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).
So 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.
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.
The initial display (shown on the left) has changed and is less verbose. 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.
That 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.
I 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.
Let’s start at the very beginning. It’s a very good place to start – Apparently.
According to the documentation, the VT180 is a VT100 serial terminal with a VT18X add on board. This add on is a Z80 single board computer that talks to the VT100 though one of its communications ports. this make for quite a nice combination for the day.
You can use the VT100 as just a terminal to your corporate mainframe or mini-computer and when required, fire up the Z80 into CP/M-80 and compute like there’s no tomorrow.
Mine is computing like Yesterday was a little rough.
At this point I am working without the VT-18X board present. I don’t think I need it yet. My understanding is that the VT100 side should come up cleanly without it. I could of course be wrong but I’m going down this road for a while.
The photo above shows what I’m dealing with. The keyboard has sensible lights on but I don’t know yet if that means anything.
My first job as always is to try and re-seat all of the socketed chips on the board. I’ve don’t that and it made no difference. Oh well.
Next step, are the voltages correct? The VT100 has an Intel 8080A processor on board and that needs care and feeding.
Off with its head.
One thing I’ve always liked about DEC equipment is the way they built it. Their kit always feels like it was ment to br built and stay together but they also kept in mind that you will need to service it.
The photo on the left shows one of four plastic “poppers” that hold the case halves together. There are also four bolts to hold the steel frame in but that’s it. Dead easy.
Check the power supply.
Using the diagrams in MP00633_VT100_Schematic_Feb82.pdf I checked the voltages on all of the pins. Everything is OK with the exception of the 12V line. I can see that there should be 12V on the orange wire but there is hardly anything there at all 0.4V and just ain’t enough.
Is that my problem?
Hmmm. The CRT uses the same 12V supply and you can see from the top photo that the CRT is working. When I tested the voltages I didn’t have the CPU board in and the CRT didn’t come on. When I plug the CPU board in, the 12V comes back.
Doh! Red herring.
On the eve of ‘challenge I have retrieved my VT-180 from the “shelf of good intentions”. I have taken photos of the two main boards and powered the machine up to see what happens.
The first board basically the Z80 board. The second is a VT-100 terminal (if I remember correctly) and they talk to each other over the flat ribbon cable in the second photo.
At the moment I don’t have the processor board in place as I think the VT side should come up without it. However, the current state of play is less than ideal.
Roll on Tomorrow 🙂
I was given this VT-180 machine a few years ago and after a quick look, I put it on the shelf to get round to later.
Retrochallenge 2016/1 is the later it’s been waiting for.
My aim this January is to mend this rather under rated but rather splendid CP/M-80 machine and get it to live, breathe and jump again.
Quick, to the Bat Cave.
I’ve been thinking about this on for a while. I even considered this as a Retrochallenge but I grasped the nettle and just did it.
I now have a SCSI2SD card adapter on my PDP-11/53 running RT-11 🙂
I could show you pictures but there’s nothing to see really.
This is the rather excellent page that told me how and this is where you can get your own adapter from.
This is what it looks like… Yea!
In the previous post I got the PDP-11 booting from a tape emulator into the diagnostic system, XXDP.
Next I wanted to move on a bit.
I was given some boxes of RX50 diskettes by a friend of mine. DEC kit can be very picky about floppies and generally wants them pre-formatted. The PDP-11 is no exception. It is possible to format them on a PC I believe but I don’t have a PC with a 5.25″ floppy drive.
I booted the PDP into XXDP and then used that to initialise a floppy diskette. this doesn’t format it, just writes some structure to it.
I next used the COPY/BOOT and COPY/FILES commands to make the diskette bootable and have all of the files that are on the tape.
I now have a bootable floppy 🙂
I was a complete novice with the PDP-11 and so I asked for some help on the www.vintage-computer.com forums. Thanks to the help of the folks there I got the machine to boot.
