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PDP-12 #435 at the University of Minnesota Duluth

Never tried matching NOS part to see if they age sitting.
Yeah -- I have found OOS (Old Old Stock) parts online but not yet NOS. It looks like OOS parts on Ebay cost about as much as 1-2 NS parts would, so I'm loathe to spend a bunch of vintage fans just to have the same issue. I may purchase a NS replacement and then see if there's any thing that could be done with the old one. (It stopped making grindey sounds, so... [shrug]). Thanks again. Maybe I should just print a sticker to look like the old one and put that on a new fan. :P
 
System Source replaced a lot (all?) of their fans in their DEC systems. Not sure what the replacement they picked was, but they were a drop-in as I recall.

I have some old fans but obviously nothing is guaranteed about them. Happy to send one that seems mostly functional for the cost of shipping.
 
@vrs42 and @m_thompson (or anyone else with a PDP-12): do either of you have PDP-12-specific RK05 images that you like to use for booting your PDP-12s? If so, what kinds of things are on it, or how is it configured? We were thinking it might be nice to have a couple of standard Swiss Army knife disk images with necessary handlers, helpful utilities, etc. and then we can use other disk images when we want to run various things. But we're not really even sure what should be on a "PDP-12 specific system disk."

Or, if you don't have something like this, how do you prefer to work? Thanks again!
What I seem to have that is PDP-12 specific are all variants of two images from @djg's website:

It's on my maybe-someday todo list to adapt the PiDP-8 images, which are nicely updated, to contain some PDP-12 stuff, games, etc. Maybe also to put together a variation on diagpack2. It seems to me that it's been an issue for a while that all the known pack images have issues and limitations.
 
I also have a long term plan as part of Console Serial Disk to make machine specific diagnostic images. I have started on the 8/E, F, M version but it is not even close to ready to distribute and it would do you no good. the 12 would fail the E specific tests.
 
What's the origin of the OS/8 RK05 images that report as OS/8 V3T in RES/E, such as the diagpack2 image in os8diskserver? Based on what I could find V3T seems to be OS/78 for the DECMate systems, which doesn't include some of the handlers for earlier PDP-8 systems. So are these images some type of franken-OS/8 (mixing OS/78 w/ handlers from an older OS/8) or is there something else I'm missing? Thanks ^^
 
System Source replaced a lot (all?) of their fans in their DEC systems. Not sure what the replacement they picked was, but they were a drop-in as I recall.

I have some old fans but obviously nothing is guaranteed about them. Happy to send one that seems mostly functional for the cost of shipping.
Hi @antiquekid3, if you have an extra vintage fan that can start up and run for 15 minutes without making grindey noises, I'd love to pay you shipping for it.

If we had a large number of failing fans, I would be OK with replacing with modern fans, but since this is the first one we've had go grindey in 7 years, I think there's something nice about keeping things vintage. I'll email you at gmail and we can discuss off-forum. Thanks!
 
What I seem to have that is PDP-12 specific are all variants of two images from @djg's website:
PDP-8 File list for os8
It's on my maybe-someday todo list to adapt the PiDP-8 images, which are nicely updated, to contain some PDP-12 stuff, games, etc. Maybe also to put together a variation on diagpack2. It seems to me that it's been an issue for a while that all the known pack images have issues and limitations.

I also have a long term plan as part of Console Serial Disk to make machine specific diagnostic images. I have started on the 8/E, F, M version but it is not even close to ready to distribute and it would do you no good. the 12 would fail the E specific tests.
Something like this would be really cool. Machine-specific diagpacks would be awesome, and a PDP-12 "starter" system disk would be super helpful. One of my long-term hopes is to have a set of tools, images, documentation, etc., that I can just hand to students interested in playing with the '12, and part of that would include a handful of curated images for stuff like this. Thanks.
 
Something like this would be really cool. Machine-specific diagpacks would be awesome, and a PDP-12 "starter" system disk would be super helpful. One of my long-term hopes is to have a set of tools, images, documentation, etc., that I can just hand to students interested in playing with the '12, and part of that would include a handful of curated images for stuff like this. Thanks.
One problem people have with restoring or even keeping these machines alive is around running diagnostics. The service guy would show up to a dead machine and work his way through finding exactly what was wrong and then solving it. This would usually involve a little work with the front panel switches and then feeding diagnostics in via either the console teletype or the high speed paper tape reader if available.

