Tymshare built IO channels for the 940 so as to attach reliable IO devices. Initially IBM sold disks and tape drives that were more reliable that any other manufacturers that we were aware of. Howard Steadman designed our channels. The ‘plug compatible’ sector arose to allow other companies to sell IO devices to users of IBM computers. I suppose that IBM published the channel interface specs for legal reasons. They were quite accurate and complete.
Our purpose was the opposite: attach IBM IO devices to non IBM computers.
When we first approached IBM to buy disk drives they were concerned that their familiar diagnostic routines, written to run on the 360, would be unavailable to IBM’s disk maintenance people. I proposed the following deal: Tymshare would not call a disk broken except when there was a very competent programmer present with tools and knowledge of the software to explain why we thought the disk was misbehaving, and also able and ready to write new test code to track down the problem.
The IBM rep with whom we talked had once been a field engineer and his eyes lit up — he had repaired disks in the field and had needed just such help.
He recommended that IBM lease us disks, and they did (2319’s). A few years later we began to use plug compatible disks and tapes from other companies as well. Our prices to Tymshare customers for disk storage made our cost of disks negligible.
We graduated to PDP-10s later and bought IBM compatible channels for the PDP-10, from a small company (Systems Concepts), in San Francisco, with people from Stanford. Later a related company built Foonly, a PDP-10 clone, that we did not use.