Commit d33e923f authored by natalie's avatar natalie 💜

add some info about the speech eprom board

parent 2de1a61d
The A3 board contains the speech data EPROMs and relevant bus interface logic.
The board can support up to 8 27C256 EPROMs. On my specimen, only three are populated. All EPROMs share common address/data busses, /OE, Vcc/Vpp. The address bus is buffered through several channels out of three MC14053B buffer ICs. Data bus connected directly to the backplane connector, and seeing as Vpp is exposed on the backplane connector, this board was likely programmed after assembly.
On my specimen, /OE appears to be shorted to ground. I do not actually see where this is happening, as I can follow the trace across all the EPROM footprints and down to the backplane connector. It is possible that an EPROM has an internal short, or that the board was damaged during my reverse engineering efforts. In my case I just left it alone since /OE being stuck low is not a problem for dumping the data.
Both halves of a CD4556B are used to select EPROMs. Since my board only had the first three positions populated, I did not trace out the second half of the demux. I would assume this is just sitting on the CPU bus as standard address decode logic in the device, although the particulars don't really matter for externally dumping the roms. Standard input applies on the CD4556B - treat A and B as bits 0 and 1 respectively, which selects the appropriate rom.
Working left to right, bottom to top:
IC1 MC14503B
IC2 MC14503B
IC3 CD4069UB
IC4 MC14503B
IC5 CD4556B
IC6 MC14503B
IC7 MC14503B
Backplane pin numbers are specified as labeled on the connector shell.
| Signal | Backplane Pin |
| --- | --- |
| O0 | 16 |
| O1 | 7 |
| O2 | 6 |
| O3 | 58 |
| O4 | 4 |
| O5 | 5 |
| O6 | 21 |
| O7 | 20 |
| A0 | 74 |
| A1 | 73 |
| A2 | 75 |
| A3 | 76 |
| A4 | 77 |
| A5 | 78 |
| A6 | 68 |
| A7 | 69 |
| A8 | 67 |
| A9 | 71 |
| A10 | 72 |
| A11 | 70 |
| A12 | 51 |
| A13 | 52 |
| A14 | 53 |
| DEMUX_A | 54 |
| DEMUX_B | 55 |
| DEMUX_!EN | 57 |
| /OE (CPU /RD?) | 3 |
| Vcc | 1 |
| Vss | 40 and 80 |
| Vpp | 10 (needs to be double-checked) |
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