It’s come up as an idea once or twice, but there’s currently no driver or other functionality that would let Ignition expose data by acting as a Modbus slave/server.
I think you can make this work, but it would require using Ignition’s other capabilities to enable this. Let’s assume you have PLC_A_TAG and PLC_B_TAG representing the data you want in each PLC, and you want to get PLC A data to PLC B.
First possibility is using an Expression tag. These allow writing back data to another OPC tag. To make this work, create another tag such as PLC_AB_TAG whose expression is just the tag that you want to read from (i.e. {[~]PLC_A_TAG}). Below the expression, check the box to Write value back to OPC item? and set this up to reference the other tag path (i.e. [~]PLC_B_TAG). Now each time the value updates in PLC A, the expression tag will reflect this and get written to PLC B.
Second possibility would be to setup a Value Change tag event on the tag you want to read from. In the example above, you would edit PLC_A_TAG and add a script to write PLC_B_TAG with the currentValue of PLC_A_TAG. So in the Value Changed event script you would add to the end something like:
system.tag.write("[~]PLC_B_TAG",currentValue)
Each time the value changes in tag PLC_A_TAG, PLC_B_TAG will get written with the updated value.