Because Console Serial Disk has a server connected to the console interface, it is possible to feed paper tape diagnostic images into the machine and replace either the HSR or Teletype reader. One goal is to get all the diagnostic paper tape images onto the CSD system. This would make the basic restoration require that the the only peripheral that needs to be present is the console port. Enough of the machine needs to work for the RIM loader to function. This means the first 4k of memory and the basic instructions have to mostly work. If you can get Instruction test 1 to load it will validate quite a lot of the machine. It tests AND, TAD, and the OPR instructions pretty thoroughly. It does not test ISZ, JMS, DCA, or JMP. I think I counted only two JMP instructions in the whole thing. That is why you also need to run the tests for those instructions.

The PDP-12 needs to run all the 8/i tests and the LINC specific tests. At the ages of these machines it is probably a good idea to run all the diagnostics frequently. One reason it isn't done is that it is kind of a pain to do so, I have rekeyed the ins test 1 for the 8/e and I am going to fix the handful of bugs I found and then convert it to run under OS/8 as a confidence test. The other tests will follow. Should be able to get the testing time for a confidence test down to just a couple of minutes. Memory tests will take quite a bit longer but if the addressing test passes and you can do writes and non destructive reads to a few locations, for the most part core is working. Semiconductor failures in modern devices require quite a different kind of testing.

The idea is to get testing down to just a few minutes or (even seconds) at CSD boot time. Not unlike what a POST is supposed to be.
 
What's the origin of the OS/8 RK05 images that report as OS/8 V3T in RES/E, such as the diagpack2 image in os8diskserver? Based on what I could find V3T seems to be OS/78 for the DECMate systems, which doesn't include some of the handlers for earlier PDP-8 systems. So are these images some type of franken-OS/8 (mixing OS/78 w/ handlers from an older OS/8) or is there something else I'm missing? Thanks ^^
Well, sort-of. "Franken-OS/8" is uncharitable, because Bill does take his PiDP curation seriously, but yes, the images are pieced together from later distributions of OS/8 and OS/78 and various support patches in an attempt to provide the latest and greatest of everything.

I can't speak directly to the origins of diagpack2, and I don't think that's Bill's work. I have used used it enough to know that it has newer stuff and fixes some issues with diag-games-kermit.
 
I can't speak directly to the origins of diagpack2, and I don't think that's Bill's work. I have used used it enough to know that it has newer stuff and fixes some issues with diag-games-kermit.
diagpack2 was a disk pack that somebody stopped by with and I imaged. Copy on my site is dated 2000. Did a quick search but couldn't find any details on it. I don't think we knew who the original user was.
Based on files DEC released diagnostic packs but it may have been modified by customer.

FIELD SERVICE PDP-8 DIAGNOSTIC SYSTEM
developed by the 12 Bit Group of New England District Support.
Version 5
March 1981
Developed by:
Rick Moore
 
Hi all, sorry for the long silence! Zach and I were busy in July and August working on a research project with a deadline.

With the functional memory expansion in place, @ZachyCatGames and I are moving on to the next big item on our to-fix list, the tape drives.

tl;dr: Our machine seems to be able to read tapes, but can't mark them. Any ideas why running mark12 would result in spooling everything onto the takeup spool and nothing else? Are we just doing something wrong?

Longer version:

As I recall it, once upon a time, the top tape drive worked reliably. The bottom tape drive had an issue where a brake did not enable as it was supposed to, so it would sometimes keep spinning due to momentum and unspool the tape. Most of what follows is about our top tape drive, which was always the more reliable of the two.

As I understand it, Warren made dumps of all our tapes, and we have copies of them (does anyone else?). He seemed to have used a version of dumprest that supported LINC tapes... (@djg , @vrs42 , @m_thompson or @DougIngraham -- do you know anything about that? We seem to have sourcecode for it.)

Back then, marking tapes worked; we recorded a video of mark12 in action (narrated by Warren).

When Warren was still here with Dawson there was a major short issue that fried a bunch of FCs, which -- to their knowledge -- they repaired. But, at some point before Dawson graduated, the top tape drive became unreliable. (I don't remember if the unreliability was in any way obviously related to the big short.) We could use the tape drive, but it seemed like it would randomly corrupt tapes, which would become unreadable.

Later, in 2019, Dawson, Julian, and I tried to figure out the issue with the tape drive. We ran the diagnostics, but got stuck on a failing Tape Data Test with some errors with the XOB. It turned out that one of the bits of the XOB was only getting set probabilistically. We created a small program to set the bits of the XOB manually and we scoped the XOB pins while that program ran. Weirdly, we could "fix" the issue by lifting the backplane door slightly (more like putting a little upward stress on it). You can see that in action on an oscilloscope in this video. We sort of ran out of time / steam once the school year started, and then of course the pandemic happened a few months later.

