Welcome aboard.

These are the random jottings of Andy Collins, a Fifty-something bloke who still thinks a blog or website is a good idea.

Here you will find small nuggets of information about Hovercraft, retro-computing and other stuff that amuses me or appeals to my sense of wonder.

Recent posts

DEC PDT-11/150

22nd May 2018

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 … Continue reading DEC PDT-11/150

Multicomp revisited.

2nd January 2018

Way back in April 2015 I wrote about building an FPGA computer based on Grant Searles Multicomp. To be honest, it’s not had a lot of use. It has sat, largely unloved, in a cupboard awaiting redisovery. Well. in a roundabout way I have rediscovered it. Back in September last year, I exhibited a few … Continue reading Multicomp revisited.

So another Retrochallenge comes to a close. What did I do lately? Well, in the last 24 hours I have tested all of the 74 series chips, with the exception of the 74LS163 (my tester won’t touch them). All were fine, no problems at all. I swapped the 74LS 163s around to see if the … Continue reading RetroChallenge 2017/10 – Last day

Well progress has been slow but thanks to some helpful and knowledgeable people (here), progress has been made. So what do I know now that I didn’t know before? Well, I burned a ROM that just tried to access an imaginary I/O port. When I ran that and watched the _IOREQ_ line, I saw it … Continue reading RetroChallenge 2017/10 – Slow progress

So here we are, two thirds of the way through the month and the Nascom-1 is looking just as it did when I started 🙂 But now, take a look at this picture… This is the Nascom with a ROM in it that fills the screen with NULLs. I’ve just pressed reset. Now look at … Continue reading RetroChallenge 2017/10 – More clues but what do they mean?

Oh now this is getting a bit disappointing. It was a bit too much to think the tri-state buffers had failed and changing them one by one made no difference. Hmm. I think that the screen memory is OK because the noise on the screen shows each of the bits both set and cleared so … Continue reading RetroChallenge 2017/10 – Mid challenge – all the red herrings.

So here’s a thing. I recently met up with an old acquaintance who I hadn’t seen in years and as we’re both retro-computing fans we were talking about old machines. I have just received an email from an old acquaintance of mine who is a dedicated retro-computing fan and has spotted my plight in these … Continue reading RetroChallenge 2017/10 – Purely by coincidence

Having checked the clocks and finding things look well, it’s time to see if the ROMs are OK. The Nascom uses 2708 EPROMs. These are 1k by 8 bits and must have been one of the first EPROMs available. The problem with them is that they need three different voltage rails and most modern EPROM … Continue reading RetroChallenge 2017/10 – More things it’s not

In my last post I was suspecting the clocks. Now I’m not so sure there is a problem with the clocks. Having traced them through everything looks fine. I’m not sure what I was doing wrong but the symptoms haven’t changed but I’m seeing bus activity now and also the _CS_ on the main 2708 … Continue reading RetroChallenge 2017/10 – Still looking for clues

Yesterday I checked the voltages with my DMV and everything looked OK. Today I checked with the ‘scope just to check for ripple and they all look pretty clean to me. Clocks I also saw that there was a running clock but overnight I became increasing worried that it wasn’t a very good clock. Here’s … Continue reading RetroChallenge 2017/10 – and the beat goes on

A short step from where you are.