B board · sheet 6/9 · manual page 16

The second line buffer sheet

The other half of the pair, and the swap that makes both of them useful.

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 sprites work

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

What this sheet is for

7N, a CXK5808P, and 8N, a TMM2108, with counters at 9L, 9M and 9N. Structurally the same as the previous sheet.

Reading them side by side is the fastest way to understand either. Where they differ is only in which phase of the swap they are wired to.

The swap itself

The 74LS257 multiplexers at 6L and 7L are the changeover. One line-timing signal decides which buffer is connected to the fill side and which to the read side, and it toggles every scanline.

There is no copying and no moving. The buffers stay where they are and the wiring swaps around them, which is the cheapest possible way to do it.

Every chip on this sheet

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

6L74LS2577L74LS2577M74LS867NCXK5808P8M74LS868NTMM21089L74LS1639M74LS1639N74LS16314L74LS21RA61K x 8 resistor arrayUNMARKED_27374LS273
← open the schematic for this sheet