Picking things back up with my student Kyle (after the pandemic), we started the process of testing all the flip chips (thanks to @vrs42 updated FCT) because we discovered we could no longer boot OS/8 or run diagnostics (longtime readers will recall this was an issue of minicom inserting LFs into tapes being sent to the '12). The teletype and diskserver seem to be pretty much rock solid these days (thanks to @ZachyCatGames and you all), and what's more, we now have 32KW (thanks again @vrs42 !)... so we are now looking at the tape drives again.

Here's where things are at. We recently were able to run Tape Control Test 1 & 2 successfully, but we haven't been able to run Tape Data Test or Tape Data Exerciser, because we don't have any marked tapes we are willing to potentially sacrifice, and -- importantly -- we are unable to mark new tapes. When we try to mark tapes, Mark12 seems to start normally, but once we select a format and press "mark", it simply spools all the tape from the main spool to the takeup spool. (Incidentally, this happens on either drive.)

Thinking this might be our mysterious probabilistic issue with the XOB, Zach wrote an updated diagnostic punch-in program that makes sure that the XOB has the contents of the RSW, or it halts. That program is:

Code:
4020    0516    RSW
4021    1041    STA [1]
4022    0001    AXO
4023    0021    XOA
4024    1441    SAE [1]
4025    0000    HLT
4026    6020    JMP 20

That program ran without halting for over an hour with various settings of the XOB bits. So, perhaps our intermittent issue with the XOB was resolved in all the flip chip removal / replacement. (Or maybe it's somehow just in a stable state right now.) But if the XOB issue is gone, what's keeping us from being able to mark tapes?

We got curious and tried to read (with write lock on) some of our older tapes, and while some tapes seemed to be a little flaky, we were able to read directory listings and files from a variety of sources. So, it seemed like read was working reliably on our top drive, and iirc, @ZachyCatGames was able to read with the lower drive as well.

So, today, we tried running dumprest on some of our old tapes to see if we would get identical images to what were backed up by Warren. When we compiled and ran dumprest, it also seemed to just spool everything onto the takeup spool, (except for one tape that seemed to get stuck in a loop trying to read a particular region, which we stopped -- I presume this is the dreaded "shoeshining"?). Later, @ZachyCatGames was able to boot one of our original OS/8 tapes (again with write lock on). So maybe we weren't running dumprest properly?

This brought up two more questions...

1) do people wipe and reuse old tapes? Or do you tend to leave them in pristine original state because they are artifacts? The tapes we have been trying to mark are four Scotch brand dectapes that Warren sourced for us back in the day. But it occurred to me that maybe we can't mark them because they're bad in some way, and we're failing simply because we are marking unmarkable tapes?

2) Where do people source dectapes? I would love to get to the point where we have enough dectapes for students to play with, but I'm not sure if there's a cost-effective way of doing that.

Any and all hot ideas welcome! Thanks!
 
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PS: We have had more interest in the PDP-12 in the last week than in the last 4 years combined... several students have stopped in, gawked at the machine, played ADVENT and laughed, punched in the RIM loader, and more... I'm hopeful that we'll get a crew of people messing around with the machine this year, which would be wonderful. So, thanks again to all of you who have helped so much in getting our PDP-12 fixed, kitted out, and doing cool stuff. I'm deeply grateful.
 
You should make a set of PDP-1 Spacewar! controllers like those at the RICM. I can send you the modified source code that can use the PDP-1 controllers.
I definitely plan to! I think I have the source already, but I'll ping you if I don't.
 
As I understand it, Warren made dumps of all our tapes, and we have copies of them (does anyone else?). He seemed to have used a version of dumprest that supported LINC tapes... (@djg , @vrs42 , @m_thompson or @DougIngraham -- do you know anything about that? We seem to have sourcecode for it.)
Since I have a lot of Warrens stuff I probably have copies of the images as well. I remember we were trying to code a CRC32 to run fast enough to keep up with DECtape but we came to the conclusion that the 8 was just a bit too slow. We spent hours working on that code and I managed to find only a couple of percent. I have not done anything with it since his passing.
When Warren was still here with Dawson there was a major short issue that fried a bunch of FCs, which -- to their knowledge -- they repaired.
I got a call from Warren the night this happened. There was some mention of the need for adult supervision because it shouldn't have happened. I don't remember the exact details, it was something perched on the top that fell into the backplane. I remember something like nearly 100 ic's needing to be replaced. I learned this lesson when working on CDC Cyber 174 and my metal watchband brushed the backplane. I had a minor burn, and some gold/copper plating on the watch band from the wire wrap pin that vaporized. Oddly, in that case the diagnostics that were running at the time didn't even hickup.
This brought up two more questions...

