Resurrecting a PDP-12 in Duluth

Over the last couple days we have been trying to track down what else isn't quite right after the magic smoke incident... The M221 boards had a lot of problems. These are the main registers. On all of them (except bits 0 and 1, that board was fine) E18 was the problem. E18 is a 74H00. These were replaced. We tried to send characters to MTTTY again but only received NULLs. Checked pin R1 of M07. This is the trigger that loads data in. This was good with pulses about 500μs apart. Checked M07U1, which is AC11(1). Something is holding it at low. This is a problem. Testing Flip Chips: M706 in M/N06 is good. M707 in M/N07 is bad. M707 in M/N10 is bad. M706 in M/N09 is good. Warren noticed that one of the ICs on M707 in M/N07 was getting hot. We took the thermal gun to it and it came in aroun 130F. This is a bit too toasty. We ran the computer for a while and ran the thermal gun up and down the Flip Chips installed in the machine. Nothing seemed out of the ordinary. Aver...

On Saturday we started working on the second core. Timing the rising edges of the Field 0 and Field 1 pulses was as far as we got before a probe slipped in the backplane and shorted a pin, letting out some smoke. We powered it off and then back on again to see things weren't working correctly. Inspecting further, we found the probe was connected to -30V. I pulled out cards in the area while Warren tested them on his tester. I pulled out an M216 in slot E9 and saw it had some crispy looking pins. Warren tested it and confirmed that it was dead. We replaced all three 7474 chips and put the card back in. We tried sending Warrens Tune program (for tuning the cores) but we got no sound from the speaker. We tried to run it and it wouldn't run. Strange. Warren determined that the SMA (7500, skip on minus AC) and SPA (7510, skip on plus AC) were not skipping. So he put together a little program to use while we were tracking down the source of this. 0200 / 7604 LAS...

Thursday night we worked on the second tape drive a bit to try and find the source of the logic problem. We swapped Flip Chips around between both drives hoping the problem would move to the top drive. The problem left the second drive but did not appear in the first drive. Odd. It would appear that the Flip Chips were just dirty and not making full contact. We proceeded to clean all the Flip Chips in both drives. This fixed our logic problem. We then put a tape on the second drive to run a format test. We noticed the left reel wasn't positioned correctly so the tape wasn't straight and made rubbed on the side of the track.  We adjusted it and then ran Mark12. It worked fine and said it was a good tape. We then ran the tape data test and it also ran fine (this test will just run forever until it finds an error. So Warren said it would be wise to stop it while it's reading the blocks backwards, since it can't write backwards, that way there is little chance of messin...

We decided to run some tests with the top tape drive to see how well it works. (Warren brought some almost never used tapes that we used for this). We ran the tape formatting program, Mark12. It failed the first time, but the program maybe didn't send correctly, so we sent it again. It worked! The tape was properly formated for LINCtape. See video below. We ran some diagnostics for the TU55 and they ran fine. We will start tracking down the problem with the second tape drive. Warren ran FOCAL in the PDP-8 simulator on simh . It turns out that when he ran the same code in the simulator it also ran an extra time, 101 or 1001. It turns out that the code wasn't exactly right. After it runs for the set number of times, it runs one extra time as it goes back through the code. Adding another line that tells it to stop fixed this. So, FOCAL does work!

 I forgot to mention that Monday night we started running some of the "straight-8" diagnostics. We ran the following: maindec-801-1 maindec-802 maindec-803 maindec-08-D03A maindec-08-D04B maindec-08-D07A maindec-801-3A (closed MTTTY at lease two times, large file) maindec-08-D0BA   All of these diagnostics ran correctly. This is odd because of the problems with FOCAL. We will continue running diagnostics to try and track down that issue. It could be that the Flip Chips aren't clean and properly seated. Some of them have been cleaned, but more need to be.   Tuesday we decided to try and get 8k of core running with one of the cores that Warren brought to test. We put the first one in and ran the 8k memory checkerboard program. It failed a lot of tests. We turned off the computer and then turned it on again to run again and it passed all the tests. Strange. We ran the test a few more times and it failed consistently on a few addresses. Warren...

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Warren came up from South Dakota to help us get the serial ports working and do some more diagnostics. Sunday night he made a cable to connect the serial card to the serial port on our computer tower. We are using Warrens modified copy of MTTTY (Multi Threaded TTY) as our TTY emulator. It is an old Windows program so we are running it in Wine and it has been working pretty well so far.  After On Monday... We toggled in two simple programs. One was a send/receive program and the other was a send. The send would take the data input on the right switches and display the character or number associated with that number in MTTTY on the PC. The code for that is as follows: 0200 / 7604  LAS 0201 / 6046  TLS 0202 / 6041  TSF 0203 / 5202  JMP 0204 / 5200  JMP  The LAS instruction loads the AC with the switch register. TLS Sends the AC to the serial port. TSF will skip the next line if the flag is set. JMP will jump to 202 and then the next JM...

