Micro­power

  

Volume 1, Number 4 – December 1981

Page 7 of 33

BIT 5
When this bit is 0, it holds pin 20 of the 2708 at 5 volts, during normal READ operations. When set to 1, it allows pin 20 to pull up to 12 volts, for ‘Write Enable’.

The power supply is fitted with a multipole switch on the D.C. outputs. This should be fitted in ALL cases, and should be used to switch off the programmer when changing chips. If the programmer is turned on/off with a mains switch when a chip is being inserted or removed, the chip may be damaged, because the low voltage supplies decay more quickly than the 26 volt line; this can result in a 26 volt pulse being written into address 0 of the EPROM, possibly PERMANENTLY!

COMPONENTS FOR THE PROGRAMER.

Transistors
TR1, 2BC548
TR3BC558
TR4, 5BC109
Diodes
4 x 1N4148
Integrated Circuits
IC1CMOS 4049
IC2TTL 7406
IC3CMOS 4040
Resistors
4 x 10 Kohm
1 x 33 Kohm
1 x 180 Ohm
1 x 47 Ohm
1 x 1 Kohm
6 x 4.7 Kohm
Capacitors
1 x 0.001 μF
5 x 0.1 μF

COMPONENTS FOR POWER SUPPLY

Transformer
15 – 0 – 15 volts, 6VA
Diodes
D1, 21N4002 or similar
Z126V, 1 watt Zener
Resistors
2 x 4.7 Kohm
1 x 1 Kohm
1 x 10 Kohm
1 x 15 Kohm
Single pole mains switch
Five pole low voltage switch
Rectifier
1 A Bridge, 50 PIV
Voltage Regulators
78L12 (+12V, 100 mA)
78L05 (+5V, 100mA)
79L12 (-12V, 100mA)
79L05 (-5V, 100mA)
Capacitors
4 x 0.47 μF
2 x 0.22 μF
2 x 4700 μF, 25V
2 x 470 μF, 64V

The full circuit diagrams of the programmer and power supply, a Veroboard layout of the programmer, and the software for its operation will be given in the next issue of the magazine.

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