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Z80 Sloth IV    This board was given to me in 1999 by my friend Geoffrey Barkham. It is a Z80 single-board computer designed by Steve Rimmer and the subject of a construction article in the June 1985 edition of Computing Now! magazine. Subsequent articles in the November 1985 and October 1986 editions completed the description.

The board, shown below, is an early version that included a prototyping section in the bottom right. Subsequent boards omitted this to reduce costs since all those plated-through holes were expensive. For many years, the computer ran Geoff’s home security system and included the ability, in the event of a burglary, to automatically telephone his friends with a tape-recorded message asking them to call the police!

The 80-pin chip on the right is the Z80 CPU. On its left is a 28C64 8K EEPROM. The board was designed for a 2716 2K EPROM but I modified it for the larger, more convenient chip. Next to the left is a 2016 2K static RAM followed by an 8254 counter timer. The 80-pin chip on the left is an 8255 PIA which is driving the board’s output. Here, most of the output pins are driving six 7-segment LED displays that are mounted on the board’s other side. The driver chips for the display are the 14-pin and 16-pin chips on the left. The display section of the board is isolated by the ribbon cable and can be cut off and mounted separately.

At the bottom of the board is a small prototyping area, omitted in later versions, but currently used for a 5-volt regulator and smoothing capacitors. Power comes in on the left - either 5 volts on the red wire to directly power the board or 8 volts on the yellow wire to feed the regulator.

On the top row are a 4 MHz crystal and, to the left of it, an 8-pin 555 timer. A jumper allows the Z80’s non-maskable interrupt (NMI) to be derived from either the 100 Hz output of the 555 timer or the Z80 controlled output of the 8254. Another jumper allows the 8254 clock inputs to be configured. The board was designed for a 2 MHz crystal and an 8253 counter/timer but these have been replaced with a 4 MHz crystal and the faster 8254 counter/timer.