The PDP-12 uses 120 tiny lights in its console -- here's an edited still from one of our demo videos showing the PDP in operation: If the PDP were built today, they would almost undoubtedly use LEDs due to their longer lifecycle and other properties (assuming the machine was otherwise and inexplicably unchanged!). However, during the 1969-1972 era when the PDP-12 was being built and sold LEDs were only just coming into their own for use in devices, most commonly as the well known red seven segment displays in calculators. Many of these were so small (due to manufacturing and cost constraints) that they were paired with magnifying lenses (now I understand the vintage calculators of my youth so much better). In any case, LEDs were not seen to be as having enough brightness, economy, reliability, aesthetically pleasing characteristics, or some other desirable property, so the PDP uses small incandescent bulbs. Of course, over the course of 50 years, incandescent bulbs fail. And...

Peter and I recorded a little programming demo today. I am currently shopping around for some 40 pin idc cables and DB9 RS232 ends so we can start using the serial connections on the computer to load in programs and (eventually) load an OS. Michael put us in contact with Warren, who has been a huge help as well. He sent over a wiring diagram for the RS232 connections to the serial cards that will be very helpful. Still need to get around to replacing the bulbs. Edit: Huge thanks to David Gesswein at http://www.pdp8online.com/ for writing the toggle in programs we have been using. 

I ran a number of the toggle in diagnostics that Michael sent me. They all test different functions of the PDP-8 instructions for the PDP-12. I have done no testing in LINC mode yet. JMP: Passed (tested the JMP instruction) Increment AC: Passed JMS: Passed (tested the JMS instruction) Write Zeros: Passed (this wrote all zeros to the memory) Checkerboard: Passed (this wrote 7777 and 0000 in alternating addresses) Group One Microinstructions: Passed. I originally thought this failed but it was because it was using a BSW instruction, 7002. I figured out later that this is not part of the PDP-12 instruction set. It's actually a PDP-8/e instruction. Group Two Microinstructions: Passed. Heres the code for the JMP test as an example of the instruction set:   200 / 5210   JMP 210 201 / 7402   ERROR HALT 202 / 5206   JMP 206 203 / 7402   ERROR HALT 204 / 5212   JMP 212 205 / 7402   ERROR HALT 206 / 5204   JMP 204 207 / 740...

After the power supply was running I had a friend help me put it back in the case (it's about as heavy as it looks). I left the CRT and the tape drives disconnected as I haven't gotten around to doing anything with them yet. Fingers crossed, I flipped the switch. The fans came to life and the console lit up. No smoke! Michael gave a good rundown of the switches and some basic tests we could run. All of the switches should be forward, towards the computer. These are zeroes or off. The one that doesn't matter is the PDP-8/LINC switch. That just changes the processor it's running on. We left it running in PDP-8 mode. To read from memory: Set the left switches to the memory address you want to read. The Least Significant Bit (LSB) is on the right. Hit the EXAM switch. The contents of that address are displayed in the Memory Buffer lights. Pressing STEP EXAM will step to the next address and display the contents of that address in the MB. This worked fine for as f...

Our PDP-12 came with these options and modules prewired from DEC: EM12/EP12: Basic 4k Memory and PDP-12 Processor MC12: Memory Extension Control TC12: LINCtape Controller KW12A: Real-Time Clock VC12: Graphics/Display Controller KE12: Extended Arithmetic Element ASR33: The ASR33 Teletype. (We don't actually have the physical Teletype) DR12: Relay Buffer DP12-A/B: Teletype Dataphone/Teletype Dataphone EIA voltages Our system was originally wired for 8k of memory. We are still missing a 4k stack. The only other part we were missing was a M216 Flip Chip. We found a lot of Flip Chips on eBay that had a M216 in it.  Michael got me in contact with Vince who, a number of years back, made some TTY cards for PDP-8/I and PDP 8/L machines. One is a baud rate generator (M452X) and the other two are serial interfaces (W076X). These will allow us to use an external terminal with the PDP-12 instead of tracking down an ASR33 Teletype.

Testing continued on the power supply. I borrowed a few 4Ω 255W resistors from the Power Systems lab so I could try and load test the supply.      With four of these resistors wired in parallel we get 1Ω. Because of Ohms law this means we will get a 5A current from the 5V output. This would give about 25W of power, (P=V*I). 900W is more than enough to satisfy this. The result of having the load was a drop in the output voltage. The 5V line dropped from its normal 5.07V to around 4.96V. I hooked the oscilloscope back up to measure the +8V and +5V lines as they power up. Compared to before this was a much better graph. There is a much lower spike in the +5V output, it jumps to 5.8V and then quickly settles down to 4.8V to 4.9V (according to my multimeter). The power supply appears to be working just fine now. No more random tripping and no more burning resistors. Next on the agenda: Powering up the machine and running a few basic tests. Posting a lis...