The INS2651s are programmable USARTs, much like the hardware in an average MCU (and more generic than an 8250/16550 SIO). These interface via SN75188 RS-232 line drivers and MC1489 receivers.
The signaling is definitely RS-232, but that other DIP40 is a HDLC/SDLC controller:
http://datasheet.elcodis.com/pdf2/75/73/757320/p8273.pdfAs for what the software is doing, you'd have to debug the ROMs and figure it out for yourself. Seems like communication routing, packet switching, passing messages from here to there. Definitely not enough power or memory to do any real processing on the ports, nor an output for data logging purposes or whatever.
As for the 10mF cap, with all the roasty-toasty old NMOS chips on there, that board alone will consume about two amperes at 5V. 'Twas the days before CMOS VLSI, and boards were hungry!
Tim