Building a RISC OS machine from a dead PC

Hi everybody .

I’ve been wanting to build a RISC OS machine for sometime but never really had all the parts I wanted to do the build I had in mind.

Recently the motherboard in my 486 PC has died beyond repair. The motherboard was very specific to that case “Non standard” so the case was useless. Now I have a nice old small PC case with a PSU and 3.5 floppy drive and I want to use this for a RISC OS build.

I plan on using a Raspberry Pi for this or something better. I would like to use something better than a SD Card to store the OS. I find the SD cards can be pretty temperamental when using them as hard drives. I know that there is a way of getting a Pi to boot from USB rather than the SD socket.

I can power the Pi from the 5volt line of the PSU and there are 34 pin to USB adapters for the floppy drive. I’m not sure how useful the floppy drive would be on a raspberry Pi but it be nice to have regardless. I think the floppy drives were wired differently on the original Acorn computers. I plan on downloading the OS from the “RISC OS Direct” site.

I would like to be able to remove the Raspberry Pi logo’s in the Pi version of RISC OS and have acorns instead so that it looks more like a RISC OS machine. If I can get something like an ATX arm board then that would open my options up a bit more but I’ve not seen any. The Pi is about the most suitable board I can think of and its cheap and easy to obtain.

I can get Ethernet, USB, VGA, extension cables so that I can mount all the ports from the Pi on the back of the PC case. It will also have PS/2 ports for the keyboard and mouse which will also be mounted on the back of the PC case. I have a PS/2 three button mouse and plenty of nice old keyboards to choose from. Having RISC OS on a CF card sounds like a nice idea. There is something satisfying about CF Cards and old operating systems.

Has anybody else built a RISC OS machine from an old PC case yet? I’ve been looking for some photos for inspiration.

I know that there is a way of getting a Pi to boot from USB rather than the SD socket.

Some people (I’m one of them) like to keep the ROM on SD, and then use a USB HDD for everything else. That’s pretty close to how real Acorn machines did it :)

I would like to be able to remove the Raspberry Pi logo’s in the Pi version of RISC OS and have acorns instead

Only the “Raspberry” theme has the Pi logo for the Task Manager. If you change to a different theme then you get a different icon. The exception is the BootFX module which gives a Pi-themed startup screen, but it can be *Unplugged.

Similarly to Chris, I boot my Raspberry Pi from SD, but I have a SSD, connected via a USB-SATA interface, as HardDisc4. The SD card only ever gets written to when I update the OS, or change the CMOS settings. Neither is done often so the SD has proved reliable.

One other point: be very careful about running such a low power computer from a PC power supply. Almost all PC PSUs require a minimum load, otherwise the output voltage may rise significantly above 5 volts. The RasPi and peripherals may not provide sufficient load, so you could be stressing them with overvoltage.

Almost all PC PSUs require a minimum load, otherwise the output voltage may rise significantly above 5 volts.

Or alternatively, in some cases refuse to start up at all, and deliver close to zero volts, or in one case in my experience, ~125mV. (That particular PSU also had +12V and +/-15V – which also delivered ~125mV until a sufficient load was placed on the 5V line. Didn’t respond to 12 or 15V line loads at all until the 5V line was loaded. Weird…foldback switch mode PSUs for you.)

Edit: rereading this and thinking about it a bit more than I did all those years ago, I realize that it probably wasn’t so much refusing to start, as shutting itself down (due to overvoltage on the output), and then trying again repeatedly – and averaging ~125mV, producing a series of overvoltage spikes fast enough to look steady on my little voltmeter. Truly aggressive for whatever load(s) it had!

One other point: be very careful about running such a low power computer from a PC power supply. Almost all PC PSUs require a minimum load, otherwise the output voltage may rise significantly above 5 volts. The RasPi and peripherals may not provide sufficient load, so you could be stressing them with overvoltage.

I’ve never looked at the detail, but I’d imagine that working with one of the mini ITX cases and more modest PSU that fits would be the best bet

On some power supplies, there are two pins that need to be shorted in order to signal to the unit “power good” or “motherboard active” or something.

Other supplies have an always on 5V, maybe that’s enough to run a Pi?

The Titanium motherboard has supply requirements of 8530mA @ 3.3V; 640mA @ 5V; 0mA @ 12V max and I powered it with a silent Streamcom ST-ZF240 ATX power supply. I loaded the 12V line with a flashing LED array to load the power supply.

Or alternatively, in some cases refuse to start up at all

It has to start up in order to know whether there is a load that it can handle.

Other supplies have an always on 5V, maybe that’s enough to run a Pi?

Those supplies are often only rated about 500 mA, which wouldn’t be enough to run a complete RasPi system with peripherals.

It has to start up in order to know whether there is a load that it can handle.

Indeed. Bad wording on my part. See my edit about what’s most likely really going on when a PSU delivers (close to) zero volts into a too-light load.

On some power supplies, there are two pins that need to be shorted in order to signal to the unit “power good” or “motherboard active” or something.

PS_ON#, which is pulled high by the PSU and should be connected to AUX_0V in order to turn the main PSU on.

