Category Archives: Digital Equipment Corporation

DEC PDT-11/150 – Part 8. Try new media.

So the belts I fitted in the previous post got the drives spinning but the only floppy disk I had wouldn’t read.

The retro computing community is a wonderful thing and a quick post in the vcfed forums got me two offers of help from members keen to try and get the old thing up and working again.

I now have a fresh copy of the much needed boot disk but sadly, no improvement in the symptoms.

More thought needed.

DEC PDT-11/150 – Part 7. The disk drive belts.

I ordered a pair of 590mm belts from http://www.beltingonline.com/ andΒ  they arrived very quickly. The original belts were 588mm so 590mm should be fine. However, they are too small and don’t fit.

How can a belt that is 2mm bigger than the original be too small? The answer is in the detail or more specifically the tolerances. It says on the website the belts between 500mm and 980mm can be out by +-1.5%. So there’s the problem. The bigger belts may actually be smaller and these ones are.

The motors in the drives are mounted on slides and so can be adjusted to compensate for belt variation but even with the motor adjusted as far as it will go, the new belts still wouldn’t fit.

Back to the supplier. Next I ordered two 600mm belts and these too arrived very quickly.

These fit just fine.

Try again.

So how are we doing now? Mixed results I’m afraid.

The in depth software test is still telling me that there is a problem with drive 1. However, things are a little different with drive 0.

With no disk in drive 0 I see the image on the left, “Read error”. I also see it with an unformatted disk in drive 0 and even with the only PDT disk I have. There are differences however.

With either no disk or an unformatted disk, the message appears very quickly. With the system disk, it retries seven or eight times before showing the message.

I am reasoning then that the drive is able to read something but doesn’t like what it’s reading. I am taking this as progress.

 

DEC PDT-11/150 – Part 6. Looking at the disk drives.

I am now at the point where the disk controller is passing the RAM test but is hitting another fatal error. The Manual says there is a problem seeking to track 0 and I should replace the defective drive.

That’s more than a little harsh and frankly isn’t an option.

Take a look inside.

When I unscrew the rear cover on the bottom disk drive I am greeted with this image..

It seems unlikely that someone packed a spare drive belt πŸ˜‰

There is nothing so obvious looking in to the top drive but on removing the drive I get a similar story.

This is the top drive bay with the drive removed.

You can clearly see that both drive belts have fallen off their respective pulleys.

So what has happened is that the belt have lost their condition over the thirty or so years that they’ve been on the drives and just don’t grip any more.

What to do next?

As you can imagine, new belts from DEC are not an option, so I need to improvise a little.

Snipping through one of the belts and measuring gives a length of 588mm and so a diameter of 558/pi which equals 188mm.

Record deck belts are available around that diameter and I have one on order but the suggestions on-line say they aren’t a good option as they tend to be stretchy.

I have also found a flat industrial belt at 590mm length and so I’ve ordered a couple of those.

I’ll let you know how I got on.

 

 

DEC PDT-11/150 – Part 5. The new disk controller RAM arrived.

In the last post I had found a faulty RAM chip on the disk controller board and ordered a replacement from a supplier on ebay.

Well, the part arrived Today and one fitting, the error lights on the disk controller went out… For a while.

I can confidently say that we are past the RAM errors and it is trying to boot but the drive makes a lot of stepping type noises and doesn’t settle down for quite a while. Then the some LEDs on the main board and the controller board light up.

This is what we have this time.

This time, according to the manual, they don’t mean RAM error. This time they mean.

1, 4&5 on the main board.

This manual says that this “Verifies that the second pass of the disk drive diagnostic was successfully completed. If an error is indicated, refer to Table 5-4 for the possible fault indicators on the Disk Controller module test phase 2.”

1 on the disk controller.

The manual says “Verifies the position of the disk drive 0 head after a restore command is issued.” and the course of action is “Replace defective drive 0 or module.”.

Replacement isn’t an option so I’m going to have to go in and see what’s occurring.

 

DEC PDT-11/150 – Part 4. Self tests and the drive controller

I have been running some tests and learning a lot about this little machine and its ways.

