Sunday 12 May 2024

Minstrel Expansion Bus

The next of the new things I want to look at is the Minstrel Expansion Bus.

This plugs into the rear expansion connector on the Minstrel 2, Minstrel 3 or ZX81. It provides four 2x23 pin headers and a pass through rear edge connector.

The expansion bus pinout is identical to that used on the Minstrel 2 and 3 and the ZX81. The ZX80 is the same but without the /ROM_CS pin - you aren’t using this on a ZX80, are you?

You can construct this to be passive and power all the cards from the host machine. This is fine if you aren't using much power.

Alternatively, there is the facility to add it's own 9V in, 5V out power supply.

This follows the same design as the host machine. 9V is regulated down to 5V using a 7805 regulator. You can add a heatsink if you plan to need lots of power on the 5V rail, or fit a switch mode replacement.

You should only fit the 5V jumper you have not fitted a 7805 on the board.

If you omit the 9V jumper, 9V needs to be connected to both the host and the expansion bus.

If you fit the 9V jumper, you can connect 9V to either the host or the expansion bus.

This can be useful with a ZX81 to provide a better power connector, with the 9V feeding both the internal and external 7805 regulators.

You also gain a nice green power LED.

The kit includes all the parts, omit them if you don't need them.

The pass through connector means you can connect normal edge connector devices such as a RAM pack.

You can then add up to four expansion cards in between.

It doesn't have to be a RAM pack, you might be lucky enough to have a ZXpand or ZXPand+.

Here you can also add the expansion cards.

And can still power everything via the 9V DC connector on the Minstrel Expansion Bus.

Might look odd not having the power going into the ZX81, but it's more reliable that way.

You now have all the bits to load 3D Monster Maze on your ZX81.

Or maybe Minoss Knossoss, to make use of that joystick module.

The expansion bus is designed to be the same width as the ZX81 main board.

Which is of course the same size as the Minstrel 3, where it fits in very nicely.

Here I am using the power connector on the Minstrel 3, but either could be used.

You can also use the passive version, with all the power coming from the host.

That shows the option of soldering the expansion bus direct to the edge connector on the Minstrel 3, if you want a more permanent solution.

Or fit an edge connector if you want to be able to remove it.

The Minstrel Expansion Bus is available in assembled, kit and PCB only versions. If ordering assembled, let me know which options you would like.

But what can you plug in?


Well, one of the cards I was using was David Stephenson's ZX-IO, a three port memory mapped IO card - post on that to follow. Thank you to David for sending the PCBs and the 8255.

More info here

Minstrel Joystick

This is a simple Kempston compatible joystick interface, but fully decoded at address 31 (0x1F).

More info on that here:

Also available in assembled, kit and PCB only versions.

That makes up for the lack of joystick on the Micro ZXpand Minstrel.

Speaking of which, that is available here:

Behind the Scenes Bonus

When I was testing that out, I was using an unmodified ZX81, and the picture via UHF wasn't great on my LCD monitor.

I could have cheated an just used screen shots from a Minstrel 3, but I decided to only slightly cheat and use a composite video feed from the ZX81.

Here I am tapping the composite video feed to the modulator and using the transistor buffer and video connector on a Minstrel 3 board to feed the monitor.

It was interesting to see the "jail bars" you get on a real ZX81 that are "fixed" on the Minstrel 3.


The full range of Minstrel and Mini PET kits and accessories are available form my SellMyRetro store.

All the links can be found here:


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