PWR_OK (often referred to as “power good”) is a signal from the PSU, which indicates that the supply is “good”. It gives an “early” warning (where early is probably about a mains cycle) that the supply is about to go out of spec. It would probably make the PSU unhappy if a signal was driven into this pin!

Its one of those old style PSU’s with its own power switch. I will do some research to see if I can make some kind of dummy load for the PSU so that I can use it to power the Pi. I forgot to ask about HD, Power L.e.ds I’d like to rewire those to make them work with the Pi.

Its been a long time since I used RISC OS I remember using it at school. I’ve always wanted the Acorn A5000 but prices are a bit to high. I love how the case looks and it sounds like an amazing computer. I’m going to build the next best thing.

I’ve downloaded the ROM for the SD card and I have downloaded the OS I’m going to use an SATA SSD to install the operating system. I’m assuming the the ROM image on the SD card will tell the Pi to boot from USB where the hard drive will be connected… am I right or is the ROM a basic RISC OS that loads up first where you click the drive icons to access the hard drive?

I am a bit clueless how the system works but I have some kinda idea how it might work.

I have a box of Raspberry Pi’s does it matter what Pi I use or does it have to be a Pi 4 for the RISC OS Direct download? Can I use any Pi?

The ROOL version of the OS works on all full-sized Pis. I imagine that Direct is the same but I can’t say that with complete confidence.

Instead of using dummy loads with some old inefficient power supply, I’d just rip it out and use a Raspberry Pi wall wart. One of the main points of the Pi is it uses next to no power, so you can leave it running 24/7. It rather defeats the object to use more energy heating up a dummy load.

Instead of using dummy loads with some old inefficient power supply, I’d just rip it out and use a Raspberry Pi wall wart. One of the main points of the Pi is it uses next to no power, so you can leave it running 24/7. It rather defeats the object to use more energy heating up a dummy load.

1. That was the main idea behind my suggestion of one of the smaller form factor chassis
2. I believe the OP was intending to use storage devices that take somewhat more than a mere wall wart can deliver – which barely matches the power demands of the later Pi models

Well a powered USB hub would be needed for that which comes with its own rather chunkier wall wart.

Well a powered USB hub would be needed for that which comes with its own rather chunkier wall wart.

and the neater solution of powering the USB hub from the PC PSU isn’t deemed sensible?

and the neater solution of powering the USB hub from the PC PSU isn’t deemed sensible?

There is still the potential problem that the total load may still not be enough to satisfy the PC PSU.

I stress that it’s a potential problem (no pun intended) in that no-one has stated what the minimum load requirement of the PC PSU is, nor has anyone stated what minimum load the OP’s kit will impose – and let’s not forget that that figure needs care in measurement, because the load will vary.

I don’t know if it’s a problem for the OP, but a multi-output PSU typically exibits two problems:

• the main rail, normally 5 volts, requires a minimum load so as to remain in specification;

• even if that minimum load is applied, other outputs may actually be lower than the specification. It often requires more load on the main output to keep the other outputs in spec.

Running everything off a single rail, powered by a PSU that is not excessively over-rated, is the best way to get a reliable result.

Some of the old AT power supplies are ok for these kinds of purposes, I know from previous experiments that PC PSU’s can deliver decent current with useful voltages 12volts, 5volts, 3volts for powering things however the PSU’s don’t often last long when used in that way from experience mainly the more recent ATX PSU’s the circuitry is designed for powering motherboards rather than used to power single devices. I’ve had success with much older AT power supplies in the past. I haven;t studied the inner working of one in great detail yet. I’ll also be powering a hard drive and a floppy drive as well as the Pi itself and a USB hub inside. Although I’ll be adding some ports to the back of the PC case there will be extra USB ports inside for the internals, the Pi only has 2 to 4 USB ports depending on what Pi I use. I can not simply use a Pi wall power supply because it wont have the power needed to power hard drive and floppy. Also it would look pretty silly with a tiny wall adapter as the power supply. I could crack it open and solder a 3 prong mains cable to the Pi adapter with it all mounted inside the Pi but even then I’d still need power to power the other stuff so thats not an option. I’ll take my chances the the computers PSU I can modify or rebuild the PSU entirely if I have to in the mean time I’ll see how it works.

Here is what will be used
PS/2 to USB adapter for PS2 mouse and keyboard input
USB to SATA adapter for the hard drive
USB to Floppy adapter
USB extension cable for the USB ports on the back of the PC case.
USB hub for the internals that will also be mounted inside .

Running everything off a single rail, powered by a PSU that is not excessively over-rated, is the best way to get a reliable result.

Most of the list above will work off just 5 volts. However I seem to recall that the floppy would need 12 volts as well. A CD drive certainly would. Most small form-factor discs are 5 volt only.

There is another issue with the USB floppy drive – I don’t think RISC OS, can use one, as it expects unusual sector sizes, and the USB drives are fixed.

I can not simply use a Pi wall power supply because it wont have the power needed to power hard drive and floppy. Also it would look pretty silly with a tiny wall adapter as the power supply. I could crack it open and solder a 3 prong mains cable to the Pi adapter with it all mounted inside the Pi but even then I’d still need power to power the other stuff so thats not an option.

