MiniB, the floppy disc sized BBC micro

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On this page

• Introduction
  
• Specification
  
• Software
  
• Frequently asked questions
  
• Documentation
  
• Availability
  
• Downloads
  
• Building a parallel port cable
  

Introduction

Where possible it has been designed with more modern, easy to obtain, surface mount components offering not only considerable space savings but cost savings too.

Architecturally the microcomputer is based heavily on the BBC Microcomputer series, and indeed a lot of software which will run on a BBC micro will also run on MiniB - including the original unmodified BASIC language.

Specifications

• 6502A processor, at 2MHz (slowed to 1MHz during i/o accesses)
• 6522 versatile interface adapter for timers and interruptable latched i/o ports
• Interface for a 20x4 LCD display
• 32kbyte high speed SRAM
• 128kbyte in circuit programmable flash ROM (16k for main operating system, 112k for other ROM based applications)
• PS/2 keyboard interface
• Real time clock (year 2000 compliant), on a shared I²C bus
• Full implementation of the BBC micro's 1MHz expansion bus for add on boards
• Power supply monitor/reset device

All system control logic is implemented in a single 32 macroblock programmable logic device from Lattice Semiconductor.

The completed board with accompanying display

Software

The flash ROM which is not used for the operating system appears as 7 'sideways' ROM slots which can include filing systems, languages, and ROM filing system format ROM images. The original BBC BASIC ROM works without modification on the board, though to make use of it you must own one of the original physical ROMs from which the required binary can be extracted.

The MiniB OS provides the following commands (shaded items are accepted but no action taken at present)

MiniB FAQs

Can the MiniB OS be upgraded after the board is built?
Yes, the flash ROM can be programmed in circuit either by using a simple programming header from another BBC micro, or the MiniB can reprogram itself with the built in command SRWRITE.

Which PS/2 keyboards will work with MiniB?

Any keyboard which supports 'Type 2' keycodes. This is the vast majority of keyboards available, plus any keys which don't appear in the keymapping will be ignored (for example, the 'Windows start menu' key). For more custom applications the keyboard need not be a full 104 key matrix, for example a simple PIC and keypad can be combined to produce a PS/2 numeric keyboard for applications such as electronic key locks.

Prototype PS/2 numeric keypad

As it doesn't come with BASIC installed, how do I run BASIC programs?
For the MiniB OS to start there must be at least one language ROM installed. In a normal BBC Micro this will be BASIC, but this is not supplied due to copyright restrictions - instead to get you started there is a simple language ROM which just starts a command prompt. Once at the command prompt you can reflash a sideways ROM with your own copy of BASIC using the built in flash tools.

What about compatibility?

Documentation

• Memory map
• Supported OS calls
• Survey of headers and connectors on MiniB

Downloads

• OS 0.41 (implementing all of the documented features)
• GOS 0.15 (this is a very simple language ROM which just supplies a command prompt)

The programmable logic fuse maps are supplied here for reference purposes:

• JEDEC fuse file for the Lattice 2032 PLD, for use with rev 1.05 PCBs
• JEDEC fuse file for the Lattice 2032 PLD, for use with rev 1.10 and 1.11 PCBs

Once in RAM it is a simple matter of using the SRAM utilities which, despite their name, are actually flash ROM programming utilities. For example the flash utility SRWRITE is the same as in a BBC Master. They are used as follows:

1. Turn MiniB on
2. Plug in the printer port download lead
3. Type *UPLOAD on the MiniB keyboard
4. Run the printer port transfer software on the PC and enter the ROM image name
5. Wait for the transfer to complete then press MiniB's Escape key when done
6. Verify that the report of the number of bytes transferred is correct

A sideways ROM upgrade sequence for a 16k image would then be performed by entering

• SRWRITE 3000 +4000 8000 9

An OS upgrade sequence for a 12k file would then be performed by entering

• SRWRITE 3000 +3000 9000 F

• SRWRITE 3000 +4000 8000 F

Building a parallel port cable

The protocol is very simple: a single data bit is clocked with nSTROBE and the host PC will then poll until the MiniB acknowledges by dropping the nACKNOWLEDGE line on the printer port.

The nSTROBE, DATA, and nACKNOWLEDGE on the data lead

Due to using the VIA port A lines are being used a bit like a serial port with handshaking the datarate is lower than that which could be achieved if the true 8 bit parallel port was used, unfortunately that would mean losing the LCD display!
Fortunately as the typical size of the flash images are quite small the fact that the datarate is only around 530 bytes/s doesn't impose excessive inconvenience - a 16k ROM takes just over half a minute.

The lead is easy to build as it uses only 3 conductors plus ground.

The upload software is supplied for the PC along with a tokenised BASIC program on which versions for other hosts can be based. Under Windows 2000 and Windows XP the parallel port registers are protected and will need to be made accessible using the free program 'Userport' by Tomas Franzon, or equivalent.

Availability

These computers are available now, and are offered in various different options depending on how confident you are at soldering. For most installations, you'll need an LCD display so you can see what you're typing plus a PS/2 keyboard from a PC supplier: