Can I use the OPC-UA server to read/write the same values from a PLC that are also being read/written by another OPC server? I already asked this question over on plctalk.net but also wanted some feedback from here.

The PLC is a Micrologix 1000 on ethernet through a 1761-NET-ENI. There is another system using Wonderware and OPCLink reading and writing to this PLC. I want to be able to use Ignition with the OPC-UA server to read and write to this PLC as well. I’ve already tried it and it seems to work but really would like some more thoughts on this before I implement it.


There’s generally not a problem with this. The manual for your PLC/ethernet module/what-have-you probably lists a maximum number of concurrent connections allowed. Don’t exceed that and you’ll be fine.

The only thing to worry about would be asking for too many tags at too fast a rate. It seems that the more simultaneous requests a device receives, the slower the average response time. What constitutes too many and too fast is unique to your situation, but you’ll know it if you see it because your tags will have red stale overlays in Ignition.

Thanks Kevin,

I’m not doing any messaging from this particular PLC so it should then only ever have two concurrent connections. The ENI supports more than that. I’m only looking at roughly a dozen tags using Ignition, Wonderware is reading about 150 tags at 250mS.


Are you able to connect to a Micrologix 1000 with a NET-ENI module successfully?. I am trying to with no success, the driver setup on the Gateway says that it is connecting but it doesn’t do anything beyond that… The driver only lists Micrologix 1100 and 1400 as options, but I tried it anyway.

I developed an application using a NET-ENI module and tested the communication with a SLC5/03 without problem. The customer had Micrologix 1000, though.
When I arrived to customer’s site, I was not able to communicate with the PLC, the EIP was detected and device was stuck in “Connecting” state.
I changed the driver to RSLinx and it worked, the amount of tags was the same, configuration was the same (except for the IP address). So it seems that there is an issue with this PLC.

this is an old topic but Ill add my 2 cents. Id stay away from the net-eni. I wouldnt use one under any circumstance. You will be much better off using a digi one iap.

Yup… Net ENI is nothing but trouble. :thumb_left:

Do you have any tech notes or can you advise on the settings of the Digi One IAP when adding a slc 50/4 i am able to see my plc in rslinx but i am unsuccessful adding to OPC UA.