About the Plus 1 Mini

The Plus 1 Mini is a modern alternative to Acorn's original Plus 1 expansion unit. It provides two Plus 1 compatible cartridge slots that support both large and small form factor carts (the rear slot is limited to small carts if a PiTube co-pro is fitted) and implements all Plus 1 signals.

In recent years original Plus 1 units have been fetching ever increasing prices, often exceeing £100 for a unit in reasonable condition. Once you add an SD card interface, memory cartridge, tube interface, etc, the cost can become prohibitive for many.

The Plus 1 Mini is designed to provide a cartridge expansion unit with as many useful features built-in as possible, keeping the cost much lower than buying a Plus 1 and a handfull of cartridges. Below is a run-down of the Plus 1 Mini's features.

CARTRIDGE SLOTS On top of the Plus 1 Mini are two cartridge slots which are physically and electrically compatible with the slots on Acorn’s original Plus 1 interface. Each slot can accept either a ‘short’ or ‘long’ cartridge, although the rear slot will be limited to a short cartridge if a Raspberry Pi co-processor is fitted.

SD CARD The Plus 1 Mini provides the capability to use an SD memory card to store programs and data. To do this it uses the popular MMFS ROM, which enables the vast majority of Electron software to work with a memory card by emulating a floppy drive system. One SD card can contain over 500 disk images which when accessed appear to the Electron as a floppy disk.

JOYSTICK PORT On the left side of the Plus 1 Mini is a 9-pin port supporting Commodore/Atari type digital joysticks. The Plus 1 Mini hardware emulates the original Plus 1 analogue joystick system, so most games written to support that standard will work without issues. In addition, compatibility with the third-party Slogger and First Byte joystick interfaces is provided. Lastly, a keyboard emulation system is also available for games that do not support joystick use. This works with around 60% of Electron games.

SIDEWAYS RAM By default the Plus 1 Mini adds 64KB of sideways RAM to the Electron, in sidways banks 4-7. An option is available to disable the cartridge ports to free up sideways space for an additional 64KB of memory, giving 128KB total.

THE TUBE INTERFACE A 40-pin header is available which can be used to internally mount a Raspberry Pi Zero or Zero 2 in the Plus 1 Mini and have the Pi function as a TUBE co-processor using the PiTube Direct software. This is a terrific package which implements a range of useful emulated co-processors, such as a 6502 @ circa 800MHz, 68000, ARM, Z80, etc. Please note, the header only permits the use of a directly connected Pi Zero or Zero 2. Other Pis will not fit. Please note, the Plus 1 Mini does not include a Raspberry Pi.

COMPATIBILITY The much increased functionality of the Plus 1 Mini can in rare cases cause unavoidable resource clashes with some cartridges. The Plus 1 Mini implements the ability to disable the SD card interface, PiTube port and cartridge slots to enable such cartridges to work correctly. Also, there is 'safemode' which disables all of the Plus 1 Mini's hardware except for the cartridge slots, effectively turning it into an original Plus 1. All of these options can be set in hardware using DIP switches or controlled via star commands.

UPDATES All ROMs on the Plus 1 Mini are contained in a flash chip which can be updated in software. The main CPLD programmable logic chip can also be updated by connecting a Raspberry Pi to the JTAG header on the Plus 1 Mini PCB, in case of future updates.