In which I fight an uphill battle with barely functioning hardware so I don't lose my precious bytes.
As I started looking into the standard for QR codes, and browsed some tutorials (this one is excellent), it dawned on me that generating QR codes is by no means simple. Putting in all the fixed elements (like the three corner squares and smaller internal squares) is easy enough, but the data stream needs to be snaked through this in a very particular (and peculiar) manner, and even coming up with the data stream requires a fair amount of algebra. This wasn't going to be a Sunday afternoon project.
That meant I needed some way to save my code on the Beeb while I was working on it. Remember, it has no internal hard drive; typical storage options are 5¼" floppies and cassette tapes. Tapes are a bit of a pain to work with: your programs are stored much in the same way as songs, so you need to remember or write down where on the tape they are stored, and fast forward/rewind to the right location. It's also pretty easy to accidentally overwrite (part of) an existing program.
So let's look at the disk drive first. I knew that it worked for reading floppies, otherwise this whole project wouldn't have gotten off the ground, but I realized I hadn't actually tested writing. I dug up a disk with some unused data on it, and tried to format it. Formatting will also check whether the disk is still okay, so I ran less risk of losing my work.
This turned out to be a no-go. The *F80 and *F40 commands found in some old manual both resulted in the error “Bad command”. The DFS Manual I found online offered *FORM, but it didn't work for me either. Maybe the disk ROM installed in this particular machine is of a different breed, I don't know. (Yep, that's how you installed “device drivers” lacking an internal disk drive. You would use your fat fingers to open up your £1000 computer and poke an extra chip into it. Those were the days.)
Anyway, lacking a format command, I figured I'd just delete the existing files. How hard can it be? *DELETE oldfile should work, right? Nope, Disk read only. Is it? These old floppies have a small notch on the side, and you can cover it with a sticker to make the disk write-protected. There was no such sticker on this one, but I tried another one just to be sure. Disk read only. Another? Same problem. I tried adding and removing stickers to make sure I hadn't gotten the logic backwards, but that did no good either. According to the computer, every disk I threw at it was unwriteable.
At this point, I wasn't sure where the problem was, but I had a few hypotheses. How would you build a device to detect whether the notch is covered? I'd either have a little spring-loaded lever that moves into the indentation when the disk gets inserted, or I'd have an optical system with a light and a photoresistor to detect whether there is line of sight between the top and the bottom. In the former case, the spring might have become too weak over the years to overcome the friction. In the latter case, there might be dust inside the drive, blocking the light's path.
Carefully, because this drive was critical to the entire project, I unscrewed the cover. There was a little black block of plastic around the notch, but without any mechanical parts. At the back, I could see tiny slits where I presumed the light would go through. I couldn't see any obvious dust, but I blew into them anyway. It made no difference.
The drive identified itself as a Canon MDD-221. Online, I found a couple of maintenance manuals and even circuit diagrams. There were three wires on the flatcable that went to the sensor, labelled G, 5 and WA. I guessed that G would be the ground and 5 would be the +5V line to power the LED. That left WA for detecting the photoresistor's resistance. This was on the PCB on the bottom of the drive, whereas most action takes place on the top one; a green wire also marked WA went from the bottom to the top. Armed with this knowledge, I found the right lines in the circuit diagram, and confirmed that there were indeed an LED and photoresistor inside the black box. The WA line went into pin 9 of an IC labelled J4, and from another pin of that IC came a line marked W. PROTECT. It took a while to realize that the other end of that line just went straight to the flatcable between the disk drive and the computer. In other words, if I could wire the right pin to be either high or low voltage, I could override the write protection mechanism.
I'm not an electrical engineer. I can unscrew things and look at them, maybe even poke at them with a multimeter, but deep understanding of how such circuits work is beyond me. Besides, I didn't want to experiment and risk breaking my last and only disk drive. So at this point I chickened out, put the drive back together, and dug up the old tape drive and an unused cassette tape. I had used the tape drive before, and it worked fine for loading in most of my old games, like Hopper and Asteroids.
But somehow, the tape drive also thwarted me: it was as though the data was never written. Neither LOAD nor *CAT (abbreviated *.) would show it to me, even after waiting long enough for three copies of my tiny test program to have passed by on the tape.
