IBM 3274-61C

A very odd acquisition, I grabbed two of these for their floppy drives initially, but decided to keep them intact for now since they are probably very rare.

What this unit does is provide connections for IBM 3270 data terminals and related components like printers via SNA. The controller provides intelligent multiplexing and data management functions in order to allow a group of terminals and equipment to communicate to a central mainframe such as an IBM System/370. There were other versions of controllers made for local connections, but the 61C is made to work via a remote link such as a serial line, X.25 over telephone, etc.

A common application for something like this would be to allow a government office to provide mainframe access to desk agents providing customer service, or for an accounting firm to allow centralized database access to accountants from their office desks, etc. Essentially any case wherein offices would require multiple terminals to communicate with a central data system is an application for something like this.

Most interestingly, this model (and some others) uses an 8” floppy disk on which to store its “firmware”. The unit starts by reading the disk at boot time in order to establish its operating parameters. The convenience of the disk is that alterations to the machine’s functionality can be provided simply by having a systems engineer modify the disk on a computer and reinsert it.

I have no other compatible hardware to try with these, so they sit for now, although I do plan to borrow one of the 8” disk drives for archiving old disks after I build the interface so that I can use it with a desktop PC such as an IBM 5150.

I removed and restored one of the drives already; replacing the worn out rubber roller in the pinch bail lever that clamps the disk, cleaning and lubricating the mechanism, and cleaning the driver up overall. It operates smoothly!

Notice in the photos you can see some late SLT ICs in here! IBM really made headway with the SLT idea back when computers were largely built of hand-constructed printed circuit boards with ordinary through-hole electronic components. SLT came to replace IBM’s previous system SMS Standard Modular System. (Check out the Burroughs Plug In; another innovative density-conscious idea from those days.)

Check out this neat site; someone else has one of these! ilikepie’s IBM 3274-61C.

Some photos of one of my units.

The front

No Lid!

Disk Drive

More Disk Drive

Cards with SLT Chips