Modbus Ethernet connection produces bad quality tag

[Beth F]

I am using Labview DSC with the IAS OPC Server to connect to hardware using modbus ethernet. I have configured 4 modbus objects at 1 IP address. I try to read 4 tags (one from each object), but only 3 work correctly - the other one shows bad quality in the tag monitor. It's not always the same one that is bad when I restart the tag engine, but always one out of the four.

I also tried in Lookout and had the same symptoms and recieved the following alarm: "Socket communications error 10061:Connection refused"

All of the tags are configured the same and all of the Modbus objects are configured the same, why can't I read all 4 tags?

[Khalid]

Hi,

Quick question: why do you have 4 Modbus objects if they are all reading from the same PLC? Any reason you don't want to read all 4 tags using one Modbus Object?

I am not saying what you're doing is wrong, but it's not typical either.

Regards,

Khalid

[Beth F]

It's not technically a PLC, there is one master controller and 4 connected devices, each with it's own identifier.
The hardware is part of a process that is already running - I can't change the setup, I just want to do data acquisition using DSC.

[unclebump]

Try downloading a demo of the Kepware OPC server program to test the modbus devices. You will be limited to a two hour runtime. It has a quick client built in that will show you the tag data in real time.
www.kepware.com. I use this program to read data from multiple plc's over ethernet networks, but not at the same ip address. Then the DSC module reads the data directly from the Kepware program. Just my opinion, but I find Kepware much easier to setup than the lookout drivers.

[Khalid]

Oh OK, that makes sense. If Lookout too exhibits this problem, then I suspect the issue to be with the Modbus Object. To confirm this, you may want to try using another OPC Server (E.g., Kepware, like UncleBump suggested.)

What happens if you try reading 3 (or more) tags but from only 3 devices? Do they all read fine? I am just wondering if it's an issue of bandwidth here.

Regards,

Khalid

[Beth F]

Thanks for replying so quickly. I tried the Kepware OPC server and had the same problem.
FYI if I only use 3 Modbus Objects, all 3 work fine.
Everything is pointing to network issues (maybe too much traffic?), so we will look into it from that end.
Thanks for your help.