1) do people wipe and reuse old tapes? Or do you tend to leave them in pristine original state because they are artifacts? The tapes we have been trying to mark are four Scotch brand dectapes that Warren sourced for us back in the day. But it occurred to me that maybe we can't mark them because they're bad in some way, and we're failing simply because we are marking unmarkable tapes?
Those four tapes came from my collection. They were probably originally labeled hailstone and were probably written once and read just once. I've got a couple of hundred more similar tapes. The physical tape might be considered an artifact by a museum. For me, once I get the bits off of it I am happier. In a lot of cases it is the data that is important. Far more important that the object itself. Museums don't seem to understand this.

DECtapes are quite reliable. It is said you can punch a 1/4" hole in one and still read it.
2) Where do people source dectapes? I would love to get to the point where we have enough dectapes for students to play with, but I'm not sure if there's a cost-effective way of doing that.
I bought one back in 1975 at the college bookstore for the enormous price of $25. I still have that one. The others came when I bought the Straight 8. I paid something like 14 cents per pound to rescue that stuff from going to salvage.

There are some on ebay right now asking around $20 each. Compared to what I paid in 1975 this is a bargain. I could have eaten cheap fast food for over a month on that in 1975.

Check the outputs of the head amplifiers. Make sure you have good signal for the rest of the controller to work with. I've not had problems with the heads but Mike Thompson has. I discussed what we would do if this came up with Warren and we decided we would try to fix it. Unpot it, unwind and rewind it and then repot. There really is no other option if a head winding opens or shorts.
 
I got a call from Warren the night this happened. There was some mention of the need for adult supervision because it shouldn't have happened. I don't remember the exact details, it was something perched on the top that fell into the backplane. I remember something like nearly 100 ic's needing to be replaced. I learned this lesson when working on CDC Cyber 174 and my metal watchband brushed the backplane. I had a minor burn, and some gold/copper plating on the watch band from the wire wrap pin that vaporized. Oddly, in that case the diagnostics that were running at the time didn't even hickup.
There was a story from the LINC days of someone having to do a demo the next morning and the thing was behaving extremely oddly. So they phoned up the LINC team and described the symptoms. There was much head-scratching and finally the answer came back "The only place that could explain all those symptoms would be the core memory, but we can't work out what mechanism would result in all those behaviours. Sorry, can't reproduce! Give us a ring if you find anything more!"

So the researcher sighed, and stripped down the memory (which was deep in the machine, beneath all the flipchips). He took one plane of the array out, tilted it for inspection, and a single metal washer fell off and clanged to the floor. He reassembled the machine and everything worked perfectly.

DECtapes are quite reliable. It is said you can punch a 1/4" hole in one and still read it.
Tom Stockebrand gave both an impromptu talk in the early 90s and an interview a few years ago where he talked about the design of LINCTape. The mark and timing tracks are duplicated on the two outside tracks of the tape, so that you can compensate for shear/skew. The remaining tracks are interleaved in redundant pairs spaced as far apart as possible (so one is edge and middle, the next is kind of centred to straddle the middle of the tape, and the last is the other side of the middle and the other edge). Since it's ¾inch tape, yeah, you can damage five tracks' worth and still get a good read.

Wes had specified that he wanted a scaled down version of the enormous (and monstrously overpowered) tape drive from the TX-2, such that you could fit the tapes in a lab coat pocket. He also told Tom that the tape drive had to get a good read through "a layer of blood" as the LINC was designed for biomedical research that sometimes got messy. At one demo for the NIH funders, Wes tipped an ashtray over the read/write heads, wiped it roughly clean with his finger, and spooled up a tape: he got only one read error and it vanished the next time he re-read the tape. This was good, as Tom recalled that he'd specified "The user should only experience one read or write error in their entire lifetime of experience with the machine!"

Tom said that the speed of the tape made the air between it and the head have an equivalent viscosity as 30-weight motor oil in a slow-moving surface. The only problems came when the thing reversed direction, so on the TX-2 he asked them to make it impossible to write to the ends: that huge drive was more like DECTape, and could read and write at full speed in either direction, so they just sacrificed a few spaces as "turnaround points" to reverse the torques on the two motors.
 
He seemed to have used a version of dumprest that supported LINC tapes... (@djg , @vrs42 , @m_thompson or @DougIngraham -- do you know anything about that? We seem to have sourcecode for it.)
I don't have a copy.
l;dr: Our machine seems to be able to read tapes, but can't mark them. Any ideas why running mark12 would result in spooling everything onto the takeup spool and nothing else?
Haven't looked at 12 tape logic so not sure it will match. TU56 have adjustment for drive speed and I think the TD8E had an adjustment for the data rate. For the first marking I think it just writes for the correct number of blocks so if tape is fast or data slow can run out of tape. Could also be some problem in counting blocks so it never thinks it wrote the correct number.

