The controller I own, a 3174-51R, is the basic model with no options. That means that its only link to the mainframe is a serial connection (V.35 or similar). It has only 1MB of storage onboard and a single 1.2MB 5.25" floppy drive. Up to 9 terminals or printers can be attached to the coax connectors of the box.
CHOICE OF COMMUNICATIONS PROTOCOLS
This unit is designed for remote operation, that is as a box connected by a communications line from the mainframe. It can use one of two protocols for that link, BSC or SNA. The richness of capability of SNA comes with staggering complexity compared to BSC which provides for a pretty simple point to point (or multipoint) link.
BSC is adequately documented and examples of the code that drives the link are easily available from early, non licensed software such as MVS 3.8 or DOS/VS. I could dig into VTAM code from those same sources and build up a suitable specification for supporting the SNA link but I believe it to be at least an order of magnitude more work than BSC.
PICKING BSC FOR IMPLEMENTATION
Therefore, if I require a working BSC link to allow the 3174 to work with my terminals, I will have to implement this protocol and drive it over a suitable serial link. Perhaps I can link the Hercules mainframe emulator to the 3174, but that will still require some coding as Hercules carries 2703 (the communications controller) traffic encapsulated in TCP/IP. I would have to unwrap the BSC data and control a serial connection wired as a null modem to my IBM 3174.
Can you use a null modem for synchronous communications? I worked with this stuff a lot, but the answer to this question is beyond me.
ReplyDeleteI believe that this will work just fine, although i have to be sure that I sort out the control signals to build a 'null modem' link where each side believes it is talking to a modem.
DeleteAs I said, the control signals might require a bit more logic than simple wire jumpering, in which case I will whip up a bit of hardware to handle it all.