Service manual

APPEND

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Eproms, by definition, are erasable under Ultra Violet light. Since many second

hand

Eproms are available, cheaply, it is worth considering the use of such devices to save

money. Before you can utilize them you must erase the existing contents, effectively

resetting all bytes to FF.

The "window" in each device is not glass but

quartz which allows UV light to pass through

much more readily than glass does. The UV light must be within a specific frequency band

to be effective. Ordinary sunlight will erase an

Eprom but it would probably take weeks to

effect complete erasure. A "black light" tube, as used for display purposes and in discos is

no use since the

UV which it produces is much too low in frequency. The type of

light

which does work is produced by a white tube which is called an "actinic" tube because of

its ability to kill germs by radiation. You can be sure, therefore, that your Eproms are

clinically clean, after erasure!

ning

The time required to erase an Eprom depends upon its distance from the light source, the

intensity of the light, the contents of the Eprom and the physical characteristics of the

Eprom itself. As a very rough guide, an "average" Eprom will require at least 15 minutes

but not more than an hour to erase when placed one inch from, and facing, an 8 Watt tube.

It is possible to make an eraser, yourself, but due regard must be paid to all aspects of

Electrical safety, the high temperature of the tube and the danger to living tissue caused by

UV light.

WARNING. UV LIGHT CAN DAMAGE SKIN AND CAUSE BLINDNESS. The unit

must be ventilated but should not allow UV light to escape. Some excellent commercial

units are available, moderately priced, with safety interlocks and with or without a timer.

One unit even "interrogates" the

Eprom to ensure it is erased, exposes it for a further

calculated period to make sure,

then turns off. This type of unit is supposed to avoid over-

exposure, thereby increasing the

long term reliability of the

Eprom and increasing the

number of erasures which are possible. There is a limit to the number of times that an

Eprom may be erased before damage occurs. This limit varies widely from one

Eprom to

the next. Some do not survive even the first erasure, whilst others can be wiped and re-

used numerous times.