It has two layers of self-test. The first is a mandatory Power On Self Test (POST). The second layer is an optional interactive test made available by setting switch 4 on the main DIP switch block and activated by the toggle on the back panel (replaced in part 1)

In my second post I couldn’t get to the optional test no matter how I set switch 4 inside and the toggle switch on the back. Instead I was getting an octal number and being dropped into the ODT.

I naively thought that this was some kind of RAM value. It wasn’t. This was in fact an error code from the POST. The value I was given, 170732 means there is a RAM fault on the main board. I have spent some time re-seating all of the socketed chips and I think that’s what fixed this.

So the reason I couldn’t get to the optional test suite was that the POST was failing.

Past the POST.

Now that the basic POST is passing, I can get beyond that to the attempts to boot I showed in the last post.

At this point it doesn’t make any obvious attempt to access the disk drives. I would expect a little graunching to find track 0 or something similar but there is nothing.

Test mode.

I can now try setting the DIP switch and toggle to enable the test mode. With this done and a reboot I get nothing on the terminal at all. Flick the toggle for normal mode and it’s back to the boot prompt. Set the toggle to test and nothing.

The PDT-11 has a number of status LEDS on both the main board and the disk controller. Setting for test mode again, opening up the lid and separating the boards shows us a number of clues.

The main board switches are showing 01111 which means “1 Disk controller module” or “2 Cable G1”. The cable looks fine so it’s the disk controller module.

A look across to the disk controller and its lights are showing 0011 which meansΒ  that the LSB RAM IC is faulty. If that’s true, switching the two RAM chips over should move the problem to the MSB.

A quick switch later and the error code on the LEDs has changed to 0010 which means the MSB is now faulty.

These chips are 4bit x 256 bits and some new ones are on order. πŸ™‚

DEC PDT-11/150 – Part 3. Healing hands.

I was preparing to share with you all my efforts driving the ODT with PDP11GUI.

I have been using it to assemble Marco-11 assembler code and squirt it across to the PDT’s ODT. The problem is that when I came to fire up the machine to do a bit more and maybe take some screenshots I can’t get to the ODT because the machine is now asking for a boot floppy!

So that’s how you get it to try and boot. You write about it on your website and shame it into trying. I am part delighted and part deeply suspicious. Computers don’t heal by themselves.

I kind of miss the ODT to be honest. At least with that I could make some progress and fire octal code at it.

So what happens now?

Not much. It doesn’t seem to touch the disk drives when I answer its question. It just tells me there is “No boot on volume” and asks me again. It doesn’t even seem to rattle the disk.

The funny thing is, when it only went to the ODT, it seemed to get the drives to seek to track zero and it’s stopped doing that now. Curious.

 

DEC PDT-11/150 – Part 2.

In the last post I described the PDT-11/150 a little bit and the state of play. It was showing signs of life but not talking to me.

It turns out the cable wasn’t right. Switch to a new cable, fire up GtkTerm and we’re on the right lines.

What we have here is the ODT display. It doesn’t look much but it shows us that the machine is alive and conscious.

Main switch settings.

There is a five switch DIL pack on the top board that sets various options for the machine. The manual gives the following explanations:-

IT seems a little odd that the switch labelled “Auto Baud” should be set to off to turn Auto Baud on. Really? Ah well.

Mine are set as follows:-

  1. ON – Auto Baud is disabled. Baud rate is set to 9600.
  2. ON – Line clock is disabled.
  3. OFF – Enable the DRAM refresh.
  4. OFF – Disable the test mode.
  5. OFF – Disable manufacturer mode.

Setting the switches this way I can get to the prompt you see above.

I have tried getting into the test mode but it doesn’t seem to work on my machine. I wondered if the self test at power up is failing and dropping me into the ODT but the error light on the front goes out so I guess not.

I haven’t turned on the line clock as I don’t know yet if I need an interrupt handler for it. Clearly we need DRAM refresh and fixing the Baud rate at 9600 is just A Good Thing (TM).

Booting?

Now here’s a thing. I can’t find any information about how to get this thing to read a floppy. The manual describes setting the switches to get test mode. It says how to insert a floppy and how to handle them safely but nothing on how to boot the machine.

I’ve looked at both the technical manual and user guide (thanks to Bitsavers) but there is nothing.

A little test.

