So update time, first things first, gotchas:
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The power switch on the back switches the neutral not the hot, thus when the sign is pluged in and turned off accidently bridging things to ground is exciting. Keep this in mind when poking around
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Most of the custom board has latching flipflops, that latch to ground when its off. This makes it fun to figure out which are actually data lines.
I have worked out most of the pinout on the 386 side and how it maps to the 31 pin ribbon cable to the sign board. Using this and the dos source code from Shanes link you can work out where the dma dumps into the sign. Basically, store all the pixels in memory (2 bits per pixel), dump that via dma to the registers on the custom board in the sign (ribon pins 2-16 I think), and repeat. Should be able to bitbang it as its only going at 2.5mhz or so. Still have to wrap my head around some of the stranger pins and address connections but its solveable.
So far this is my best guess as to the pinout of the ribbon cable between the 386 and the custom board:
format is ribon pin, isa pin, notes.
1 B1 x ?
3 B2 x ?
5 B6 50 khz pulse on startup - nothing on data
7 B1 B3 B10 B31 x ?
9 A11 Base frequency 66.7khz 6mz between data packets 5.8mz of packets 48 separate data groups 2.5ms within data group 1.6 us packets
11 NC x ?
13 B14 x ?
15 b15 high
17 b16 Random short pulses
19 b18 high
21 b17 Random short pulses
23 b20 2.5mhz clock
25 b22 ?
27 a22 ?
29 b26 high
31 nc Random short pulses
33 b31 x
2 A2 Data octal buffer
4 A3 Data octal buffer
6 A4 Data octal buffer
8 A5 Data octal buffer
10 A6 Data octal buffer
12 A7 Data octal buffer
14 A8 Data octal buffer
16 A9 Data octal buffer
18 a23 Data - ISA?
20 a24 Data - ISA?
22 a25 Data - ISA?
24 a26 Data - ISA?
26 a27 Data - ISA?
28 a28 Data - ISA?
30 a29 Data - ISA?
32 a30 Data - ISA?
34 a31 Data - ISA?
Helfpfull stuff is here: jvflife/jvflife.c at master · kmcallister/jvflife · GitHub for the dos code seeing how the DMA transfer is formated and set up
How the ISA dma chip works: Operating Systems Development Series
ISA pinout: Industry Standard Architecture - Wikipedia
