Was the PDP-6 an "asynchronous" machine -- i.e., were all the modules sending requests & acknowlegements w/o a master clock? Some of the symptoms you mention might have been caused by faulty or extra such signals. In any case, regardless of all of Jack Dennis's work on asynchronous machines (including some of my own work), asynchronous machines have never made the "main stream". Fulcrum Microsystems almost went under in the past decade trying to bring asynchronous logic to the main steam. FWIW, IBM customers called "preventative maintenance" "CAUSATIVE maintenance". At 06:00 AM 4/16/2011, Bill Gosper wrote:
quad>As a quick aside, Wikipedia says
"The PDP-6 was infamous because of the 6205 board, a large (11 x 9 inches) board which contained 1 bit of AR, MB, and MQ (thus there were 36 of these). It had 88 transistors, a 2-sided PC etch, two 18-pin and two 22-pin connectors (two on each side of the module). Because of all these connectors, swapping this module was a major undertaking, and the mechanical coupling made it highly likely that fixing one fault would cause another."
following with
"There was also a great fear of powering off a PDP-6, since it would generally result in at least one 6205 board failing."
Is this true?
-------------- I think they were larger than a foot square. I might even have one somewhere. As I recall their commonest failure modes were cold solder joints and pulse transformers ("red turds"). And they were hugely expensive.
Often, the failures would elude DEC's Field Service lame diagnostic routines, which they also insisted on running as "preventative maintenance" (An ounce of preventation is worth a pound of curatativeness). Once, Stew Nelson proved to them the machine was broken with a program which buzzed the teletype bell whenever you turned out the room lights. Even after being shown the program, devoid of input of any type, Field Service ran their diagnostics with lights both off and on, shrugged, and went home.
Nelson etc eventually determined that Floating Subtract following Floating Divide got the wrong answer with the room lights off because their hum was needed to boost the Master Clear pulse enough to completely clear out the state left over by the FDVR(ounded)
The only great fear of powering off that I remember was when an idiot Multics operator wandered in and powered off the apparently unattended PDP-6, whereupon the AMF hydraulic arm nearly removed Gerry Sussman's head in the next room. --rwg Field Service also refused to fix a problem that Nelson demonstrated by Scotch-taping a spring-return switch in the up position while resting the console telephone on another switch, and twiddling the speed control.