STAR*TECH Journal
Custom ICs: Pros, Cons,
and Troubleshooting
By James Beck
Greater Southern Distributing Co.
BAM Engineering
Atlanta, Georgia
We technicians have, in the past couple of
years, been assalted by a new menace. 'Ibe
custom Integrated Circuit!
November 1987 [4]
us see just what they have condensed into
the smaller packages. I wouldn't count on
the manufacturers giving us this inner
view in the future, but the point I am
trying to make is, that these custom chips
are still just chips, and we can work with
them in the same manner that we've always
done.
custom ICs have actually been around for
many years now, but until recently they
have been few, far between, and relatively
sirrple devices. As a general rule, we
technicians have hated those little
rrulti-legged ronsters, with their
mysterious contents, but manufacturers
just love them to death. For good reasons,
I might add.
Fortunately rost custom ICs are socketed.
This gives us, the technicians, the
unprecedented ability to easily swap out a
whole functional block of the board in one
swoop. That is of, course, if we have a
good chip or board on band. If there is no
known good reference on hand we can still
check the custom IC's support chips and
draw a conclusion about the performance of
the custom IC itself.
custom ICs allow a manufacturer to bundle
a large nunber of functions, such as
timing, character Ra'1 addressing, video
RAM addressing, and the associated data
handling, into a single chip. This means
that they pay for the engineering once and
then the IC can be put into use on many
different boards. This cuts the overall
cost, parts count, and size of the boards.
Just because we can't see the descrete
parts of a timing chain, or the descrete
parts of a video RAM addressing circuit,
doesn't mean they are not there. It's just A
that all we get to look at is where the
•
circuit needs to connect to other
conponents such as RC.Ms or RAMs. This
poses no great problem as long as you
understand the nature of the beast.
One other feature of custom Integrated
Circuits is the measure of security they
provide. The chips make cost effective
copying of the board rore difficult.
Something the manufacturers really like.
There is one major drawback we need to be
concerned about. That is replacement cost.
What if one lousy little inverter inside
that big custom IC goes out. It is a
little tough to cut the IC apart and
replace the section of silicon that is
damaged! So, instead of replacing a
fifteen cent 74LS04 you are going to have
to cough up the bucks for the whole IC,
and that will probably run fifty dollars
or higher.
Most of the technicians I have talked to
concerning custom ICS seem to regard these
"creatures" as magical or something. This
just isn't the case. Remerrber the Midway
custom Data Bus Controller on all of the
PAC MAN boards? Some boards had the actual
custom IC while others had the equivalent
circuit laid out on a piggy back board,
showing just what they packed away in the
single IC, or grab some of the older
Konami boards, many of them have piggy
back boards plugged into sockets intended
for a custom IC.
'Ibey are funtionally equivalent, but let
To sum it up, custom ICs shouldn't be
feared or held in awe. 'Ibey should be
treated for what they are, Irodules of
logic that perform a specific task. If
anything -they should make trouble
shooting a little easier. They allow us to
look at generalized functional blocks
instead of hunting for specific descrete -
conponents.