Service manual

4
All we have to do is to find out what code number comes from the switch for EACH
channel and put into each box SELECTED BY THIS NUMBER the NEW code which we
want the PLL to see. Putting the codes into the Eprom boxes is called PROGRAMMING
and is a simple task which anyone can do, provided, of course, that he has an Eprom
Programmer! A Programmer can be bought VERY cheaply and used in conjunction with a
personal computer even if you have absolutely no knowledge of computer programming.
Bits in Lin e
Eproms work quite happily with binary codes consisting only of "1"s and "0"s, called
"bits". We use the designations "1" and "0" simply for convenience to represent high
voltage and low voltage respectively. The actual voltages are usually not critical - high will
be some positive voltage equal to, or slightly less than, the supply voltage. Low voltage
will usually be less than 1 volt. The position of each "bit" in line represents its actual
value:-
128 64 32 16 8 4 2 1
the code 1 0 1 1 0 0 1 0 adds up to 178 in decimal.
Horribl e H e x
That was a little tedious to work out but it is difficult to use decimal with Eproms. To make
the numbers easier to deal with we can use the system called Hexadecimal. Before you
shriek in horror at this word let me advise you that everything you ever read about how
difficult it is, is nonesense. It makes life very easy indeed.
"Hexadecimal uses the base sixteen". That means that you count up to 16, carry one, then
start again. In fact we use letters to represent numbers bigger than 9, as follows :-
0 1 2 3 4 5 6 7 8 9 A B C D E F
F represents ordinary decimal 15 and is as high as we go. To count higher we add another
column and start again so ordinary 16 is represented by "10" (that’s “one-zero” HEX) and
we count onwards in HEX ....
10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
..and if you've followed so far you will understand that "1F" HEX represents a 16 in the
first column plus a 15 in the second = 31.
Can you see that "20" HEX must be 32 ?