I do have source for a copy of mark12. I would see if I can assemble to get listing and then try to check to see what is going on in the code to see why to doesn't think its done. Since no PDP-12 emulator makes it harder.
http://www.pdp8online.com/pdp8cgi/d...reder/dial/unlabled1.linc;to=ascii;blk=379,39

When we compiled and ran dumprest, it also seemed to just spool everything onto the takeup spool,
Is this ran off end of tape? If so since you have listing should be able to see what is going wrong. What block is it looking for.
1) do people wipe and reuse old tapes? Or do you tend to leave them in pristine original state because they are artifacts?
After I have a good copy I will reuse tapes if needed. I can always put the original data back. I have a good number of tapes so anything interesting I tend to leave as was.

2) Where do people source dectapes? I would love to get to the point where we have enough dectapes for students to play with, but I'm not sure if there's a cost-effective way of doing that.
Its like a black hole. After you have a critical mass of them more arrive. Either ebay prices or hope for donations.
 
Hi @DougIngraham -- thanks for your reply!

Since I have a lot of Warrens stuff I probably have copies of the images as well. I remember we were trying to code a CRC32 to run fast enough to keep up with DECtape but we came to the conclusion that the 8 was just a bit too slow. We spent hours working on that code and I managed to find only a couple of percent. I have not done anything with it since his passing.
I will make sure that we have our images uploaded somewhere so they're not only here in Duluth. We also have some official DEC tapes... I assume that Warren would have uploaded these to bitsavers if we have anything unique, but maybe that was on his to-do list. We'll double check.

We have implemented a 12-bit XOR checksum here, which has been useful and can be done quickly in LINC mode using BCO (iirc). Not as good as CRC32, of course, but could be useful for catching some corruption.

I got a call from Warren the night this happened. There was some mention of the need for adult supervision because it shouldn't have happened. I don't remember the exact details, it was something perched on the top that fell into the backplane. I remember something like nearly 100 ic's needing to be replaced.

Thanks for sharing that memory! I was not in the room when it happened, but as I recall, Dawson was probing something on the backplane, and the probe slipped and touched something with 15V power and that was that. We have a lot of tags on our flip chips as a result... but because of that experience I have rigged up some covers for probe wires so that there's never any exposed metal.

I learned this lesson when working on CDC Cyber 174 and my metal watchband brushed the backplane. I had a minor burn, and some gold/copper plating on the watch band from the wire wrap pin that vaporized. Oddly, in that case the diagnostics that were running at the time didn't even hickup.
Whoa!

Those four tapes came from my collection. They were probably originally labeled hailstone and were probably written once and read just once. I've got a couple of hundred more similar tapes. The physical tape might be considered an artifact by a museum. For me, once I get the bits off of it I am happier. In a lot of cases it is the data that is important. Far more important that the object itself. Museums don't seem to understand this.
Oh! Well, in that case, thank you! They do look pristine and barely used. I agree that the data is more important than the object itself, which is why we have been reluctant to overwrite any of our old tapes until we can verify that they were captured correctly.

DECtapes are quite reliable. It is said you can punch a 1/4" hole in one and still read it.
Maybe someday I'll try this as a demo. :LOL:

I bought one back in 1975 at the college bookstore for the enormous price of $25. I still have that one.
Apparently that's about $146 in today's money. I can't decide if I think that price is shocking, or a deal, considering how expensive memory and media was back then. Does your college bookstore tape still work? Just curious.

The others came when I bought the Straight 8. I paid something like 14 cents per pound to rescue that stuff from going to salvage.
🤯 -- that's incredible. I'm glad you were able to save it.
There are some on ebay right now asking around $20 each. Compared to what I paid in 1975 this is a bargain. I could have eaten cheap fast food for over a month on that in 1975.
If When we get the tape drive working, we might be interested in buying around 20 or so tapes for student use from somewhere.

Check the outputs of the head amplifiers. Make sure you have good signal for the rest of the controller to work with.
We will look into this... any advice on how to do that, or what to look for?

I've not had problems with the heads but Mike Thompson has. I discussed what we would do if this came up with Warren and we decided we would try to fix it. Unpot it, unwind and rewind it and then repot. There really is no other option if a head winding opens or shorts.
Yes, I think that we would also try to repair our heads if it turns out that's the issue. Hopefully it isn't. 🤞

Thanks again!
 
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