Can pass-through be used for device communication?

Can pass-through be used for device communication?
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  • HMI Model: cmt-SVR-100
  • EasyBuilder Pro Version:
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    Using an Allen Bradley CompactLogix PLC with an Add-On Instruction for Modbus TCP Server I am able to simulate (using a Modbus simulator) writing values from the PLC to Modbus registers. The PLC’s IP is while the Modbus Client network is 192.168.8.YYY. Can I simply use a cmt-SVR-100 and use the pass-through option to share the PLC’s Modbus Server info with the Client network? Would there be a need for the creation of a macro? Thanks for your support.

Hi @ProcessControl,

To clarify, will the CompactLogix function as a Modbus TCP server within this application? If so, you should be able to use a “Data Transfer” object or a “Macro” to transfer data from the Modbus client to the PLC:

When you have time, can you please advise how many clients will need to read from and write to the CompactLogix so that I can advise the most efficient solution?

There is only one Client that is reading the Modbus data only. I do have the option to change all the IPs of the devices on our PLC side, but this could take a while and I’m not that familiar enough with the other equipment and want to avoid any downtime if I can due to the type of process. So, since I was able to simulate the communications with another PLC and Modbus simulator at my desk, I thought that maybe I could just use a cMT-SVR to share the Modbus info out onto the other IP network. Thanks


Thank you for the quick response! Since you only have one Modbus client I think the most efficient solution would be to use the cMT-SVR as a Modbus gateway. To do this the cMT-SVR will utilize the following drivers:

  • Modbus Server
  • Modbus TCP/IP

While using the cMT-SVR as a Modbus gateway, communication from the Modbus client will be routed directly to the PLC. Here is an example:

Note: When the client makes a request to 0x-1 ~ 0x-4000 within the SVR, the request will be routed to the “Mapped device address”. In this example, the address is 0x-1 ~ 0x-4000 within the CompactLogix. These registers are likely defined within your AOI.

Note: It is also possible and it may be easier to use the “Rockwell EtherNet/IP (CompactLogix) - Free Tag Names” driver. While using this driver, you will not need the Modbus Server AOI.

Here is a link to a tutorial in which we demonstrate how to configure a Modbus Gateway: Click Here

Sorry, a little confused…the Modbus Gateway demo is for Modbus RTU to Modbus TCP. Since both my Server (Allen Bradley PLC via Ethernet) and Client (Modicon PLC over Ethernet) are both TCP do I still need a conversion driver? I just need to pass-through from one IP to another IP. Please clarify, thanks.

No need to apologize, in our tutorial we use an application in which a Modbus TCP/IP device needs to communicate with a Modbus RTU device as an example. However, this approach can be adapted to nearly any situation in which a Modbus TCP/IP client needs to access another device.

In your case, we need to route communication through the SVR since the network conditions are different. It is not possible to use the pass-through feature in the way that you have described because the pass-through feature is only for use in applications where the PC will pass-through the HMI in order to connect to an external device such as a PLC. The solution in this case would be to establish a Modbus gateway that allows a client on the 192.168.8.Y network to communicate with a PLC that has an IP of The Modbus server driver is necessary.

Here is a description of our Ethernet pass-through feature from our user manual:

Config is still not clear, however today I tried simulating using two separate PLCs but no results. Tried numerous configurations and tests but with no positive results. Now I don’t believe I can communicate to the cmt anymore, how can I preform a reset?
Have “so” many questions but so time consuming posting back and forth there has to be a better way.

I did find a doc assisting me with resetting the cmt-SVR. I am going to try everything again and will report back. Thanks.

Understood, thank you for the update!