Star Tech Journal

4
STAR*TECH JOURNAL/DECEMBER 1982
VIDEO
GAME
SPECIALIST
We can repair most current
video game P.C. boards at the
lowest prices.
We are also available for
consulting and designing video
game software and hardware.
We can turn your ideas into a
new video game.
Contact us first for your
electronic supplies: PROMs,
I.C. chips, and all game board
related parts.
SEGNGREMLIN'S
ROM LINE (PART 3)
Clock Circuits
In this installment of the ROM Line, we will
discuss typical clock circuits in microprocessor-
based games.
As you remember, a computer requires a
master timing signal to keep all its actions
occurring at the right time and place. For
example, an accurate and stable clock times
the occurence of the "memory read" and
"memory write" signals from the micropro-
cessor. Obviously, these 2 signals must occur
at different times; if they occurred together,
mass confusion would result.
A typical clock circuit is shown in Figure 1.
Yl
15.46848 MHZ
..------i □ 1----~
R45
330!]
I.AD.,inc.
171-01 Jamaica Ave.
Jamaica, NY 11432
212-739-0758
Note that it contains a crystal, Yl; this is
because the signal generated by a crystal is
more stable than that from other types of
oscillators. The crystal's signal is amplified by
the inverter IC to a level that is capable of
driving the microprocessor.
In circuits containing the 280 micropro-
cessor, there is only one clock input needed and
is designated by the symbol ct>. In servicing
video game boards, checking for this signal is
one of the most important steps toward repair.
This signal must be present, and must be of
sufficient voltage, to operate the microprocessor.
QUALITY REPLACEMENT
SEMICONDUCTORS
AVAILABLE FOR THE
ELECTRONIC GAMES
INDUSTRY,
Let's look closer at timing signals and see
their importance. A computer works in a series
INCLUDING THE FOLLOWING
MANUFACTURERS:
• GREMLIN
• MIDWAY
• STERN
• WILLIAMS
-------hi
_ _ _ _ _ __,,X
lo
74S04
Figure 1
TECHNICIAN COMPONENTS GROUP
• ATARI
• BALLY
• CINEMATRONICS
• EXIDY
• GOTTLIEB
U68
74S04
74S04
The " ct>" symbol refers to the master clock
signal, as discussed above. Note how all the
other timing signals occur in relation to this
master signal. The next signal, A0-Al5,
represents the memory address selection on the
280's 16 address lines. Since the address lines
can be either hi or lo, the timing signal indicates
this with 2 lines:
Once the memory address has been selected,
the next signal, MREQ (Memory Request),
goes lo to indicate that the address is valid.
Then, the RD (READ) signal goes lo to
activate the memory circuit and force the
stored information onto the data bus. Notice
that the WR (WRITE) signal is inactive during
this time.
R46
330!]
C39
680pf
of steps, which form the computer program.
The point to remember in understanding the
computer's operation is that these steps are
acted on in a precise order. Figure 2 shows the
280 processor timing diagrams for a memory
read and memory write.
Finally, the data bus signal shows that
during the end of the MREQ and RD signals,
the stored information has been placed on the
data bus and can now be read by the 280 and
acted upon. The signals occur in the same
sequence for a memory write, as shown, except
that the READ signal remains inactive during
this time. The signal labeled "WAIT" is used
to coordinate the flow of information from
memory and input/output devices to the 280.
The WAIT line forces the 280 to "wait" for a
data transfer from these sources.
In this installment, we have seen the
importance of the clock signal in all computer
systems. It is this signal that keeps all computer
events occurring at the right time and place. In
the next issue, we'll learn about the 1/0 ports in
a computer.
MEMORY READ OR WRITE CYCLES
AVAILABLE FROM YOUR LOCAL
TCG DISTRIBUTOR
Memory Write Cycl
Memory Read Cycle
Tl
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T2
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T3
T2
T3
ct>
COMPANY NAME
YOUR NAME
AO
TITLE
Al5
MREQ
iio
ADDRESS
WR
CITY
STATE
ZIP CODE
PHONE NO .
DATA BUS
(D0-D7)
CLIP AND MAIL TODAY TO:
WAIT
IN
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NEW-TONE ELECTRONICS/TCG
DEPT Gt/44 FARRAND STREET
BLOOMFIELD , NEW JERSEY 07003
Figure 2
-.IL-=-
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