This is what I did…
Downloaded the TU58 emulator from Don’s site here. This enables a PC to emulate a TU58 which is a tape drive unit that talks to the PDP using a serial port.
I downloaded a disk image for the TU58 emulator to load and present to the PDP when asked, from here.
I used Win Xp on my rather old laptop and used Hyperterm on COM1 (19200) to act as my terminal to the PDP.
I started a command prompt on Xp and then started tu58em with the options -p5 -s 9600 -r 11xxdp.dsk. This starts tu58em talking on COM5 at 9600 baud with the disk image mounted as read only.
I had COM1 connected to the console port and COM5 to the printer port.
I started the PDP, watched it go thorough its self test and the numbers 1 to 9 and when it started rattling the RX50 I pressed ctrl-C to break the boot. It asked for a command and I used…
and watched as it started up.
One point I didn’t pick up on until I saw it was that it has both DD0 and DD1 are on the same port. I thought they would be on two serial lines (SLU) and they are not
Now I’ve booted up the PDP I’ll sit down with the manuals and boot myself.
I picked up this PDP-11/53 in Feb 2014. You can read a little about it here.
Somewhat unorthodox to say the least but strangely practical. It appears that a previous owner has taken the card cage from a DEC chassis such as a BA-23 and mounted it in a 19″ cabinet. There is space above the card cage for a couple of full-height 5.25″ drives.
At the back.
I haven’t followed the internal wiring through yet and so I haven’t found out why there are two mains sockets. When I run the machine I connect to the socket on the main PSU and use the switch on the back.
The card cage is completely exposed as you can see. Currently there is plenty of space for expansion.
Above the card cage is the space of drives and to the left of that an aluminium plate with 2, 25 way serial connectors. These are connected to the two serial ports on the processor board. One for the console the other is labelled “Printer” but is a general SIO. These are known as SLU0 and SLU1.
My machine has the following cards…
- M7554 – CPU – KDJ11-DA Q J11 CPU 15MHz, 512-Kbyte RAM, 2 SLUs, LTC
- M7551 – CA – Memory – MSV11-QC Q 4-Mbyte 22-bit parity/CSR MOS RAM
- M7555 – Disk controller – RQDX3 Q MFM Winchester and floppy disk controller (RX50/RX33/RD50-54/RD31/RD32/RD33)
- M7516 – Ethernet – DELQA-M Q Ethernet interface (replaces DEQNA)
- M7546 – TK50 – TQK50-AA Q TMSCP controller for TK50 tape unit.
Towards the end of 2013 I started to think getting hold of a PDP11 of some form might be fun. I know there are excellent emulators out there but nothing is the same as actually using the real thing. It’s partly the “getting”. It’s partly the intangibles. Flicking the power switch and hearing the clunk of the switch, maybe relays, the fan. Watching the self test go through for the first time. On an emulator it will pass. On the real machine it might not. All or these things and more make me want to use a real computer rather than an emulator.
That’s not to say I won’t use an emulator. Let’s be pragmatic here. I’m sure as I write this that one will be an important part of getting the PDP-11 up and running. The easiest way of migrating files between different disk and tape formats seems to be to use an emulator such as SIMH.
Where to begin?
I am a long time fan of the Vintage Computer Forum and also the CCtalk mailing list. Every now and then something will come along on one of these two sources. Every six months to a year, someone’s collection will reach “critical mass” and they reach the point where a major clear out is needed. In the last few of these I’ve read about, the people concerned has accumulated large amounts of “Big Iron” and it seems as if they were just at the right place at the right time to get shed loads of stuff given to them.
So it seems with my PDP.
I saw a posting from someone with an industrial unit full of piles of stuff, Sinclair, BBC, Atari, etc. etc. and so on. There were a couple of things on my wish-list and so I got rid of a few pieces in my collection to make space and raise some funds and set off on a bit of a road trip.
I’d already inquired about the possibility of a PDP-11 but it was not my primary mission but when I got there and saw piles and piles of stuff it became clear that I’d probably have a DEC in the boot when I left.