What is it?

This is a self assembly Z80 based computer kit. It is compatible with the Jupiter Ace computer released in 1982. It has been designed using modern 74HC series logic to be fully compatible with the Jupiter Ace.

This version optionally includes a built in 40 key keyboard using tactile switches. An overlay PCB sits over the switches to indicate all the additional functionality each key provides.

The Z80 has now been discontinued, but I have secured stock from one of the last batches of Z80 CPUs. All kits will be supplied with Z80s from that batch, date code 2427. While stocks last as they say.

This is a tried and trusted design, I have been selling these for 5 years now. The kit comes with all new modern parts which should mean every kit built works first time (now using fully tested New Old Stock dual port RAM chips as these have also been discontinued).

Why did you make it?

I wanted a Jupiter Ace but I could not afford a real one so I made my own.

What makes it special?

The upgrades from the original design include a full 49K of RAM on board, with 2K video RAM. The ROM has been expanded to use 5K of unused space. Up to four ROM images can be selected from the included ROM chip.

Dual port video RAM is used to simplify the design, in place of 4 RAM chips and switching logic.

The original design involved a lot of counter and decoding logic to provide the video, this has been replaced with a microcontroller. That allowing PAL and NTSC modes and a properly formatted composite video signal with a backporch

White text on a black background was originally used on the Jupiter Ace, or change the junmper and you can have black text on white, as used by the ZX80 and ZX81.

The Z80 can be clocked independently of the video circuitry. Four options are provided:

• 3.25MHz (as used on the original Jupiter Ace)
• 6.5MHz (double speed)
• RC2014 clock 1 (main clock on 40 way connector)
• RC2014 clock 2 (alternate clock on extended connector)

Useful if you want to use a serial module which provides a 7.37MHz clock, or if the 3.25MHz or 6.5MHz clock is selected, they can be fed to the RC2014 bus for use by other cards. The supplied Z80 is rated for 8MHz operation.

Unlike similar computers of the era, the ZX80, ZX81, ZX Spectrum etc. which used BASIC as a programming language, the Jupiter Ace (and therefore the Minstrel 4th) used the Forth language (see what I did there?)

Forth is very powerful and considerably faster than BASIC, although it is not as straightforward to use at first. There are lots of resources online to help you learn Forth, and lots of games ancient and modern.

• https://github.com/markgbeckett/jupiter_ace
• https://www.jupiter-ace.co.uk/
• https://gldm.itch.io/
• https://petrih.itch.io/

There are four ROM options included: Original Forth ROM for best compatibility with the Jupiter Ace Patched Forth ROM with additional utilities and serial save load with RC2014 68B50 serial card MPF-I Z80 monitor assembler / disassembler ZX80 Integer BASIC modified to run on Minstrel 4th hardware

Power is by 9V DC 2.1mm jack. Current consumption is around 100mA. Jumpers select either centre negative (ZX Spectrum, Commodore 16, guitar pedals) or centre positive (everything else).

The kit includes a 7805 linear regulator. No heatsink is required for normal operation, but if you plan to add a lot of expansions, you could fit a 7805 switching regulator.

Options

This is available as a kit or an assembled and tested version.

All through hole parts are used making this kit suitable for an intermediate level kit builder upwards.

The standard kit includes turned pin IC sockets for the Z80, microcontroller, ROM and RAM chips. Turned pin IC sockets are optionally available for the remaining logic ICs.

The standard version includes the keyboard. It is optionally available without the keyboard, making a board the same size as the ZX81, should you wish to use your own keyboard.

What is included

Only the Minstrel 4th mainboard kit (and keyboard if chosen) is included. No modules or backplanes etc.

Expansion

One of the best points of the Minstrel 4th is that it can be expanded using RC2014 modules. Most modules should work, apart from any that contain ROM, RAM or CPUs. The only IO port used on the Minstrel 4th is $FE. All others are available. Interrupts are generated by the video circuitry and used for timing, display update and keyboard scanning etc.

Minstrel 4th is designed for RC2014.

A lot of standard modules and backplanes can be used, some of the ones shown in the photos are:

• https://www.tindie.com/products/shieladixon/usb-keyboard-interface-for-minstrel-4th-4d/
• https://www.tindie.com/products/dr-ian-johnson/jester-ace/
• https://www.tindie.com/products/semachthemonkey/ym2149-sound-card-for-rc2014-retro-computer/
• https://www.tindie.com/products/semachthemonkey/backplane-5-for-rc2014-z80-homebrew-computer/
• https://www.tindie.com/products/semachthemonkey/digital-io-module-for-rc2014-z80-homebrew-computer/
• https://z80kits.com/shop/tynemouth-68b50-clocked-serial-port/

Many more are available, see also

• https://z80kits.com/
• https://www.tindie.com/stores/semachthemonkey/items/