Tom Stepleton
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stepleton.bsky.social
Tom Stepleton
@stepleton.bsky.social
1.6K followers 90 following 110 posts
AI research engineer, glider pilot. Apple Lisa power user. @[email protected] on Mastodon. Most of my posts will be about old computers; occasionally I'll mention flying.
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stepleton.bsky.social
@breakintoprogram.co.uk so the problem with my Spectrum+ was the Z80, it turns out. The address bus was stone dead, +0V on all sixteen lines. Subbing in the CPU from my Jupiter Ace got it going; a replacement is now on order. Thanks for the chat on Sunday!
The display from a broken Sinclair ZX Spectrum+ computer: inside a thick green border, eight alternating black and white vertical bands with a few small, coloured squares arranged on the left-hand side.
November 18, 2025 at 1:23 AM
stepleton.bsky.social
Just in time for #PERQtober ! An adapter to replace your missing or broken #PERQ T2 landscape monitor with a VGA display: codeberg.org/stepleton/pe... Thanks to @[email protected] for early advice.
A circuit board with two DIP ICs and some passive components connects a VGA cable and two DIN-5 cables (top) to a DIN-4 cable, a DA-15-terminated ribbon cable, and a 75-ohm coaxial cable (bottom). The bottom cables go to a PERQ T2 workstation; the top cables go to mouse, keyboard, and display.
September 29, 2025 at 10:24 PM
stepleton.bsky.social
The Olivetti Programma P203 (1967) uses a circuit construction technique known as "transistor jail"
(or "cordwood" [1]. Seen at [2].)

[1] en.wikipedia.org/wiki/Printed...
[2] www.thecomputerchurch.org
August 26, 2025 at 3:24 AM
stepleton.bsky.social
Wheeling Airglow (en.wikipedia.org/wiki/Airglow... ) out to the runway at the British Human Powered Flying Club's 2025 Icarus Cup event, Lasham Airfield, Hampshire.
A group of people walk a slender aircraft with long wings down a tarmac taxiway, all silhouetted by the rising sun ahead of them. A shallow mist rises off of the airfield's grass surface.
June 20, 2025 at 8:04 PM
stepleton.bsky.social
Happy Pride #rc2014
An assembled RC2014 Zed Pro modular Z80 hobby computer, which comprises a backplane about the size of a paperback book with playing-card sized component circuit boards plugged into it vertically. This particular Zed Pro is the limited-edition Pride kit, where the component boards are coloured so that they form a rainbow when arranged in the backplane. The computer rests in the hands that assembled it, which here are sporting some fairly long, blue-varnished fingernails. All of the electronic components, including dozens of resistors, headers, and IC sockets, were placed and soldered with the fingers and fingernails shown.
June 4, 2025 at 12:04 AM
stepleton.bsky.social
#Smalltalk -80 on #PERQ --- Mario Wolczko's implementation (www.wolczko.com/st80/) running with maybe some gremlins still to sort out.
A monochrome graphical display from an old computer workstation. In a large window at the centre of the display, a Smalltalk-80 session can be seen, with a browser window partially occluded by a System Transcript and a System Workspace window.
May 20, 2025 at 12:12 AM
stepleton.bsky.social
Here are some images from ICL's looping demo that runs on the #PERQ 2T2 under PNX 5.
