Friday, 4 April 2008


It blew up and then it died!

I acquired a PDP-11 over 7 years ago (September 2000). You probably don't want to know what a pdp-11 is, but you should, so here's the low-down:

A pdp-11 is a 37 year old computer, made by a company called DEC that went bust 10 years ago. The exciting thing is that it is so old, big, slow and limited. The processor isn't 1cmx1cm, it's about the size of a Beano annual. There's about 500 chips inside! Only trained personnel ever used these things, because maintenance was such an issue. They cost over £10,000 each when new. They were unreliable.

But, these are the computers that created the internet and the world's most popular programming language called 'C'. These computers gave us email, CAT scans, fractal graphics and early office tools. The bosses of today's computer industry started their careers with these machines. They're epic, but they're unreliable!

Mine was one of the fastest PDP-11s around, a 25 year old model called a Micro PDP-11/73. In today's terms really it's very slow, about 1000 times less powerful than any computer you could buy today. It looked a bit like a tower PC, but it was big, about the size (and weight) of a large old-style radiator. I was going to have a go at trying to use it, but every time I turned it on I could do nothing with it, because the last owner had left a junk operating system on it.

Now that I had a bit of spare time, or rather, because I'm trying to clear away some rubbish, I decided to get the machine going once and for all. I tested out how to use a pdp11 using a pdp-11 emulator on my Mac and it was looking good. And then I checked out all the circuit boards that made up the pdp11, removing them all and then replacing them carefully. The machine is good, it's got 512K of RAM and controls a 30Mb Hard drive.

The clever thing was simply the effort of transferring a new operating system and everything to the pdp-11. I did that by copying it from my Mac to a Zip250 USB drive and then to a PC running Windows98 which I rebooted into DOS so that I could use a program called PUTR which can copy files to ancient 5.25" disks. So, I worked out a way of getting enough of the OS and utilities into 400K (!!!) so I could boot up the pdp11 and then install that OS onto it's Hard Drive.

Easy! But no, it took all day. Just connecting all the cables up from the back of the pdp11 to the front was a nightmare (maybe I got it wrong??!?) and then the VT320 terminal was just generating rubbish for a while. Finally I got it to display something and boot the pdp11 to the floppy disk where my OS was.

AND IT WORKED! For the first time EVER in my life I'd managed to get an ancient minicomputer era computer to run and have control over it!

So, then I copied the OS to it's Hard Disk (and that worked woo!) and spent a few hours copying over the rest of the OS using the same laborious method above. And it was lovely, I turned on the machine for a bit; wrote a bit of a Forth program. I found out it can run about 120000 Forth instructions per second, which means it runs Forth about as fast as a 386 ran QuickBasic.

I turned off the pdp11 and was just about to do a demo and film it and .... as I was trying to get the camera to work. I heard a crackle. And another. Some blue sparks from inside the casing.

YIKE! Oh, good grief!!!!!!!!!! I turned off the computer at the mains - smoke started pouring out the pdp11 while I went to grab a CO2 extinguisher (I have one!). Gee, it smells in the spare room now.

So the pdp11's an RIP-11 now :-( What a disaster!