.

Any ideas?

Created Date: September 04,2016

Created by: Frontier

Here's my settings. Again, I can communicate with the VFD, but it just doesn't go into Run mode.

P3.00: 04

P4.00: 05

P9.00: 2 (PLC is 1)

P9.01: 19.2k

P9.02: 5

Created Date: September 04,2016

Created by: Ridgeline Mach

Can you post your ladder code? I 've already done this on three projects, but to GS2 and 3 drives, so it might help us. But for now, for example with what I did, I used a simple timer, with a preset of 1, pulse a counter set to 5, and used a compare value of the counters current value, to trigger Modbus read/writes so they did not read/write simultaneously. Another thing I notice when things are working well (and I used the terminal serial port), I'll see the TX and RX lights blinking back and forth happily (when I had problems, only TX would blink).

- Todd

Created Date: September 04,2016

Created by: Frontier

All right, got it figured out!

In the manual, there's a note on page 4-51 that says "Do not attempt to both Run and Jog (P9.27 & P9.31) in one Modbus write command. ". There seems to be a typo here, so it should actually say "Do not attempt to send both Run and Jog (P9.27 & P9.31) in one Modbus write command. ". It doesn't matter if you 're only sending a zero to the jog register; the VFD doesn't want to see the register written into in the same modbus write session. The fix for my ladder above is to thange the MWX command in rung 2 to start at array index 1 and end at 5, skipping the jog entirely.

Weird, but at least I figured it out. Thanks everyone for helping me look for this.