A black and white screen image of a graphical desktop with several windows. One window shows a clock, another a bar graph with the bars bouncing around, another a graphical diagram of a heterogeneous computer network with many PERQs on it, and a final slide describing various attributes of PERQ Networking, viz. PERQ Networking * 10 MBit Local Area Network - ECMA 72 Transport protocol - File Transfer - Remote Printing/Plotting * Wide Area Networks - IBM 2780/3870 emulation - ICL-C03 protocl - Asynchronous Terminal Emulation - uucp A black and white screen image of a graphical desktop with several windows. One window shows a clock, another a world map, another a pie chart of world population by region, another the word "PERQ" in a stylised ICL font of the time, and finally a "text slide" describing various attributes of PNX, viz. PNX Operating System * Derived from Unix System III * Microcode implementation * 32 Bit virtual address * Paged virtual store * Single (32 Bit) & Double (64 Bit) precision arithmetic * Window Manager * Fortran 77, Pascal and "C"
April 22, 2025 at 11:58 PM
stepleton.bsky.social
OK, so the #PERQ 2T2's disk system will work if:
* you let it warm up for a bit
* you have the logic analyser attached to U9 on the EIO board
* ...and running
Splayed out on a wooden floor: a covers off, partially-disembowled PERQ 2T2 under-desk workstation computer. Wires snaking out of the PERQ attach to a solderless breadboard which is then attached by a sheaf of probe leads to an old Tektronix logic analyser nearby. It looks a mess but atop it all is a 19" monitor showing video coming out of the PERQ. The machine is running and shows the diagnostic code 255 on a 7-segment display.
April 22, 2025 at 11:58 PM
stepleton.bsky.social
This is pretty funny to, like, maybe three people on Earth.
Top, the Inmos logo: the word Inmos to the right of a black circle with a grid of white squares inside. It's meant to recall a silicon wafer full of integrated circuit dies. Inmos was a legendary British semiconductor manufacturer who invented the transputer architecture, an innovative also-ran. Bottom: the logo for Imo's Pizza (an infamous St. Louis chain) next to an image of a circular pizza cut into squares, as Imo's does.
February 25, 2025 at 11:18 PM
stepleton.bsky.social
I post what from This is .
A fraudulent Jupiter ACE 4000 and Jupiter ACE 4000 manual: it's actually a regular Jupiter ACE in an ACE 4000 case. Both the ACE and the ACE 4000 were inexpensive, book-sized home computers that used the Forth programming language instead of BASIC. Forth uses a postfix (aka "reverse Polish") notation, which is the joke behind the strange word order in the post.
January 28, 2025 at 10:55 PM
stepleton.bsky.social
After learning the true meaning of "we're not doing presents this Christmas", it was off to the makerspace to hastily laser-engrave and laser-cut jigsaw puzzles for everyone.
A parody clone of the cover of a Ravensburger puzzle box, advertising a "Laserburger Puzzle" with "up to 570" pieces and "Crummy Laserburger Qualtiy [sic]". The puzzle claims to have "Arduousclick Technology", referencing the "Softclick Technology" of actual Ravensburger puzzles. The puzzle itself appears to be a halftone image of a bunch of potatoes, and helpfully the original potato image appears at the bottom of the box art alongside the text "it is potates".
December 27, 2024 at 3:35 AM
stepleton.bsky.social
I would have mentioned the 4051 if you hadn't, of course! Running the 6800 at a mighty 800 kHz. They were working on this 4051 at the vintageTEK museum last year, and as nice as the 6800 was, check out these gorgeous 6820 PIAs...
A circuit board with a collection of DIP ICs from the early- to mid-1970s, with characteristic "wavy" hand-drawn circuit traces. Closest to us is a ceramic-package MC6800 with a gold cover, but behind it is a rank of five MC6800 PIAs made by AMI in white ceramic packages with grey "spider leg" traces extending from the chip die to the DIP pins. If only microchips were packaged like this today...
December 22, 2024 at 9:04 PM
stepleton.bsky.social
With U141 replaced, U159 starts pulsing. I fixed the mouse. I have a working PERQ at last! I can run manufaturer demos that are honestly pretty good for the 1980s! The honeymoon lasts a week, and then some chip in the hard drive system fails. That's what I've been fixing ever since. (fin)
A monochrome computer display with overlapping windows: one with the ICL logo, another with a "Mystify"-style bouncing lines demo, another showing a directory tree, and another boasting about the PERQ 2's floppy drive and hard drive storage options.
December 21, 2024 at 3:11 PM
stepleton.bsky.social
A brief check with the logic analyser and we had our smoking gun. You can see data going into one of the 74S225 and then one of the bits missing when it comes out. U141 is the culprit.
