cpu troubleshooting

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The Radar Scope CPU is a Z80 clocked at 3.072 MhZ. In the picture here it is a NEC D780C-1, which is equivalent to a Z80-A (rated up to 4 MHz). On another CPU board i see a Sharp Z80A soldered in. Additionally there is a DMA Controller 8257 on the board. The DMA controller is for the transfer of video data during vertical blanks.
4 Program ROMS are of type 2532 (4K x 8 bit), which makes a total of 16 K program code. For volatile storage there is 3K RAM on the CPU board.

Click this picture to enlarge CPU board. If you want to identify an unknown board and need a better picture, view this full sized CPU board image (size is 409 KB)

Power Input on P9 (TRS-02) / P8 (TRS-01)

This is according to Nintendos schematics of TRS 02-CPU The +24 on pin 9 is used for the coin counter only. You do not need to supply this unless you have a counter connected to P4 (counter) which is operated through P3 (coin). Note that on a TRS-01-CPU the power in is slightly different: Power in is P8 and the pins 4 and 5 in this picture are connected to +12 volts for the TRS-01 RGB amplifier. This means you can use the same plug for both TRS-01-CPU and TRS-02-CPU, the former will use the +12V for video output, the latter will not connect it.

The P9 plug is located on the upper right of the CPU. Seen from the component side, you will have pin 1 on the right side as in this picture

• color palette and video output stage (RGB & composite sync). This is not on the video board!
• address decoding
• I/O: dip switches for setup and harware to read them
• I/O: game controls for two players
• I/O: for a service button and a test switch
• driver for game counter
• coin input logic to create coin signal
• latches for sound output

Color Proms

The color palette is created using these proms. This picture shows them on the CPU board.

How color generation works in theory is shown very nicely on the pages of Aaron Giles. He uses PacMan and Mappy as examples, but it works the same with Radar Scope.

These are 256x4 bit Proms used in parallel: An 8 bit address is applied via the clocked flipflops 7474 at 2G and 2H. The 2J Prom then puts out the red bits and the first green bit of the color information, the 2K Prom the rest of two green and blue bits, respectively:

If you need the color PROM contents for repairing your boardset with MB 7052 PROMS or a replacement (see below) they are referenced in this table:

Replacing missing color proms

In one of my boardsets the color proms were missing. The color proms needed are MB 7052 PROMS, organized as 256 x 4 bit. Since i could not find a direct replacement for the MB 7052 PROMS i replaced them with GALs (Generic Array Logic). These are logic device IC's you can program (and reprogram) logic functions with. Basically you define what the inputs will be and what the outputs for each input state should be. I had no previous experience with this but it works great as a PROM replacement. Note: An EPROM can only be used if it is fast enough to handle the transfer speed. An old bipolar PROM is faster than an EPROM.

You need software and a programming device though. I used WinCupl Version 5.2.16 (runs on Win95 and Win98, produces a JEDEC file) and an ALL-03A Programmer to write the JEDEC file to the GAL (thanks George). If you get an "Excessive number of product terms" error when compiling you have to fiddle with the compiler options. I used the "Minimization: Expresso" setting. You can also try to optimize the pin D3 separately (with the TRS2-C-2K GAL).

You might find some other similar GAL that works. Pin CLK/I0 can be used as a clock input (not used here). The pins labeled I1 to I11 are dedicated inputs. Pins I/O0 to I/O9 can be defined as either inputs or outputs. For the replacement of a MB 7052 i used in the case of the 2J PROM: -Pins 2 to 9 are address inputs to the GAL -Pin 11 is used for the tri-state chip enable input -Pins 15 to 18 are outputs to the PROM socket Below is the wiring for the adapter you must build from the MB 7052 socket on the CPU board to the GAL. My programmer handles this GAL as manufacturer type "AMI" and device type "PEEL 22CV10".

This picture shows the necessary wiring for the 2J color PROM (as seen from top):

Also, there is a different layout for the pins: -Pins 2 to 9 are address inputs to the GAL -Pin 10 is used for the tri-state chip enable input -Pins 17 to 20 are outputs to the PROM socket

Reset circuit

The reset signal on power up is generated by the 74LS123 one shot at pos. 3G.
It can be observed at TP8.
If you need to reset the game often (as i do when troubleshooting), you probably wish you had a reset button instead of powering off and on. Such a button is not available, but you can easily create one.
I use a push button to clip onto the plus side of capacitor C53 (it is located in the middle of the CPU board) and to ground (TP1), with a resistor, like this:

When you press the button for about 2 seconds, the game will reset.

Wait circuit (freezing the game)

The circuit below is easy to build and allows you to freeze the game in action. Then you can take screen pictures during attract mode or when a game is in progress and this can be very helpful for troubleshooting.

How it works: The Z80 CPU has six control signals for internal status and control of sequencing. Among them is the /WAIT (active LOW) signal input, normally used to synchronize (wait for) slow peripherals, like memory or I/O devices. In the picture below, this input is shown in the hatched area to the right:

This is a section of redrawn schematics which shows the TRS01-CPU wait state control.

The video memory pulls the /WAIT input low to indicate that it is not ready for data transfer. The Z80 will then go into a wait state as long as /WAIT stays low.

At the time when /VRAMBUSY and /VRAMRQ both go low, the output of the NOR gate at 4E will go high. The 74LS74 flipflop is triggered by the positive edge of the CK input, so at the next clock pulse the input at D will cause Q be high and /Q be low, thus giving a low at the /WAIT input of the Z80.

During the vertical blanking interval (/VBLK is low) this flip flop will be forced to a state of Q = low and /Q = high, since the vertical blanking signal is connected to the /R (reset, active low) input of the flip-flop. This makes sense since the transfer to memory is done by DMA at this time (see explanation in 'the video system').

To hold /WAIT low and freeze the game we use a switch. Since mechanical switches do not allow for a clean transition from HIGH to LOW state this circuit debounces the switch:

The two NAND gates (here two gates from a 7400 for example) will lock each other into a stable state once the SPDT switch has been thrown. This means that the bouncing of the switch is prevented during those maybe two milliseconds when the switch contacts do not close "cleanly".

If you put this device into a small plastic box with clips for +5 Volts supply and GND and a cable for the output, you can connect its output to pin 6 of the IC 74LS74 at position 4F. This goes directly to the WAIT input of the CPU. (You could connect it directly at the CPU pin 24 but i found it easier to do at the non-socketed flip-flop)

Address bus buffering

The address bus is buffered by 74LS367 Hex buffers as shown in this picture.

If you need to check the signals, the location of the buffer on the board is indicated, e.g. address line A10 is buffered in the buffer chip located at position 4B on the Radar Scope CPU board (pin 10 is the input, pin 9 is output).

The address bus is shown on the right and the memory control lines on the left. The numbers inside the rectangle are the Z80 pin numbers.

This is redrawn from the original schematics, since most numbers are hardly readable there.