Service manual

49
Eproms
The 2716 and 2516 have 24 pins and are generally interchangeable, although slight
differences in programming parameters exist between the various manufacturers' devices.
The 2732 and 2532 also have 24 pins and have the same capacity as each other but
BEWARE: they are NOT pin for pin compatible, as the sketches at the back of this book
show!
The 2764 and 27128 have 28 pins and can be interchanged, but bear in mind that the 27128
has twice the capacity of the 2764.
The 27256, with twice the capacity of the 27128, is also compatible, having only one pin
function different. It is generally very fast to program, compared with the smaller capacity
devices!
Double again, the 27512 is probably the largest capacity device you will want to deal with,
although there are even bigger Eproms available and their prices are falling all the time.
CMOS devices are also available. These are usually denoted by a letter C in the number, for
example 27C128. The current consumption is lower than that of standard devices but they
are somewhat more susceptible to damage from static electricity and also more expensive.
Eproms usually have number suffixes such as 2764-25. The -25 refers to the speed at
which the Eprom can be read. For CB applications we are not the slightest bit interested in
the speed, since it takes a relatively long time to change channel, compared with the speed
at which a microprocessor works! Consequently, you can buy the cheapest, slowest
Eproms you can find, which will often have the suffix -30 or -45 or nothing. Remember,
too, that we mentioned earlier that a letter suffix might refer to the programming voltage.
Check this and ensure that the Eprom is compatible with your programming unit.
All Ch a rg e d Up
Whenever you are handling ICs you should take precautions to avoid the generation of
static electricity. Always touch the surface on which an IC rests before picking it up and do
the same before putting it down. This will ensure that your body is at the same electrical
potential as the surface and thereby ensure that you do not discharge current through the
IC. You can generate a large voltage simply by moving around in clothing made of
synthetic material. Removing a nylon shirt or pullover is a good method of creating static
electricity and if you touch a grounded surface after doing so you can expect to receive a
nasty shock. Imagine, therefore, what this does to an IC which contains tiny conductors
only a few atoms wide!