The I/O sheet | Ghosts'n Goblins

Draft. This sheet was read from the original 1985 drawing and cross-checked against MAME, but it has not yet been walked pin by pin against a physical board. The logic described here is what the drawing shows.

▸This sheet is part ofHow the 6809 runs the game

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

What this sheet is for

Everything the outside world does to the machine arrives here: joysticks, buttons, coins, start, and the two eight-position DIP switch banks at 1A and 2A that set difficulty, lives and coinage.

None of it is complicated, and that is the point. The whole sheet is buffers and pull-ups.

Why buffers at all

A switch is not a bus device. Wire one straight to the data bus and it would fight every other chip on the board. The 74LS367 buffers at 4A, 5A and 8A sit between: their outputs float away to high impedance until the decoder selects them, at which point they drive the bus for exactly one read.

The resistor arrays RA1 to RA3 hold every input high until a switch pulls it down. Active low throughout, which is why the software reads a pressed button as a zero.

The edge connector

The controls arrive on an edge connector that the drawing does not give a designator, so the model coins $CN-CTRL and $CN-SYS for them. The $ prefix marks a name this project invented rather than one Capcom printed, and it is never quoted as a board signal.

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