The screen of a logic analyser showing numerous signal traces. A circled area on the left shows the hexadecimal digit C going into the 74S225 memory, while a circled region on the right shows the hexadecimal digit 4 coming out of it. The binary pattern for C is 1100, while 4 is 0100. A 1 bit has been lost.
December 21, 2024 at 3:11 PM
stepleton.bsky.social
We've learnt to be suspicious of 74S225 chips: it seems they don't age very well. It's been necessary to replace two of them in the PERQ already, since they made the display look like this. (That's a whole different story.) Good thing a PERQ 2 system uses about 30 of the things...
Close-up of a computer display showing text-filled areas and blank areas with graphical corruption. The nature of the corruption is lots of black vertical bars as well as vertically-aligned dots. For the curious: the PERQ has special hardware for moving data around in memory called a "blitter". It's especially useful for drawing text to the screen, since you can store characters in other areas of memory and then copy them to the display each time you need to show a new letter. But the blitter temporarily stores the data in a 74S225 as it does its copying, and if the 74S225 doesn't store data correctly, you get corruption like this.
December 21, 2024 at 3:11 PM
stepleton.bsky.social
Anyway, why is this happening. Well, what does the Z80 code encounter on its journey between the PERQ CPU and the I/O-handling Z80? The schematic shows that it filters through these 74S225 memory chips, which basically handle the traffic jam of a two-lane highway for data merging down into one lane.
Excerpt from the PERQ EIO board schematic showing four 74S225 16-by-5-bit FIFO ICs that connect the PERQ memory data bus to the Z80 data bus. Those are a 16-bit and an 8-bit bus respectively, so the analogy of a two-lane road becoming a one-lane road is pretty apt. The bytes must take turns, and the 74S225s gives bytes a place to wait as they zipper-merge onto the data bus. There are four 74S225s here, but that's because each one is only five-bits wide: you need two of them to store a byte. (Each one has an unused bit.)
December 21, 2024 at 3:11 PM
stepleton.bsky.social
Maybe it only applies if you're lucky. I think my PERQ's Z80 was experiencing some truly Buster Keaton-level good luck. BTW @ctrlaltrees.bsky.social , this bug is why that keyboard glitch you saw was happening..
In a modified clip from Sherlock Jr (1924), Buster Keaton rides a driverless motorcycle on its handlebars and narrowly avoids getting run over by a train (repeatedly).
December 21, 2024 at 3:11 PM
stepleton.bsky.social
It's especially important because the instruction the Z80 runs to send configuration to U159 is D3, the "OUT" instruction. 53 does something entirely different. Suddenly it's clear why U159 wasn't getting set up properly. The Z80 was running corrupted code!
Just more text describing the difference between the bytes D3 and 53, as far as a Z80 is concerned. D3, which is 11010011 in binary, is written as "OUT (n),A" in Z80 assembly language and means (simplifying) "emit the A register's contents on I/O port n". OUT instructions are what the Z80 uses to send configuration commands to U159. 53 meanwhile is "LD D,E" and means "move contents of the E register into the D register", which is some data-shuffling internal to the Z80 that doesn't involve U159 or any other external device in any way.
December 21, 2024 at 3:11 PM
stepleton.bsky.social
The scene: A and I at the bench. A reads eavesdropped bytes from the logic analyser. I follow along in the code. We start finding differences. What should be C3 becomes 43 in the PERQ. 96 is 16. D3 is 53. Something is turning some high-bit binary 1s into 0s! *dramatic music, fade to commercial*
Double-spaced lines of text showing a hexadecimal representation of the program code that the Z80 should be running. In the space beneath the line, corrupted digits are noted. Extra info for you, the ALT-tag reader: there are also pipe characters that happen to mark "seams". The PERQ uploads program data into the Z80 in 31-byte chunks, and the pipe characters show the boundaries between those chunks.
December 21, 2024 at 3:11 PM
stepleton.bsky.social
But then at the end of the show, there was a breakthrough. To understand, it helps to know that (a) in the PERQ, I/O is handled by a second processor, a Z80 (b) the main processor has to download code to it to use the mouse, and (c) we know what that code should be.
