Romrot (Customer) asked a question.
GS20 VFD with Click PLC Plus Modbus over RS-485 can receive data, but not send
I have two GS20 VFD's and a click plus. I want to control the frequency of the drives through modbus but I've only been able to get one to work.
using "Receive" function works, I'm able to read data from the VFD, but "Send" keeps resulting in an error.
The other drive communicates with Receive and Send just fine and I am able to control the Frequency on that drive.
Drive parameters are the same for both drives.
P00.20 set to 1
P00.21 set to 2
P09.00 Set to 2 and 3 for the drives (the one with 3 works for both send and receive, 2 only works for receive)
P09.01 set to 115.2 kbps
P09.04 set to 15
What error are you getting?
Have you cycled power to the drive after changing the parameters? Many drives require this, not sure about the GS20. The manual didn't explicitly state that it's required, but I have run across other makes that are unclear about it, and do require it.
Are the end nodes of the RS485 network terminated with 120 Ohm resistors? This may not always be necessary. I've found for shorter runs with few devices you might get away without them. I typically have four or fewer drives on a RS485 network and run a baud rate of 19200 or 9600, and if there are terminating resistors that can be switched on at the drives, I will put the PLC in the middle of the daisy chain and turn on termination at the two drives at the ends.
The click PLC doesn't give me and error code, the error bit on the Send function just turns on. the Error code is always 0.
I think it has to be something on the drive because the other drive works for send and receive.
I don't know if it needs a resistor the drives are in the same cabinet as the click PLC the cable is less than a meter to both of them. The click PLC is in the middle and both drives are the ends. and if the cable was wired wrong I would expect the "Receive" function on the click PLC wouldn't work, but it's working for both drives.
I haven't tried power cycling, right now that isn't really an option.
How are you controlling the Sends and Receives? Click does not natively control the polling; you have to code them so they poll properly. Can you post your PLC code? I can imagine if you're performing them all simultaneously, it will not work.
The GS20 Manual has an example for using click and modbus, I followed that example.
under subroutine "Communication" you can see the logic using send and receive functions.
Water Pump Motor control
One other thing to be aware of with the Click Send and Receive instructions is that they will constantly fire as long as the rung-in condition is true. I have fought this battle before. My first attempt to solve it was to use a rising edge one-shot to trigger them, but then they sometimes don't fire at all. So I ended up using a 40ms time limit on each message instruction to limit how long they can be true. I didn't discover this issue with Yaskawa drives on my first few programs, but I later ran across a device that would lock up if I peppered its serial port too fast like that.
I think you are right to suspect the VFD. Your program looks fine and the comms instructions for both drives look identical with unique addresses for each, so that checks out.
I would recommend comparing all the parameters in both drives to ensure they are the same except for Modbus ID numbers and then find an opportunity to power cycle the problematic drive. When you do, make sure to leave it off long enough for the DC bus to completely bleed off. Large drives may take several minutes to fully drain the logic power.