This is an old post, preserved for reference.
The products and services mentioned within are no longer available.
Pretty much since the launch of the Minstrel Issue 2 ZX80 clone, people have been asking for this to be ZX81 compatible. It's taken quite a while to get there and beyond, but the Minstrel Issue 3 has now been released.
First a quick recap.
Minstrel Issue 1
Minstrel Issue 2The Issue 2 followed on shortly afterwards, and was the first that was widely available. Minstrel 1 boards must surely now be worth a least £1M each.
Minstrel Issue 3In order to move on, the next version had to be ZX81 compatible, but I had built up quite a list of requirements beyond being about to run 3D Monster Maze.
- ZX81 compatible slow mode
- It had to still be usable as a ZX80 (but not necessarily 100% compatible)
- Use the full 32K RAM chip
- Improve the composite video output levels / structure
- Improve character pixel output (make synchronous)
- Remove the less common parts (e.g. 74LS05, 74LS93, 74LS365 etc.)
- Reduce the overall chip count if possible (e.g. 2x74LS05 replaced by 1x74LS244)
- Remove some of the more 'analogue' parts of the circuit
- Still fit within the ZX81 board form factor
- Resolve issues with height of 7805 switching regulators (move part or add second set of pads)
- Reduce reliance on chip quality (i.e problems with old 74LS08 chips not working)
- Remove jumpers, replace with DIP switches
- Add turbo mode (a faster slow mode)
Minstrel Issue 3 V3.1
Minstrel add on board
Lots of things were going on at the time, so I left this for quite a while. I did look at doing a more controlled step by step change, and also the possibility of doing this as an add on board. I designed this board, I think I even ordered the PCBs, but I don't think I ever built any up. It was going to require too many track cuts and wires between the boards, it wasn't really viable.
Minstrel Issue 3 V3.2Grant Searle's ZX81 slow mode design, with a bit of optimisation, removing some things no longer required, such as the old HSync flip flops.
How the ZX80 works' (you can also see the first pixel of the second T1 is thinner due to the LOADSR pulse above). I tried various options to find a suitable way to latch the data at the end of T4 / start of the next T1, but I couldn't get it to work.
Minstrel Issue 3 V3.3With the changes above, and a few more, board version 3.3 arrived.
Tut-Tut is looking good.
RAM (during refresh cycle)
RAM (during refresh cycle)
The final memory map has RAM from 8K to 40K, with 16K-32K mirrored at 48K-64K for display use (active during the refresh cycle).
Minstrel Issue 3 V3.4
Minstrel Issue 3 V3.5OK, here we go. This is hopefully the final revision. This time, I wanted to fully manually route the board. The previous revisions I had expected would probably not be the final versions, so had done the important routing by hand then autorouted the rest and tidies up the most egregious autorouting inefficiencies.
tactile switch keyboard with overlay PCB.
2022 Update: At the time of writing, there are still a few Minstrel 2 (ZX80) and Minstrel 3 (ZX81) kits available from The Future Was 8 bit but those are the Final Edition kits, there are unlikely to be any more.