[TUHS] Re: magic, was pseudo tty history

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-fr…) 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: