Introduction and MODBUS/CLICK PLC/RS-485 problem

bimmersix

Member
Join Date
Jul 2014
Location
PHX, AZ
Posts
38
Hello,

(It's long, so bear with me)

First post, by way of introduction I am fairly new to the world of automation but have been reading just about everything I can get my hands on in my free time over the last year. I work in the environmental remediation industry where most of our equipment is still operating in the stone age - place remote machines that have to operate 24/7, then send a tech every two weeks to see if they are still on. If not, no one gets paid and no one is happy. Chances are, you won't have a clue what the problem was, either, and have to send a more highly trained tech or specialist to figure it out. Or it could have just been a momentary power outage that dropped the seal-in circuit out...

So - I had the "brilliant" idea of designing a system that is better than the status quo in this industry, which my boss suprisingly gave me a budget for. VFD to vary process flow instead of a physical recirculation valve, an HMI and datalogger, PLC control of very simple logic, and several discrete temp controllers.

Now to my question - which I've had a hard time finding anything out about - I have a Click analog PLC, 2 SOLO temperature controllers, and a GS3 (Automation Direct brand) VFD. The PLC can communicate via MODBUS RTU protocol over RS-485. I have a test station set up where this is suppose to happen before it goes into the prototype machine.

Goal: Read operating parameters from VFD, Send Frequency to VFD (RS-485) - Start/Stop will be achieved through digital inputs.
Read temperature and duty cycle from each temp controller. Would be nice to be able to change set point, but not necessary (RS-485)

1 device (VFD) worked perfectly. Added the first temp controller, worked sometimes. Added 3rd device (temp controller 2) and the RS-485 couldn't send or receive anything.

I know this has to do with correct interlock but I haven't found any good information on how to do this with send and receive from more than 2 devices, and have only been marginally successful trying to imitate that logic.

Any suggestions? What can I provide that would make answering this question easier? The cable connection is very short and noise/connection/port setup are all fine. I'm almost 100% this is just an interlock problem. Thanks in advance!
 
Tell me how you ran the RS485 cable. It needs to be a trunk line and not a star. You need to have termination resistors at the physical ends of the cable.
Is the PLC controlling the communications? Make sure it isn't trying to communicate with more than one device at a time. When you're trying to read data from one device, make sure you have received a response before you try to read data from the next device.
 
Tell me how you ran the RS485 cable. It needs to be a trunk line and not a star. You need to have termination resistors at the physical ends of the cable.
Is the PLC controlling the communications? Make sure it isn't trying to communicate with more than one device at a time. When you're trying to read data from one device, make sure you have received a response before you try to read data from the next device.

It's a trunk line, only about 3 feet of cable. 120 ohm resistors at each end.

My devices are trying to communicate at the same time, which is causing the problem (I think), but I'm not sure what the interlocking ladder logic will look like.

I've tried a system timer pulse (500ms) that advances an interlock counter and a compare NO bit on each rung that tries to send or receive from any of the devices. I guess the comm times and scan times are vastly different because it seems to work for a few scans and then gets "stuck" try to send or receive one rung.
 
Click Comms

So I have done this... I have 1 click as the master, 2 clicks as slave I/O, and 6 VFDs all on a RS485 (MODBUS RTU) network. I basically check to make sure the comm port is free and that Im not already transmitting or receiving then I use the success and error bits to trigger the next message. I always start with my receives. I go and grab all my data first. Then I go to sending out the changes. You will see in the attachment alot of the "mess" is change of state logic. Having as many nodes as I had on the network if I were constantly sending and receiving it would slow down my process too much. Thats why I added the change of state logic so I only send if I really need too. It works really well. If you have any other questions you can PM me, I will send you my info.
 
Got this working with about 98% success. Thanks again for the help. If anyone else has interlock issues, this ladder works well. I'm sure the last 2% is something I missed and I'll keep working on it.
 
UPDATE:
Increased the MODBUS comm speed from 19.2K to 38.4K and it began working with 100% success.

Big thanks to PLC GURU for the help here and via PM. Once I had the ladder written correctly, it was just a matter of experimenting with the comm settings.
 

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