I’m working on a proposal to migrate a PLC from L55 to L81E and replace the WinCC supervision system with one developed in Ignition. I have a question about the communication between Studio 5000 and Ignition: I understand it must be through an OPC server, but is it necessary to install and license FactoryTalk Linx or RSLinx, or would Ignition’s built-in OPC/driver be enough to communicate between the PLC and the Ignition HMI?
Studio 5000 is the software package used to program the Rockwell PLCs. You don't want Ignition talking to Studio 5000. You want it to talk to the PLC (which is what you asked later in the question).
As Transistor said, the native Ignition Logix driver talks to Control Logix PLCs. Just be sure you study the best practices for communicating to Logix PLCs in regards to AOIs (avoid direct reference), UDTs, Tag Groups, et al.
Also, if this is a one off replacement, you may want to consider an Ignition Edge setup (2 clients) instead of a full blown Ignition install. Depends on what the future expansion plans would be.