The Microbox II 6809 Single Board Computer

Advert from 68 Micro Journal, December 1985, describing the Microbox II

From the advert:

Microbox II is a powerful 6809 based single board computer packed
with inovative features in an easy to build form.
Running under the Flex operating system it contains 60K of dynamic ram,
8K of eprom, high resolution text and graphic displays, up to 500 sector
ramdisc, up to 512 sector eprom disc, floppy drive controller, serial and
parallel I/O, real-time clock and eprom programmer.
An eprom disc that looks to Flex like a standard write protected drive
can be programmed with anything that would normally be on floppy -
including Flex itself. A ram disc that looks like a standard
unprotected disc acts as a very fast work disc. Support for two floppy
drives is also on-board.
Exceptional monochrome graphic capabilities are provided by a NEC7220A
graphic display controller which gives very fast drawing speeds through
hardware vector, circle, rectangle, pattern and area fill generation.
The Flex operating system can be booted from any standard system disc -
configuration is carried out automatically by the supplied firmware -
and all the usual software can be used.
Microbox II can be controlled from a standard serial terminal a serial /
parallel keyboard and video monitor or a mixture of both.


68B09E microprocessor supporting 60K of dynamic ram and 8K firmware.
7220A graphic display controller supporting 128K of dynamic ram
partitioned as monochrome video display and ramdisc.
Text display of 84x24 or 108x24 characters. Or invent your own format.
Graphic display of 768x576 pixels. Very fast hardware vectors etc.
Composite video and seperate video / sync outputs.
Eprom disc using four 27128 devices. An eprom programmer is on board.
Floppy disc controller for 48 or 96 tpi single/double density drives.
Two RS232 serial ports with programmable baudrates, 50 - 19200 baud.
Centronics type parallel printer port.
Parallel keyboard port.
Battery backed real-time clock/calendar.
DIP switch selection of input source, output destination and autoboot.
Additional I/O capability via user expansion buss.
100's of Microbox II's currently in use worldwide.

The firmware includes system diagnostics, utilities, graphic primatives,
terminal emulation and auto-configuration that ensures that the board
will boot from any standard Flex system disc.
The software includes disc formatter, printer drivers, disc allocation,
alternative terminal emulators, eprom programmer routines, real-time 
clock support, graphics macros and demo, character set source and system

  *Flex is a trademark of Technical systems Consultants.

Let us just say that I was seriously in breach of the 10th Commandment when I became aware of this single board computer.

Like the TRS Color Computer and the Dragon, the Microbox uses the Motorola 6883 (74LS783) Synchronous Address Multiplexer (SAM). The CoCo and the Dragon are pretty much built according to the Application Note in the datasheet, using a 6847 to provide 16 lines of 32 characters text and up to 256 x 192 pixels in graphics mode.

Unlike the CoCo and the Dragon the Microbox II uses a NEC 7220 Graphic Display Controller instead of the 6847 CRT controller to give a maximum (monochrome) resolution of 768 x 576 -- bleeding edge 1985 technology (the Apple Macintosh debuted in February 1984 with a 512 x 342 pixel monochrome screen -- less than half the number of pixels and rave reviews).

To understand the Microbox (and for that matter the CoCo and Dragon) you need to be intensely familiar with the Motorola MC6883 / SN74LS783 SAM.

Microbox II component side Microbox II solder side
This is my Microbox II, in "as received" condition (except that I removed the monitor ROM to read the code, MON09 4.22 20.8.84 (2764, 8K). Someone had removed the crystals before I got it.

The three jumpers between the 6809 and the SAM can configure the SAM to run from a 14.31818 or 16 MHz crystal as shown on page 12 of the datasheet. Standard configuration is to run it from an external 16 MHz clock from the 74LS00 in the bottom right-hand corner (IC14D).

The one strange thing about the Microbox is that the designer (Dave Rumball / Micro Concepts) did not place mounting holes anywhere on the PCB, so there's no easy way to bolt the PCB to a case. The shape of the PCB also doesn't fit into a modern PC case (the obvious choice, I think) but it does go into an old "AT" style case (the one with the runners for the floppies). The PCB is about 300mm x 240mm (12" x 9").

Microbox II component side Microbox II component layout

Double-sided floppy support

The ETI article claims support for double-sided disks, and the block diagram on page 29 of the December 1985 edition shows a line "SIDE" from the PIA to the drive.

Pin 32 on the floppy drive connector on the PCB is however very much unconnected and there's no SIDE connection to the PIA in the actual schematics.

I asked David Rumball about it. Somewhere between MON09 4.23 and MON09 4.5, double-sided support was added. PB6 "Step Rate" on the PIA is repurposed, the step rate being stored in EEPROM. PB6 is routed to pin 13 on the 7407, and pin 12 of the 7407 goes to the floppy connector and also to a spare pull-up resistor.

(On my board, the same gate / pull-up combination replaces R37, which is the chip select to the 8255. This mod is documented in the User Notes).

Graphic Ram upgrade

The 7220 provides 18 address lines for 256k (x16) RAM. The 4164s used by the Microbox can be changed to 41256s -- they are pin compatible, with the extra address line (one line only, because DRAM address lines are multiplexed) connected to pin 1. You'd need another LS257 chip to select between the 7220 A16 and A17 depending on the state of /ROW. See Library Disk 40 above.

Pinouts of 4164 and 41256 Dynamic Ram chips


The Microbox includes an Intel 8255 parallel port which interfaces to an EPROM Disc PCB. This PCB has four EPROM sockets for 27128 to 27512, giving 64 - 256 kilobytes of storage. One of the sockets can also be used to program EPROMs.

Food for thought: here's Keith Welker's archive of Peter Faasse's 8255 based IDE interface for the 6803 and here are some notes on doing the same with a 6809 and Compact Flash. The 8255 based interface is not fast, the Xilinx CPLD interface would be better -- but the Microbox already has the 8255.

NEC 7220A / uPD7220A

Bitsavers has the datasheet.

WD 1770 datasheets and information

WD 2123 UART information

It's not easy finding WD2123 information on the web.

2004-12-28 :

Steve Ellenof found this page while looking for the WD2123 datasheet. When he eventually found one, he sent me a copy to post here. So, here's the WD2123 Datasheet, courtesy of Steve.

The Timex / Sinclair FDD3000 used the WD2123 and the WD1770.

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