PDP 11/24 - A Step Backwards

-----Original Message----- From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Chris Zach via cctalk Sent: 28 March 2022 20:57 To: cctalk at classiccmp.org Subject: Re: PDP 11/24 - A Step Backwards I'm on a nice long train trip right now but I recently got my 11/24 running again. One thing that baffled me was it would not do anything on the serial port. No ODT, no nothing. Turns out you really need to make sure the slots are filled properly. The CPU top, then the memory map, then for the next 4 boards one has to be either a properly configured MS11-PL (the 128kw board) or the memory boards specific to that type of 11/24. Or you have to put G7273's in the CD slots.

Next you need proper devices or G7273's in the next two slots and a proper terminator in the left sockets of the last slots and a G7273 in the center slots. Only then will ODT work. Another oddity is that the 5.25 inch box has +5 and +12 I think and the 10.5 has +5 and +15. There are different memory boards that work in one and not the other, or both depending on jumper settings that have to be right. Unibus drives me bonkers sometimes with the number of different voltages requires (+5, +12, +15, +20, -15, etc....) It doesn't help that the +15 and +12 are on the same pins. Plus it's possible someone screwed with some switches, make sure they are set properly (ie: default is a good start). If you're still stuck next week drop me a line and I'll fire up my 11/24 and see if I can replicate your failure. > >> >> The first will tell you that i) the CPU is basically functional, >> executing > micro- >> instructions, etc; ii) that the bus is basically functioning (for > master-slave >> cycles; DMA and interrupts will remain to be checked out); iii) that >> the console port is working. (Yes, on the KDF11-U, the console is on >> an > internal >> bus, and so in theory a machine could have the ODT 'front panel' >> working, _and_ still have a problem with the bus, but depending on >> the exact details of said problem, maybe not.) >> >> So, hook up a console, set the machine to 'halt', and power on. Is >> console ODT working? If so, congrats, you win, go to stage ii) below. > > I had a console attached. There is nothing on the console. When I > first got the machine I did get output on the console. But that was > before the PSU first failed on me, which is quite a few years ago now. > >> >> If not, you have a reduced area in which you have to investigate - >> and > you'll >> need a 'scope of some kind to make any progress. (If you don't have >> one, you're SOL. Get one.). In order i) is the CPU's internal clock >> (and thus, probably the microcode) running? (At this point you will >> need to consult > the >> "PDP-11/24 System Technical Manual", EK-11024-TM.) If so, is it >> trying to > talk >> to the console's registers? (See Section 4.6 of the TM, "Internal >> Address >> Decode".) If so, is the UART working properly? (4.7 of the TM, >> "Serial > Line >> Units".) >> >> If so, console ODT is running, you're now at stage ii): you can see >> if the > CPU >> will run. Deposit a 0777 ('BR .') in a likely location (I usually use >> 0100 or 01000); read it back to make sure the write succeeded. (If >> not, > likely >> either the UNIBUS or the main memory has a problem.) Start the >> machine; the 'Run' light should come on - if you're lucky! >> >> Depending on which bin you wound up in, further assistance s available. > :-) >> >> Noel >