IIRC, this is little more than <a href=”http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/index.jsp?topic=/rzajt/rzajtasync.htm”>Asynchronous communications</a> programming with <a href=”http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/index.jsp?topic=/rzasc/sc092507446.htm”>ICF (Intersystem Communications Function)</a> files.
You use a V.24 comm port (Physical interface *RS232V24), e.g., the old ECS comm port. I think you need to use the IBM cable in order for the system to recognize that something is attached, and connect your RS-232 to the end of that cable — or build your own cable to satisfy the IBM pin-out requirements.
(All of that is ages old.)
You might be better off if you get a RS-232-TCP/IP converter. Then use sockets to do the communications.