The I/O sheet

▸This sheet is part ofHow the CPU shares the machine

The subsystem followed end to end, across every sheet that builds it.

What this sheet is for

Everything the game can perceive comes through this sheet. 3B carves the top of the memory map into its jobs and 3A turns a CPU read into the right port answering.

The controls arrive through plain '367 buffers: 6B for the coin slots and start buttons, 4B and 5B for the two joysticks. The operator's settings sit on two banks of eight physical switches, 1A and 1B, put onto the bus by the '245s at 2A and 2B when asked. There is no input hardware more sophisticated than that: the CPU reads a byte and the bits are the buttons.

Five addresses, decoded by 3A, covering everything the machine can sense. MAME's memory map lists exactly these five and nothing else.

The control register, and one strobe that matters

Writing is done by 7B, a '138 that turns CPU writes in the C800 range into eight separate strobes. Three of them do most of the work: C800 drops a command in the sound mailbox, C804 loads the control register, and C806 produces /OKOUT, which is what starts the sprite board's raid on the A board's memory.

The control register itself is a '174 at 7A. Its bits click the mechanical coin counters, hold the sound processor in or out of reset, and set the screen flip that ripples out across all three boards. MAME's handler for the same address does precisely these things: bits 0 and 1 to the coin counters, bit 4 to the sound CPU's reset, bit 7 to flip the screen.

One genuine fragility is worth recording, because it is the sort of thing that only bites during a repair. The write decoder at 7B is not qualified by the write signal. Nothing in the hardware prevents a stray read in that address range from firing a strobe. It works because the program never does it - software discipline standing in for a gate that was never fitted.

Every chip on this sheet

Click any of them to open the schematic with that chip selected in both panels.

← open the schematic for this sheet