Re: [TUHS] UNIBUS issues (Was: Spacewar at Bell Labs)

I'm sending this reply to TUHS since the message I'm replying to has some errors, and I'd like for the corrections to be in the record close by. Although DEC did have some custom chips for QBUS interfacing, they didn't always use them (below). And for the UNIBUS, the chips were always, AFAIK, open market (and the earliest ones may have predated the UNIBUS). E.g. the M105 Address Selector, a single-width FLIP CHIP, used in the earliest PDP-11's when devices such as the RK11-C, RP11 and TM11 were made out of a mass of small FLIP CHIPS, used SP380A's for its bus receivers and 8881's for transmitters. On the QBUS, the KDF11-A and KDJ11-A CPU cards used AMD 2908's as bus transceivers, even though DEC had its own custom chips. The KDF11-A also used DS8640's and DS8641's (transmitters and receivers), and also an 8881! (The UNIBUS and QBUS were effectively identical at the analog level, which is why a chip that historical was still in use.) One _possibility_ for what he was talking about was that it took DEC a while to get a race/metastability issue with daisy-chained bus grant lines under control. (The issue is explained in some detail here: https://gunkies.org/wiki/Bus_Arbitration_on_the_Unibus_and_QBUS and linked pages.) This can been seen in the myriad of etch revisions for the M782 and related 'bus grant' FLIP CHIPs: https://gunkies.org/wiki/M782_Interrupt_Control By comparison, the M105 only had 3 through it's whole life! It wasn't until the M7821 etch D revision, which came out in 1977, almost a decade after the first PDP-11's appeared, that they seemed to have absorbed that the only 'solution' to the race/metastability issue involved adding delays. In all fairness, the entire field didn't really appreciate the metastability issue until the LINC guys at WUSTL did a big investigation of it, and then started a big campaign to educate everyone about it - it wasn't DEC being particularly clueless. Well, exactly - but it's useful to remember the differening situation for DEC from 1970 (first PDP-11's) and later. In 1970 DEC was mostly selling to scientists/engineers, who wanted to hook up to some lab equipment they'd built, and OEM's, who often wanted to use a mini to control some value-added gear of their own devising. An open bus was really necessary for those markets. Which is why the 1970 PDP-11/20 manual goes into a lot of detail on how to interface to the PDP-11's UNIBUS. Later, of course, they were in a different business model. Noel