-
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, 2 | BC548 |
| TR3 | BC558 |
| TR4, 5 | BC109 |
| Diodes |
|---|
| 4 x 1N4148 |
| Integrated Circuits |
|---|
| IC1 | CMOS 4049 |
| IC2 | TTL 7406 |
| IC3 | CMOS 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, 2 | 1N4002 or similar |
| Z1 | 26V, 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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