I flipped the tape around to try the other side. It contained my Elite save game (I think I made it to Dangerous level back in my youth), but fast-forwarding beyond that, I managed to successfully save my test program and load it back in again. Hooray!
I also noticed something else: this time, during saving, the RECORD light of the tape drive lit up. I didn't recall seeing that before. Maybe I didn't press the RECORD button hard enough together with PLAY? I flipped the tape again to keep my Elite savegame safe, rewound, and tried saving again. This time, it worked like a charm, so I decided to use this side exclusively from now on.
So, armed with a read-only disk drive and a single side of a tape to store my code in progress, I set out to discover the secrets of the QR code.
As I started looking into the standard for QR codes, and browsed some tutorials (this one is excellent), it dawned on me that generating QR codes is by no means simple. Putting in all the fixed elements (like the three corner squares and smaller internal squares) is easy enough, but the data stream needs to be snaked through this in a very particular (and peculiar) manner, and even coming up with the data stream requires a fair amount of algebra. This wasn't going to be a Sunday afternoon project.
That meant I needed some way to save my code on the Beeb while I was working on it. Remember, it has no internal hard drive; typical storage options are 5¼" floppies and cassette tapes. Tapes are a bit of a pain to work with: your programs are stored much in the same way as songs, so you need to remember or write down where on the tape they are stored, and fast forward/rewind to the right location. It's also pretty easy to accidentally overwrite (part of) an existing program.
So let's look at the disk drive first. I knew that it worked for reading floppies, otherwise this whole project wouldn't have gotten off the ground, but I realized I hadn't actually tested writing. I dug up a disk with some unused data on it, and tried to format it. Formatting will also check whether the disk is still okay, so I ran less risk of losing my work.
This turned out to be a no-go. The *F80 and *F40 commands found in some old manual both resulted in the error “Bad command”. The DFS Manual I found online offered *FORM, but it didn't work for me either. Maybe the disk ROM installed in this particular machine is of a different breed, I don't know. (Yep, that's how you installed “device drivers” lacking an internal disk drive. You would use your fat fingers to open up your £1000 computer and poke an extra chip into it. Those were the days.)
Anyway, lacking a format command, I figured I'd just delete the existing files. How hard can it be? *DELETE oldfile should work, right? Nope, Disk read only. Is it? These old floppies have a small notch on the side, and you can cover it with a sticker to make the disk write-protected. There was no such sticker on this one, but I tried another one just to be sure. Disk read only. Another? Same problem. I tried adding and removing stickers to make sure I hadn't gotten the logic backwards, but that did no good either. According to the computer, every disk I threw at it was unwriteable.
 |
| The write-protect indentation in a 5¼" floppy disk. |
Carefully, because this drive was critical to the entire project, I unscrewed the cover. There was a little black block of plastic around the notch, but without any mechanical parts. At the back, I could see tiny slits where I presumed the light would go through. I couldn't see any obvious dust, but I blew into them anyway. It made no difference.
 |
| The optical sensor that detects write-protected stickers on the disk. |
I'm not an electrical engineer. I can unscrew things and look at them, maybe even poke at them with a multimeter, but deep understanding of how such circuits work is beyond me. Besides, I didn't want to experiment and risk breaking my last and only disk drive. So at this point I chickened out, put the drive back together, and dug up the old tape drive and an unused cassette tape. I had used the tape drive before, and it worked fine for loading in most of my old games, like Hopper and Asteroids.
 |
| Attempt to make the tape drive read back what it just wrote. |
I flipped the tape around to try the other side. It contained my Elite save game (I think I made it to Dangerous level back in my youth), but fast-forwarding beyond that, I managed to successfully save my test program and load it back in again. Hooray!
I also noticed something else: this time, during saving, the RECORD light of the tape drive lit up. I didn't recall seeing that before. Maybe I didn't press the RECORD button hard enough together with PLAY? I flipped the tape again to keep my Elite savegame safe, rewound, and tried saving again. This time, it worked like a charm, so I decided to use this side exclusively from now on.
So, armed with a read-only disk drive and a single side of a tape to store my code in progress, I set out to discover the secrets of the QR code.
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