There seems no out-of-the box add-ons from splunkbase which supports PLC & HMI devices. Usually devices natively generate syslogs which eventually can be forwarded to Splunk via syslog set-up on splunk side.
I would recommend to read device documentation about how to enable and generate syslogs for specific functions that you require example firmware updates, device boot logs ,authentication, CLI executions etc.
Then send these logs to splunk: Device (syslog enabled and generating) -> syslog server (optional) -> Splunk ( syslog needs to be set-up here).
You can reach out device vendors they can point to right solution as well!!