The sync sheet

▸This sheet is part ofHow a frame is drawn

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

What this sheet is for

Capcom titled it SYNCHRONOUS and meant it. A single 12MHz crystal, kept swinging by plain inverters at 2N, is divided by the '74 at 2M into the 6MHz dot clock and the 3MHz both Z80s run on. Nothing on this board needs synchronising to anything else, because nothing was ever independent.

From there it is counting. 2K and 1K count 384 dot clocks per line; 2L and 1L count 262 lines per frame. Those two numbers are the entire video format, and the frame rate falls out of them: 6000000 divided by 384 gives a 15.625kHz line rate, and that divided by 262 gives 59.64 frames a second. MAME configures its screen with the same pair of numbers.

Everything downstream of the crystal is counting. The sheet's title, SYNCHRONOUS, is a literal description: there is no second time domain anywhere in the machine.

The counters are the coordinate system

It would be easy to read this sheet as "the bit that makes sync pulses" and miss the point. The sync decoding is a small part of it. What matters is that the H and V counts are broadcast across all three boards as ordinary signals, and everything else uses them as the definition of where the beam is.

The V count addresses the interrupt PROM on sheet 1/8. It addresses a second timing PROM on the sprite board. Its lowest bit swaps the two sprite line buffers. It feeds the vertical scroll adders on the C board. The H count does the same job horizontally, and one of its bits is what shares the character memory between the CPU and the display.

3K is worth noticing for that reason: it is a buffer whose only job is to give these timing signals enough drive to survive the ribbon cable to the other two boards.

Decoding the pulses a monitor needs

5L picks the moment in each line for horizontal flyback; 3N watches the line counter and fires at line 248 to start the vertical retrace; 5N and 5M shape the vertical sync pulse and hold it steady for its full width. 5K merges horizontal and vertical into the single composite sync wire the monitor actually receives.

3L blanks the video during flyback so you see a clean edge rather than the beam sweeping back across the picture. 4L, 3M, 4K, 4M and 4N are the quiet half of the sheet: they mark the moment in each group of eight pixels when fresh graphics must be fetched, shape the windows that decide whose turn it is at the video memory, and retime a couple of signals so they leave the board cleanly.

Every chip on this sheet

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