Tuesday, 26 January 2016

Tektronix 8560 still going strong

And now for something completely different...

I have been tidying my lab post kit production to make room for future experiments. In the process I have been skipping loads of junk. I do have trouble throwing things away that work just as well as when they were new and expensive but are now utterly out of date and superseded by much smaller,  better and cheaper things. I power them up periodically to test them and hope they might fail and give me an excuse to chuck them.

I powered up an old PC and six electrolytic caps exploded in quick succession, sounding like a distant firework display! Presumably a victim of the great capacitor plaque in the early 2000s. Remarkably it still managed to boot into XP but it had signed its own death warrant and went straight to the skip.

Several more PCs with only 256MB of RAM went when I found they wouldn't even load a supported version of Linux. Only a Shuttle with 1GB of RAM that successfully runs LXLE survived the cull.

I also have a Tektronix 8560 multi-user software development unit that is actually a DEC PDP11-73 mini-computer in a 19" rack with a Kennedy Model 9000 open reel tape drive and a Dylon Model 1015B Magnetic Tape Controller.

It was purchased by the company I worked at for £48,000 in 1984 and supported 8 software developers on dumb terminals connected via RS232. They gave it away for nothing in the early 90s.   To put that in perspective £48,000 would buy a decent house in 1984 that would have been worth a lot more in the nineties. Computers must be the worst investment ever! The rack is probably the only bit that retains any value.

It has 1MB of memory on two large cards and the processor is only about as powerful as an Intel 286, but with a much nicer instruction set. We had to write PDP11 assembler in some university workshops and it is the nicest instruction set I have come across.

It was replaced by two MicroVaxes and later PCs of course. The ironic thing is it replaced Motorola  EXORcisers, which were single user desktop computers, so things sort of went in circles.

When the machine was delivered it had a PDP11-23 but the embarrassing thing was it didn't perform as well as the 8 bit EXORcisers because, although it was a 16 bit processor with 1MB of memory, each process had to fit both the instructions and data into a 64K segment, just the same as the 8 bit machine. The 8 bit linker program was smaller so there was more room for the target program. We had to immediately upgrade to the PDP11-73 CPU that has separate instruction and data segments allowing the full 64K to be used by a process for its data. I still have the original processor: -

It runs Unix version 7 re-branded TNIX. It is built like a tank, so of course it still powers up fine, asks if the time and date is still 2006 (the last time I powered it up) and if not suggests I enter a time and date with 1982 as an example. No Y2K bugs here!

It is nearly as loud as a vacuum cleaner and smells of burning for the first few minutes. I think there must be a thermistor or a dropper resistor in the PSU that quickly gets very hot and burns off the dust.

Even the tape drive still works once I had remembered how to set it up, but it only holds about 20MB and would take something like an hour to write that much. The only thing that failed is the foam block that holds the end of the tape in place in the spool. It has started to disintegrate as foam seems to do eventually.

It is in need of a good home but I think it is too modern and not rare enough for a Museum. One day I might skip the insides and use the case to house a big 3D printer or lots of little ones.


  1. I think I've donated newer to http://www.computermuseum.org.uk in the past - might be worth an email?

  2. I'm with you. It feels like a waste to skip something that works. I on the other hand house my printers in and old commercial Coke fridge (bought off Ebay for £1.50 in working condition).