This is a bit strange isn’t it?
I haven’t used any DEC kit for nearly 20 years and here I am sitting a few feet away from my latest addition, a mid ’80’s DEC PDP-11/53 (Pictures to follow).
There was a bit of confusion as to what model it really was. It was an 11/23 for a while. Then an 11/73, 11/83 but now I’ve been through the boards one by one I’m pretty sure it’s an 11/53.
It’s not in the original rack or more properly the original rack has been boxed in to another rack. I was given another rack by the previous owner though on my way out which seems very generous.
I’m now trying to learn about PDPs and how to find boot images for the 5.25″ floppy disks, no hard drive yet.
I’ll give it a proper entry in the main part of the site when I can.
The progress has been a bit slow on the 380Z front at the moment.
As you may have read, It displays nonsense on the screen and doesn’t do much else. I wondered if this was just a fault with the display controller card or more. At startup the machine is supposed to display a boot prompt and wait for a key press. however, I don’t have a keyboard. The keyboard interface is a simple parallel port with 8 bits for the decoded key code and a strobe to say that the data is ready. To try and make some progress I made a simple one key keyboard built into a 15 way connector.
All it does is permanently tie the lines needed to send a ‘b’ and then it has a push button on the strobe line. I push the button and the 380Z thinks I’ve pressed a ‘b’.
This didn’t change anything and so I think the problem is more with the logic board.
It has been suggested that the ROMS (EPROMS) might have lost their memory. I think their original life expectancy was only about 10 years and we’re way past that now. I would like to read them and compare them with the image files that I have. The chips used are TMS-2716, 2kx x8-bit EPROM from Texas Instruments. Their power requirements are the same as the 2708s and different to non-TI 2716s. I have been able to borrow an EPROM programmer and I have the needed adapter on order from the USA but unfortunately the seller has had some problems with his supplier and it’s taking longer than I had hoped.
When it gets here I’ll be on the case.
The 380Z seems to be my system of the month and is causing me head scratching. It’s becoming one of those puzzles where you have to slide pieces around but you can’t because something is always blocking them.
I have tried moving the DRAMs on the CPU card around by one socket. I’m not sure that this made any tangiable difference. The screen rubbish doesn’t have as many ‘C’s and ‘0’s in it but left long enough it changes anyway. Did it allways do that? Not sure.
I would really like to read the EPROMS but they’re TMS2716s and they are different to the run of the mill 2716s and so the EPROM programmer I was kindly lent by a friend won’t ready them without an adapter, which we don’t have.
I have borrowed a logic analyser in an attempt to see what’s going on on the bus. As I mentioned before, in a previous post, the bus terminator card has a row of holes and each signal is brought out to one of them. I have soldered some header pins so I can pop the wires of the logic probe on to key sognals and see what’s going on.
The short answer is nothing. nada, nilque. The long answer is that the logic analyser isn’t playing ball. If I use the ‘scope I can see a clock, memory requests, M1 bipping up and down, activity on the data bus, _RD_ is low and the first eight or so lines on the address bus are wibbling as expected. It’s frustrating the analyser won’t work as that would be a great help.
The 380Z has got as far as displaying rubbish on the screen but no further. It’s quite a predicatable kind of rubbish from a sub-set of characters. Curious.
I have tried running with just the processor, VDU and the passive card that I think is for bus termination. The symptoms are the same. I started removing the chips on the CPU board one by one, using IPA to clean the pins and putting them back. After I had done about a third I just went for lifting the chips, squirting and reseating them.
I did get some different results but I had inadvertently missed a pin on one of the EPROMS. When I put it back where it belonged the machine went back to it’s old behaviour.
It was suggested to me that the EPROMS could have lost their memories by now as their expected life was around ten years so I will try and read them and verift them against some ROM images I have found through the 380Z group on Yahoo.
The bus terminator card has a row of holes for (I think) test pins, on every signal. I’m going to solder right angled header pins into these holes to get easy access to them and start probing about.
I also plan to shift the DRAM chips around by 1 socket and see if the fault changes.
I’ve been having a look at the RML308Z recently. One of the capacitors in the power supply went pop when I first powered it on. This is not unusual for machines of this age and it’s a good idea to inspect the caps before powering on a new machine. I decided to chance it and POP.
It turns out to be a tantalum on the 12v and a visual inspection wouldn’t have shown anything.
It’s very easy to open the case and look at the boards, even to take them out but to get to the PSU is a right pain. There isn’t a PSU in the usual sense. There is a portion of the machine that takes mains electricity and passes it through a collection of seperate bits, all bolted to different parts of the case.
Anything beyond a board swap is a bit tricky.
Anyway. I haven’t got a suitable replacement cap so I snipped of the remains of the old one and re-assembled. A look at the circuit diagram show this is for taking out the ripple and so isn’t vital – I don’t think.
I’ve now powered it on again and it’s nearly working…
I’ve just had the greatest piece of luck in the form of two great reto-machines. The first is a DEC VT180, a spledid CP/M-80 machine, which was based on the famous VT100 terminal. The second is an RML 380Z, another CP/M-80 machine that has been on my wish list for so many years it’s not true.
Both machines have their issues and neither is working, yet, but I’m absolutely delighted.
I’ll add them to the Retro Tech section when I get some photos and some more news.