A powered USB hub can power the Pi, hard drive and floppy. If you can’t get one with a separate power supply, simply mount an extension lead inside the case where he PSU would go and plug the wall wart in to that.

There is another issue with the USB floppy drive – I don’t think RISC OS, can use one, as it expects unusual sector sizes, and the USB drives are fixed.

RISC OS uses unusual sector numbering (from zero) compared with DOS (from one). Most USB drives can’t read the 0th sector, which makes them useless for RISC OS format floppies.

I’m not sure if you’d get anywhere trying to use a USB floppy drive as a 1.44MB USB stick (not readable on other machines), or even as far as reading DOS floppies, but I suspect it’s not going to work out of the box. A better use for a floppy drive would be one of the USB ‘retrocomputing’ interfaces that talk to floppy drives in a much more flexible way (Kryoflux is one, but there are quite a number of others). I’m not aware of any software for those existing for RISC OS, so it would need to be written.

I’ve worked out the solution for power supply. The AT power supply works fine with some simple modifications. The floppy I will come back too a bit later on in the build, the floppy drive will remain on the PC case either way as its part of the look of the RISC OS machine that I want to create.

I’m still confused about booting… I’m aware I can boot RISC OS from an SD card but its not reliable over a period of time. SD cards easily get corrupted and in my opinion aren’t the best things to install an operation system on to.

I’ve read about flashing a RISC OS ROM to an SD card.

What is the ROM? Will I need firmware for this?

When I was at school I used RISC OS 2 which was less than a few MB in size and was on a ROM chip. Why is the RISC OS for the PI so large in file size even for just the basic OS without the extras?

I’m still wondering how I’m going to be able to boot from the hard drive or install RISC OS to a hard drive.

What exactly do I need for the SD to get things running?

What exactly do I need for the SD to get things running?

Here’s the usual way to do it. First download “RISC OS Pi” from here. Then flash this to your (minumum 2GB) SD card using Method 2 here or similar. This will put on the SD card:

• Rasperry Pi firmware (nothing to do with RISC OS)
• ROM for RISC OS 5.28
• !Boot application for RISC OS 5.28
• Various applications for RISC OS 5.28

I know you want to move this onto a USB drive, but it’s recommended to leave the first two (Pi firmware + RISCOS ROM) at least on the SD card. Now put the card in the Pi and turn on and check that everything’s working (e.g. desktop appears on your monitor, mouse, keyboard work). The SD card will appear in the bottom left hand corner with an SD card icon.

Once you’ve got to here you can now move !Boot and other applications to your USB drive by followng these instructions. The !HForm application is found on the SD card inside Utilities.Caution.

When I was at school I used RISC OS 2 which was less than a few MB in size and was on a ROM chip. Why is the RISC OS for the PI so large in file size even for just the basic OS without the extras?

The RISCOS Pi SD card has Firmware + ROM + !Boot + Applications. Take a look at the download page again and scroll down: The ROM itself is a lot smaller.

I’ve described using RISCOS Pi because that’s what I use, and I hope that gives you an idea how things fit together, but I imagine if you use the RISCOS Direct SD card instead it wll all work mostly the same.

Recently the motherboard in my 486 PC has died beyond repair. The motherboard was very specific to that case “Non standard” so the case was useless. Now I have a nice old small PC case with a PSU and 3.5 floppy drive and I want to use this for a RISC OS build.

It sounds like a fun project :-)

Thanks for the info. I have followed your instructions and downloaded the following and I have printed out the instructions. Unfortunately after some hours of faffing about with SD Cards I haven’t got anywhere. I have over 15 SD cards in a draw and the computer would only read one of them which is a 2GB. I flashed it with Ecther and the RISC OS Image file but returned an error at the end, its the only SD card that seemed to work.

I know all my SD cards cant be bad because they were all in working order. I don’t know how they have gone bad from sitting in a draw it makes no sense. I decided to test them on my phone and some work but are all corrupted for example a 16GB SD card showing as 56MB. I will have to trouble shoot how to format corrupted SD cards. The computer would not see them at all. I tried to read them in Linux in terminal and various disk repair applications with no luck.

I don’t know why Raspberry Pi couldn’t have made them bootable from USB. It would have saved so much time faffing about with SD cards. I will get the in the end.

I’m puzzled. It’s highly unusual for anyone to have trouble with SD cards at this stage, i.e. just starting out putting RISC OS on them. After lots of use, sure, but that’s not your situation.

It’s possible that they are not being corrupted. The RISC OS image produces a partitioned card. There is a small FAT partition, probably 64 MB, which may be what you’re seeing; and the rest is FileCore formatted – this part is not normally given a formal partition table entry, so Linux for example wouldn’t see it.

Having flashed the cards, have you actually tried booting them in a Pi without doing anything else first?

It is definitely possible to boot a Raspberry Pi from a USB drive, but it normally isn’t necessary. Many (most?) people boot from SD and use another storage medium as HardDisc4 – that includes all but one of mine, and that other one is a special case. My main desktop Pi uses an SSD via a USB-SATA interface.