Micrologix 1400 Modbus RTU Slave

Jmeadows7676

Member
Join Date
Oct 2013
Location
Alabama
Posts
158
I am having an issue with my Modbus Communications. I have a PLC 5 running a Prosoft 3100-MCM as Master. It is polling a total of 3 Devices. 2 Airtek Air Dryer Control Centers (Old Air Dryers) and a Micrologix 1400 (New Air Dryer). The issue is slow communication about 4 secs for round trip. If i pull up the Channel Status 0 on Micrologix i see there is about 10 Message Received to 1 Message Send. I have double checked all settings and everything seems to be right. It's a Daisy Chained Network with proper resistors on both ends of the line. During troubleshooting i noticed if i took the 2 Airtek Control Centers out to the Modbus scan the Message Received and Message Sent on the Micrologix match with no blinking error light on the Prosoft 3100-MCM card and speed was good. If i took the Micrologix out of the scan and just had the Airtek controllers it was the same, no error light and speed good. I can't wrap my head around the issue.:mad: Any help or insight will be greatly appreciated.

See Attachment for Channel 0 Status



Thanks,

Channel 0 Status.JPG
 
Last edited:
I would check your termination resistors.
The other thing to do is to reduce the speed (baud) of the transmission. This sounds counter intuitive but the signals will be less susceptible to noise. Try 9600 to see if communication errors reduce. Ideally the communication cable will be run by itself and grounded to only one end. (Usually grounded to the panel side.)
Hope this helps you out.
Regards,
 
I don't get the Messages Sent/Received counts. The counts appear backwards. I would expect more messages sent than received from the perspective of the master, which presumably is reporting the message count.

Slaves do not speak unless spoken to, so where are 10x as many slave responses as Master polls/queries coming from?

When the counts are reversed it means that slaves are not responding, probably because the master's message is not complete and intelligible and therefore a slave will ignore it, or a slave is (or multiple slaves are) offline and not responding. The suggestion of noise is a reasonable cause.
 
pretty sure it is counting messages received from the master that was intended for other slaves on the 485 bus.
 
I would check your termination resistors.
The other thing to do is to reduce the speed (baud) of the transmission. This sounds counter intuitive but the signals will be less susceptible to noise. Try 9600 to see if communication errors reduce. Ideally the communication cable will be run by itself and grounded to only one end. (Usually grounded to the panel side.)
Hope this helps you out.
Regards,
The Resistors are 2 watt 120 ohm. One on the Master and one on the last slave. Like i said in earlier post it is a daisy chain network. The Cables are ran in conduit with no other wires
Thanks for the reply
 
Here is a pic of the configuration in the Micrologix 1400 as a Slave. Already @ 9600 baud rate. Is there anyway to confirm that it's a noise problem??

Channel 0 Config.JPG
 
pretty sure it is counting messages received from the master that was intended for other slaves on the 485 bus.

I agree. The "Messages Received This Slave" is the exact same amount as Messages sent indicating that the 1400 has responded each time it has been addresses by the master. The "Messages Received" is all of the messages that the channel 0 port has received.
Given that you only have three devices but 10 times as many messages received I would be looking at one of the other devices. It looks like one of them is having to be polled multiple times before it responds (correctly anyway).
If you pull any one of the devices out and things improve I'd be looking at either duplicate addresses or to much loading on the lines. If you have access to an oscilloscope you could try and look at the 485 signal when all three devices are on versus when only one or two are on and compare that to the spec. You might also try pulling one of the resistors out just to see what happens. I would also check the condition of the wires between devices. If this is an old installation (and the wires have not been replaced) then the resistance in the wires will be higher than when installed. Most of the time it's a nominal amount but you could have some corrosion somewhere. Corrosion will increase resistance and possibly cause a short that isn't enough to completely knock out the communications but enough to load the lines down making the signal coming from the master low.
The more I think about it, the more I point towards line condition as causing your problem. If I had to guess I'd say that the device furthest down the line is the one having to be polled repeatedly.
 
Thanks for your comments on the message count. Somehow I missed the thread title - the 1400 is slave. I thought the 1400 was the PLC master and the count made no sense. As a slave, the msg counts makes a lot of sense.

I will try putting an isolating repeater half way down the chain, to break a chain into two segments, because sometimes a slave will have common mode on the driver lines that makes it noisy.

Another approach is to turn off a device and see what happens to the throughput timing or in your case, the message count, to see if the ratio changes more than the absence of one node would make it change. There can be problems running with comm though, given that it is automation, after all.
 

Similar Topics

I want to learn about modbus communication and I saw on another thread about inexpensive modbus relay units so I found it on Amazon along with a...
Replies
46
Views
7,081
I have a spare Micrologix 1400 Series B I bought used on Ebay awhile back. I had thought of it as a spare PLC for my train project but I screwed...
Replies
4
Views
1,192
Good Day! Would anyone be able to explain what is happening with the COP's and MOV's in this Modbus poll routine? If anything I am confused by...
Replies
6
Views
1,700
I been tasked with setting up communication between a heating cooling unit manufactured by Advantage Engineering and a Microloagix 1400 plc. The...
Replies
6
Views
2,454
Hi, I am trying to communicate between a MicroLogix 1400 and a Click PLC. There is a great video from AD that shows the simplicity of...
Replies
2
Views
1,737
Back
Top Bottom