[TUHS] magic, was pseudo tty history

[Aron Insinga]

Yes, the problem with the large boards made DEC stick with the small 
Flip-Chip cards for many years.  (Single height, single width Flip-Chips 
are about the same size as IBM SMS cards; they both used the same 
wire-wrap backplane and they both used the card edge to form an integral 
connector.  At least for the PDP-8 they got to use some double height 
single width boards for things like the AC and registers bit slices.)

(FWIW, I am curious about how that Sylvania backplane came about.  A 
Sylvania idea?  IBM?  Gardner-Denver?)

The problems with the PDP-6 and its large boards were one reason DEC 
co-founder Harlan Anderson left. 
(https://videogamehistorian.wordpress.com/2014/12/11/historical-interlude-from-the-mainframe-to-the-minicomputer-part-3-dec-and-data-general/)

The PDP-11 and KL10 projects finally got around the fear of large boards 
at DEC.  (IIUC, the very large boards in the Data General Nova were one 
reason for its low cost; I wonder if that helped too.)  Maybe because 
they had 7400 TTL & Motorola ECL (respectively) ICs on the boards 
instead of discrete transistors.
https://bitsavers.trailing-edge.com/pdf/dec/pdp6/F-67_circuitInstr_May66.pdf
These important bit-slice modules described on pp 32-33 of that manual:

* 6205 AR, MQ, MB, and MI flip-flops.  These were 36-bit registers. It 
is mentioned that it is 3x the area of DEC's then-standard modules (each 
of which had 1 22-pin connector) and to provide enough connections, it 
had 4 22-pin connectors, two on each end, side-by-side.  Gordon Bell 
once described this module as "Bell's Folly.

* 6206 MA, PC, and IR flip-flops.  These were 18-bit (memory address) 
registers.  It is mentioned that it is 2x the area of DEC's 
then-standard modules and to provide enough connections, it had 2 of the 
22-pin connectors, one on each end.    (I don't know if this was also a 
significant problem source, or if that was only the 6205.)

The problems with these modules was that one end could plug into the 
backplane, but a bus cable had to be run across the back to connect to 
all of the modules of the same type.  When a module had to be removed, 
it often resulted in breaking another module or the cable (I forget if 
it was one, the other, or both).

I believe that the effort to construct the large hand-soldered wire-wrap 
backplane of the PDP-6 encouraged the company to look into the wirewrap 
backplane for Flip Chips in the classic PDP-8 (and PDP-7).  This was 
absolutely critical to getting the PDP-8 down to its price point.

Dave Gross [RIP] at DEC (a TX-0 and PDP-1 hacker at MIT before he joined 
DEC) once said that one problem was the PDP-6 design started with 
germanium transistors but switched to silicon transistors.  (I haven't 
looked at the module design transistor types in the above-referenced 
manual to verify this.)

So by the time of the PDP-8 and especially the KL10, I think they had a 
lot more experience with silicon transistors and the transistors 
themselves were better.

So, with virtually the same architecture & instruction set as the PDP-6, 
the PDP-10 (KA10) was a big winner.  There were a lot of them on the 
ARPAnet.  It was not the only time that DEC's first product in a space 
did not do well, but a successor did very, very well.

- Aron



On 8/16/25 21:56, John Levine wrote:
> It appears that Larry McVoy <lm at mcvoy.com> said:
>> On Sat, Aug 16, 2025 at 08:21:30PM -0400, Aron Insinga wrote:
>>> The PDP-6 had a sign on it that said something like "This machine
>>> is old and flaky so don't touch it unless you know what you are doing."
> PDP-6's were flaky even when they were new, due to large circuit cards
> with unreliable connectors.  I gather a standard diagnostic technique
> was to tap all the cards with a rubber mallet to reseat them.  The KA-10
> used much smaller and more reliable Flip Chip cards.
>
>> Wasn't there a PDP-<something> at MIT, I think, that had a switch labeled
>> "magic" and "more magic" that had wires that went nowhere but it only
>> worked when set to "more magic"?  I'm sure I have the details wrong but
>> I have a pretty strong memory of that.  Anyone able to confirm?
> Probably this one:
>
> https://boingboing.net/2022/08/11/a-story-about-a-weird-magic-switch-at-mit.html