Micro­power

THE Hardware

The author’s unit is built in two parts. A diecast box, 4.5″ x 6.5″ x 2″, is used as a base and contains a small power supply that provides +5, +12, −5, −12 and +26 volts (the −12 volt line is not needed by the programmer, but it is easy to include and makes the power supply useful for other circuits). The electronics are built on a piece of 0.1″ matrix Veroboard, that sits on top of the box. Power could be taken from the computer supply, as only a few milliamps are needed, but a 26 volt generator would still be required. By providing a completely separate power suply the unit can be made much more portable.

Two EPROM sockets are fitted to the unit; one is for 1K chips, the other for 2K chips. To avoid the possibility of damage the sockets should be Clearly marked with the chip type, and pin 1 should also be marked. The use of zero insertion force sockets is strongly recommended, to prevent wear and tear on the chips and sockets. Only one socket should be used at a time.

The Z80 PIO has a low drive capability, and buffering is desirable. Pin 20 of the 2708 (WE) rises to 12 volts during programming. Since this is well above TTL levels, the use of a high voltage rated buffer is necessary. The 7406 is suitable, as it is rated at 30 volts. Double buffering is used in the unit as this provides the required number of signal inversions, low loading of the PIO, and the necessary voltage isolation. Buffer IC1 is a CMOS 4049 hex inverter chip; IC2 is a 7406, backed by a couple of BC108/9 transistors to make up the required number of inverters.

IC3 is a CMOS 4040 12 stage binary counter, capable of counting from 0000H to 0FFFH (0 to 4095 decimal). In this unit a count of 03FFH is used for 2708s and a count to 07FFH for 2K chips. The spare output might be used in the future to extend the unit to 4K chips. Port A of the PIO is used for data, and port B for control. Normally port A is set to input and port B to output, but during programming both ports are output.

PORT B Bit Assignment

BIT 0

Controls the 26 volt programming pulse through the high speed switch formed by TR1, TR2 and TR3.

BIT 1

Controls the WE/CS signal on pin 20 of the 2708 socket, and the OE signal on pin 20 of the 2516/2716 socket.

BIT 2

A pulse from this bit is used to increment the address counter.

BIT 3

A pulse from this bit is used to reset the address counter at the start and end of each routine.

BIT 4

Used to switch the PGM input on pin 18 of the 2516/2716 socket.