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The QL has a few interesting design flaws, quirks, and factory faults. A rather obscure one is a problem with the write protection circuit. While on the Spectrum, Microdrives signal a writable cartridge to the Interface 1 by asserting 9V on the WR.PR line of the Microdrive bus (and don’t assert anything for a write protected cartridge), write protection on the QL is handled completely within each Microdrive unit. Here’s an excerpt from the QL schematics showing the write protection part:

QL Write Protection


First let’s look at what happens when the QL writes to a cartridge:

Write Protection

Now let’s see how this can be prevented. If the cartridge in the active drive has its write protection tab broken off, switch S1 goes into the WR.PROT position, causing two things to happen:

What Could Possibly Go Wrong…

This works really well, as long as all components are present as shown in the schematic. Unfortunately, that’s not the case in all QLs. The trouble spot here is D22. This diode is located on the drive PCB. The designation D22 applies to drive 1 on an Issue 5 QL, for drive two it’s D23, and on Issue 6 QLs it’s D31 and D30, respectively (yeah, that’s right, backwards…). Now to the problems:

The Fix

For use with OqtaDrive, the interference with the RD/WR signal is most relevant. A clear indicator of this problem are errors during format, sometimes format may not work at all, and overall flaky behavior may be observed. A quick way to check whether your QL suffers from this fault is to place cartridges with protection tabs present in all hardware drives and see whether that improves the situation. At the same time, this is also a simple workaround, if you do not want to open up your QL. For a permanent solution, it’s however best to add/correct the missing or incorrectly installed diodes. Alternatively, you can try adding a diode into the R/W line, with anode connected to the Arduino pin. This may solve the problem, but hasn’t been confirmed yet. If you try this, let me know of the results.