Star Tech Journal

the BAK board from which the final color and
sync comes from. A ribbon cable comes from
this board to the CPU board where a single
plug Is the only connection to the outside.
This single plug makes It easy to service the
board outside of the cabinet Remember all
the plugs on Donkey Kong boards and how
difficult It was to troubleshoot the board out·
side the cabinet without a special harness?
This Is not the case with the use of the single
plug.
Anyway, the CPU board could be operated
Independently from the Video and BAK boards
If It weren't for one signal (which the CPU
board requires to operate) being generated
on another board. The sign al to which I refer la
the VB line which comes from the Video
board. Thia line Is lnputto the CPU (Z80) to the
NMI llnevlaa 74LS74 todetermlnetherateof
program execution. Literally, what the VB line
serves to do la to generate an Interrupt which,
to the Z80, la unmaakable. When the CPU Is
interrupted, It updates the video presentation
to both monitors and strobes the player Inputs
and anything else which needs to be taken
care of which concerns external communlca·
tlon to the outside world. To show you how
you can operate this CPU board by Itself
without the other two boards connected, I put
together a simple circuit which can be con-
structed on a stand-alone board and connected
to this board or any(Z80) based system which
uses a similar logic setup - that Is uses the
vertical blanking line to tell the computer via
the NMI line or the INT lines of the Z80 to
update the program execution. I have had a
lot of fun with this board. I made and have
used it on many popular video games (hint,
hint) to speed up the game or to slow it down, if
you catch what I'm saying.
.
For this game, if you construct the circuit I
suggest and use it on this game, I warn you it
will mess up the video because the Video
board-generated VB signal is synchronized
so thatthe CPU update will not come at a time
when the video circuitry is generating a picture.
Thia is not the case with some other games
which use the INT line of the Z80 to serve as
the Interrupt signal, because the INT line Is
maskable whereas the NMI line Is not. If you
use this circuit on other games which use the
Z80 I NT line, you will have a clear picture. But
this depends upon the game. Nevertheless,
the Punch-Out!/ CPU board can be operated
along with no other boards connected with
the circuit connected as described below:
Construct the circuit shown In Figure 1 or
Figure 2 and verify Its operation with either
scope or logic probe. The circuit shown utlllzes
the versatile 555 timer oscillator chip as a
variable frequency square wave oscillator
which, when constructed with the shown
resistor and capacitor values, will have a
variable frequency from about 25 hertz to450
hertz, the output being available from pin 3 of
the 555. The 555 Is a very common chip
available from Radio Shack or some old logic
boards (so are the rest of the components). I
would suggest constructing the circuit on a
general purpose PC board used by experi-
menters also available from Radio Shack.
There should be three wires coming from your
board. These are the power wire (+5vdc), the
ground wire, and the output wire from pin
three of the 555. After you have built the
board, connect the power and ground wires to
appropriate supplies on the logic board. For
example, the +5vdc wire to pin 14 of chip 1 C
on the Punch-Outfl CPU board and the ground
wiretopln7 of chip 1C. Wlththeoutputwlreof
the circuit shown, verify the circuit la pulsing
along with a logic probe or a scope connected
to this wire. If it Is, then 11ft pin 1 of 1 C on the
Punch-Dutil CPU board and attach the output
wire of the circuit to the lifted pin one. Thia will
then allow you to vary the speed at which the
CPU board executes the program by varying
the pot on the circuit board. Thia will allow you
to disconnect the othertwo boards for trouble-
shooting purposes. To restore the board to
using its own VB signal, simply remove the
output wire of the circuit and resolder the pin
1 of 1 C to Its solder pad.
This part of the article concerns the
functional operation of the CPU board. It Is a
general description of its notable points.
Because the CPU board requires nothing
except the VB line from the other two boards,
It can be considered on the overview to be an
output to the other two boards and the Inputs
from the external world. The Z80 Is the main
processor and an RP2A03 MPU (custom)
generates the music and sounds. The RP2A03
(4H) operates with Its own memory at 4K and
4L and Is told what sounds or music to gen-
erate by the polling method. The Z80 places
the data that the RP2A03 will use Into the
74LS374 chips at 2H and 2J and when the
NMI line of the RP2A03 goes low, It will get
the data from the 2H and 2J chips and act
upon It. Note the NMI line of the sound CPU Is
driven from the same VB.line as the Z80 and
the Z80 does not directly Interrupt the RP2A03.
The final sound comes directly from the
RP2A03 on pins 1 and 2 and goes to be
preampllfled to the LM324 op amp at 2 N. Two
of the four op amps are utilized on the LM324
chip to amplify the sound. The other two op
amps are used by the vocalizer chip, a
VLM5030 at 6M. the VLM5030 la driven
directly by the Z80 to generate the voices.
The vocalizer CPU operates on its own
memory at 6P. When the voice chip Is busy
generating voice, It disables the Z80 Input at
6L by raising Its busy line, pin 30. This same
line also goes to a 74LS240 at 5R which la
read by the Z80 so It can know that the
vocalizer is not taking data from It. The main
CPU operates independently from the sound
and vocalizer sections so that the RP2A03
and VLM5030 can be removed from the board
and the main CPU would still operate. Thia
makes troubleshooting the board easier,
knowing that the Z80 requires no information
from these sections to operate the video and
Inputs properly. Inputs to the logic board are
taken in parallel from the chips at 1 R, 2R, 6R,
and 5R. The Z80 operates on memory located
at8L, 81<, SJ, 8H, SF, 8M, 6D, and8C. The8C
and 8D chips are SK x 8 byte RAMs and are
not interchangeable. The other RAM is a
+5vac
OUTPUT
2
DIS'-~
556
TH~
v+
CV
TH
RESET
CV
5 OUTPUT
6
TR\,
7
GND
DIS
13
12
10
OUT
9
~~
\\\--
8
<\~5
555
11
RES
TR
,c G-
o V\
D~
';>\~I»--
\~~S\,..
Ri5
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FIGIURE 2: SGUARE WAVE OSCILLATOR CIRCUIT (ALTERNATE CIRCUIT
USINC3 BBB [INSTEAD DF 555] WITH OTHER PARTS BEINC3
THE SAME)
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