Although I can’t boot from floppy, I can fire in code using the terminal and things are starting to look brighter.

Here is an example from the ODT page on wikipedia.

This fires the letter ‘A’ at the console port as fast as it can and only stops when you press the reset switch.

Tada!

 

 

DEC PDT-11/150

This is an unusual one. The PDT-11/150 is one of a series of “smart terminals” based on the PDP-11/03’s LSI-11 chip set and so is basically a little PDP-11 atop a pair of massive 8″ disk drives.

It came to me from a friend who presides over a massive collection but is still, like the rest of us, unable to keep everything. His collection is mostly home machines and quite game focused and this business machine doesn’t fit and so I got a message.

This is just the kind of thing I love. The left-field, unloved and under appreciated machines like this are just the cut of my jib.

Details

Looking on the net there is not a lot of details on this one but according to the manual has 16 or 30 (sic) k-words of RAM, 2Β  8″ floppy drives and 6 serial ports. The processor is the LSI-11 in 3 chips.

From the front the machine is dominated by the floppy drives. The processor and all it’s additions are under the small “lid” and behind the smart label…

The state of things.

It’s not working… I don’t think there is much wrong with it as when I apply power, the disks make moving noises, the LEDs on the front light and go out but I can’t get any life out of the serial port and onto my PC that’s running a serial terminal.

Things to do.

The run mode switch on the back had been bent and become a bit flaky. I’ve replaced it with a new one (RS part number 7109551) but it didn’t make it run properly. It did give me a feeling of satisfaction though πŸ™‚

Retrochallenge 2016 – End of the line. All change.

Well, another RC comes to an end an if there’s any truth in the saying “It’s not the winning, it’s the taking apart” then all is well. If, however, it is the winning then we have another “success deficit” on our hands.

So far I have the VT-180 in pieces. I looked at a few signals on the video board and compared them to those marked on the schematic but they all looked fine (see the photos).

160116-IMG_20160116_181840 160116-IMG_20160116_181754 160116-IMG_20160116_181725So next up I wondered if the 2114 static RAM chips have failed. I’ve seen them go on my DEC Rainbow and also on a Commodore PET a friend of mine has, so it’s quite common really. It would explain the garbled output.

One thing I like about DEC kit is it’s well made. The downside is that they used a lot of solder and getting chips out of any DEC board I’ve ever worked on is very difficult. So, as the last few hours of RC2016-1 tick away I have3 chips out, new ones on order and crud to clean out of the holes in order to add sockets.

The nice thing about Retrochallenge is there’ll be another one passing by any minute now.

Retrochallenge 2016 – Week 1 done. Some progress.

So here we are, one week in to RC2016-01 and what do we have to show for it?

As you saw in my previous post, I have taken the lid off the VT180 and it sits as a bare metal cage while I try to figure out what’s wrong.

151231-IMG_20151231_174248The initial display (shown on the left) has changed and is less verbose.Β 160106-IMG_20160106_194827 I’m not sure what happened to trigger the change.

The screen display is no longer in sync and rolls very quickly.

In order to help with the repair I have taken the VT100 board out of the card cage and I have it plugged in to the cable that was powering the card edge connector on the card cage. It’s clearly straight through so should be fine.

160106-IMG_20160106_194705That means that I can get to the board with my meter and ‘scope etc. I have also removed the AVO (advanced Video Output) board and the small inter-connect board. This hasn’t changed the symptoms at all.

The first real diagnostic work was to measure the voltages from the PSU. They all seem fine.

I have been able to use a logic analyser (borrowed – I really want one of these) and I’ve been taking a look on the bus.

have attached three screen shots taken of the logic analyser screens. I have 18 channels and so I have the data bus, the lower 8 bits of the address bus and the synch pin. All signals are from clips on the 8080.

I have the triggers set to stable high on sync (start of new instruction) and 0 on the address bus. I put the analyser to wait for trigger and turned on the VT180.
The three shots are from the same run and I’ve just stepped through the time a little to show what happens at start up.

vt180-digiview-3vt180-digiview-2vt180-digiview-1I get this with or without the keyboard. With the keyboard, all of the lights are lit. When I first started on this repair, a few, normal looking lights came on first and then, after a while, they all came on. It’s a funny old world.