The PERQ's EIO board, which handles all I/O for the PERQ except for the video display. It's a large circuit board with over 200 separate DIP ICs on it. Arrows point to the board's Z80 microprocessor as well as U159, the 8254 programmable interval timer that generates the clock pulses for the mouse.
December 21, 2024 at 3:11 PM
stepleton.bsky.social
I'm so grateful to @binarydinosaurs.co.uk for the chance to exhibit, but regret being so absorbed in the troubleshooting! I didn't socialise much and I feel a bit bad about it. Sorry to fellow exhibitors; I do really like seeing you all there and wish I had made more time to chat!
A closeup of my electronics workbench exhibit at the Centre for Computing History's Retro Computer Festival in November, 2024. The logic analyser is displaying a schematic (it's basically a Windows 2000 machine, after all). The oscilloscope is on but idle. The PERQ has its CPU board extending outward from its installed position, showing many dozens of chips. Directly in front, on the anti-static mat, illuminated by an overhead lamp, the "solder side" of the EIO board, the large circuit board that handles nearly all of the PERQ's I/O. Some thin, barely-visible "flying leads" have been tacked to locations on the board, providing electrical access to signals that will soon be measured on the logic analyser. There was a whole exciting festival going on around me when I snapped this photo, but it was easy sometimes for just this scene to fill my entire field of attention. Extra for ALT tag readers: at this stage we are pursuing a red herring, investigating whether incorrect address decoding logic is preventing the Z80 from sending signals to U159. This was not the problem...
December 21, 2024 at 3:11 PM
stepleton.bsky.social
BTW, this troubleshooting is happening in public at the wonderful Retro Computer Festival at the Centre for Computing History in Cambridge. Only have a broken computer? Exhibit yourself trying to fix it as a kind of live diorama, a bit like a blacksmith pounding on hot iron in a "living museum".
A person in a blue plaid shirt and beanie gazes at my electronics workbench exhibit at the Centre for Computing History's Retro Computer Festival in November, 2024. Arrayed on a table atop a blue anti-static mat: a logic analyser with mouse and keyboard, a multimeter, an oscilloscope, and of course the PERQ itself with most of its covers off. Atop the PERQ, a screen and keyboard plus a whiteboard proclaiming "Tom vs. PERQ" and the goal of fixing the mouse.
December 21, 2024 at 3:11 PM
stepleton.bsky.social
So let's find that pulse-maker. I've probably got the #1 spot in 2024 for Time Spent Looking at PERQ Schematics, so it's not hard to trace the signal back to p8-15 of www.bitsavers.org/pdf/perq/per... and U159 in particular, an Intel 8254 programmable interval timer...
An excerpted page from the PERQ schematics, with highlights pointing out U159, the Intel 8254 programmable interval timer, and the TABLET CLK clock signal that it generates. The skew away from the vertical is present in the scanned schematics document; it's not artistic license, in other words...
December 21, 2024 at 3:11 PM
stepleton.bsky.social
The mouse isn't a normal mouse, it's more like a graphics tablet. Its dedicated "mouse pad" is a circuit board that detects the mouse's location. To work, the mouse needs to receive steady signal pulses from the computer, but it wasn't getting those. Our problem lies elsewhere...
The PERQ's "mouse pad": a fairly empty-looking, square circuit board with a few clusters of DIP ICs at its upper edge. Elsewhere, a grid of circuit traces spreads across the otherwise-bare board. Current flowing through these traces generates magnetic fields that are detected by a coil inside the mouse.
December 21, 2024 at 3:11 PM
stepleton.bsky.social
About a month ago I fixed the mouse on my circa-1983 #PERQ 2T2 computer: a key feature for an early graphical workstation. It was a wild troubleshooting journey --- in the end, there was nothing wrong with the mouse itself. You might not believe what it was...
An electronics workbench with a PERQ 2T2 computer resting atop it: its system unit is not quite as large as the kind of refrigerator that fits beneath a desk. Most of its covers are removed. Highlighted in the image is the PERQ's three-button mouse. Text on the image says, "Issue: mouse not functioning, other peripherals seem to work ok".
December 21, 2024 at 3:11 PM