80-Bus News

There are a number of ways in which redundant EPROMS can be erased before reprogramming – such as exposing the uncovered window to bright sunlight for a few days or for a rather shorter time to short-wave ultra-violet radiation. It should even be possible to use a series of flashes from a xenon photographic flash tube although I haven’t tried this method. All commonly avaliable EPROM erasers tend to use a quartz UV tube and the design which follows uses this method, allowing up to 6 devices to be erased at one time.

The time taken to erase an existing program from an EPROM is dependent upon the wavelength and intensity of the radiation which is employed, the type of EPROM and, to a lesser extent, upon the age and type of device; generally, an exposure time of 15 – 20 minutes with a standard 8 watt tube 1″ from the EPROM is sufficient although double this time can be given without undue damage being caused. A shorter time will not fully erase the contents to FF (hex). Some EPROMS may take much longer than 20 minutes, due to their method of manufacture, but these are the exception rather than the rule and can I sometimes apply to some Texas types. Sadists who erase their EPROMS with the aid of a combined UV/heat lamp are asking for trouble since the devices will be at risk of death from heatstroke – and unless they take precautions, their eyesight may be affected permanently and adversely by the high level of UV radiation (up to 300 watts).

A programmed EPROM has a pattern of charges placed in its memory locations (which are effectively twin gate MOS FETs), where, owing to the excellent insulating power of the silicon dioxide layer between the two gate electrodes, it can be retained for very long periods of time (in excess of 10 years according to most manufacturers). The effect of UV light is to allow this charge to leak away – which is why a programmed EPROM must have the ‘window’ covered; it is made out of a high-silica glass which allows UV light to pass through – most ordinary types of glass block the short wavelength (253.7 nm) ultra-violet. Additionally, EPROMS don’t like high temperatures (above 70 deg. C) which is another good reason for keeping them cool while erasing them and for fitting your computing engine with a decent fan if it acts as a part-time convector heater.

A typical device in the 27XX range requires a radiation dose of about 15 Joules/sq. cm (1 Joule is 1 Watt/​second) at an intensity of about 0.012 Watts/sq.cm and for a new UV tube, this would be given by about 20 minutes exposure at 1″ from the light source. Tubes lose emission intensity as they age but this is not likely to be much of a problem in this application since the exposure time can be increased if needed. The only drawback is that in order to check the state of erasure, the devices must be taken out of the eraser and checked in the programmer, and replaced if necessary for another dose of sun tan! A device which will not erase properly is likely to have one or more of the following attributes:

1. Made by Texas – try a longer erasure time (up to 2 hours).
2. a dirty window – clean off the muck with a solvent and try again.
3. open or short-circuited – had it been plugged in the wrong way round when-it was last used?

If the last is true, then throw it away. Incidentally, new, unused EPROMS should be blank, with FF in all locations. If not, your dealer should change