Troubleshooting Modbus communication

[DCIM_Support]

Hi All!

I need an advice how to troubleshoot Modbus communication.

I have DCE 7.4.3 installed on virtual machine and some modbus devices connected via Moxa MB3480 gateway.

There is several problems:

1. Devices lost communication with DCE in random time period usually from 2 to 5 minutes. Maximum online time is 34 minutes. I've tried to set different timeouts and retries, but its not work.
2. DCE cant add some similar devices. I have 12 Liebert HPM conditioniers and DDF file for them. I can add 5 units, others indicate communication lost error.

I need a guide how to throubleshoot this issue. I've read about packet capture, but have no idea where and how I can run it.

(CID:122095060)

[DCIM_Support]

Hi Valentin,

Random lost comm messages are difficult to track down. If it's 2 to 5 minutes, that sounds like one or 2 polling cycles. That could be anything from network issues / traffic to the device not responding. Troubleshooting those issues is usually best done by using fewer devices per gateway. If you're using the supported ESX100 or 150 gateways, there are configuration options for I believe over a hundred devices however I have heard that any more than 4 or 5 devices and you may start getting communications issues. I can't really comment on the gateway you stated is in use.

A packet capture can be done using Wireshark or, if working with your local tech support and you have root access to DCE, there is internal documentation on how to run a packet capture. It's pretty much the same as external documentation on packet captures done through a Linux interface. Since this is an external facing post, I'll not go deeper into that.

Wireshark can be downloaded and installed to your computer free of charge. You would then need either a hub connected to DCE or the gateway and you'd connect your laptop to that hub as well. If you don't have a hub available, someone with access to the switch would need to be able to mirror the port on which DCE or the gateway resides. This is because when connected to a switch, you'll only see the packets destined for your system and not for DCE or the gateway. A hub does not do this.

Packet captures can get pretty big so you may want to put in some capture filters so you're only getting packets for specific IPs. It won't be too bad if you're capturing during a discovery failure but if you're letting it run and waiting for some intermittent lost comm, that could take a long time and a lot of disk space. 

Steve

(CID:122095088)

[DCIM_Support]

Thank you Steve!

Unfortunately, i have no hub availiable (is it still produced?) as well as switch with port mirroring. I'll try to catch it on my laptop and virtual DCE.

I saw that there were long periond with good communication (several hours) at night. May be problem caused by EMI interference from conditioners?

(CID:122095593)

[DCIM_Support]

Hi Valentin,

It could indeed be interference or possible slight currents induced on the serial line. Without a packet capture, I can't really know if we're requesting or if the device is responding or if either is doing it correctly. If you have a laptop and run a small VM version there, and if this system sees the same issues, yes running wireshark locally may yield some usable data.

Steve

(CID:122095605)

[DCIM_Support]

Valentin i have similar issue

I have tested with EGX and with a similar moxa, with no more than 5 devices, but both give me random communications failures
Does anyone have a setup guide?

(CID:122095376)

[DCIM_Support]

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