7 Jul 2019

Sega PCM chip 315-5218 reproduction part #1

I can't promise to update that blog very often in the coming months but tonight I found time to write a new article!

That chip handling samples playing can be found on many Super Scaler games from the mid/late 80s:
- OutRun/Turbo Out Run
- Model X
- Model Y

And some may also know that it's unfortunately quite unreliable and no equivalent/replacement exists so only solution was to pull a working chip from a donor board (which proves expensive to the point of not making it worth it).

I started this project cause I got a Frankestein OutRun/Turbo OutRun boardset (top board from OutRun with EPROMs swapped with Turbo OutRun ones but jumpers left configured for OutRun, bottom board from Turbo OutRun) with many faults including that damn PCM chip (last hurdle in the repair).

There's little information about that chip but what's available is highly useful:
- Pinout is available in OutRun's schematics
- MAME states some earlier Sega games (Enduro Racer, Space Harrier, etc.) use an almost 100% TTL version of it
- Schematics for Enduro Racer are available!

Those were good entry points so I started by comparing schematics and identifying functions:

Pink = clock circuitry
Red = sample RAMs
Blue = sample ROMs
Grey = banking mechanism
Yellow = PCM logic

TTL version:


Then I identified signals directions and equivalence:


A0-7 = A0-7 (from Z80)
SD0-7 = PD0-7 (from 280)
!SRD = !PRD (!RD from Z80)
!SWR = !PWR (!WR from Z80)
!SRESET = !PRS (!RESET from Z80)
!EE0 = !DACS
GD0-7 = GD0-7


!SWAIT = !WAIT (!WAIT to Z80)
RS02-11 = RS02-11 (to DAC1022)
LGT = LGT (to MF6 filter for left channel)
RGT = RGT (to MF6 filter for right channel)
SA1-7 = SA1-7 (RAM)
GA0-15 = GA0-15


DD0-15 = DD0-15 (RAM)

Some other signals also have direct equivalence but don't have names in the TTL version (!WE0 & !WE1 for RAMs, etc.)

Obviously there are few differences:
- TTL version can address only 32/64k for sample ROM (A14/A15 are used for chip selection). No banking.
- Clock circuitry is all external in the TTL version.

But we'll see that in the next part...

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