I am trying to connect an allen-bradley contrologix plc via a prosoft modbus module. The DCS is redundant ac864a. Do I need 2 ci867 or1? I am assuming this has been done before. Any help would be appreciated.
I would say it´s a matter of redundancy and resilience.
If your DCS is 800xA with dual PM864 there are redundancy in case of hw-failure and also the big benefit of online upgrade. I.e without having to shut down the application (800xA system).
However, if you are heavily depending on the ControlLogix PLC for application performance, you might consider redundancy also for your CI-modules (867). Having said that, are the ControlLogix PLC and/or Prosoft module redundant? If you opt for redundant CI (and wiring), then you might be able to online upgrade with communication up and running (if this is supported by your configuration/application).
You have to analyse risk and benefits relative cost to and complexity of your setup.