This pinout is took a while to complete. As part of an effort to connect a Teradrive to a SegaCD unit, I had to trace the expansion port. This is it. Some pins are labelled by destination rather than signal, for example 87/315-5364. This means it runs to pin 87 of the IC 315-5364. Also, this refers only to the model 1 Genesis/MegaDrive. There may be minor differences in the model 2, for example the audio in the 2 does not require the patchcord, so I assume there's a small difference there.
Some of the pin names may sound a little odd, they're based on corresponding cartridge port pins. The majority of the pins on the expansion port, for example the data and address lines, connect to the same pins on the cartridge port. /C_OE is Cartridge Output Enable, which sounds a little odd on the expansion port, but I'm sure you'll adjust.
| pin | Name | Description | - | pin | Name | Description |
|---|---|---|---|---|---|---|
| A01 | GND | Ground | - | B01 | GND | Ground |
| A02 | A19 | Address 19 | - | B02 | /DISK | 1) |
| A03 | /CAS0 | - | - | B03 | A01 | Address 01 |
| A04 | D00 | Data 00 | - | B04 | A02 | Address 02 |
| A05 | D01 | Data 01 | - | B05 | A03 | Address 03 |
| A06 | D02 | Data 02 | - | B06 | A04 | Address 04 |
| A07 | D03 | Data 03 | - | B07 | A05 | Address 05 |
| A08 | D04 | Data 04 | - | B08 | A06 | Address 06 |
| A09 | D05 | Data 05 | - | B09 | A07 | Address 07 |
| A10 | D06 | Data 06 | - | B10 | A08 | Address 08 |
| A11 | D07 | Data 07 | - | B11 | A09 | Address 09 |
| A12 | GND | Ground | - | B12 | GND | Ground |
| A13 | D08 | Data 08 | - | B13 | A10 | Address 10 |
| A14 | D09 | Data 09 | - | B14 | A11 | Address 11 |
| A15 | D10 | Data 10 | - | B15 | A12 | Address 12 |
| A16 | D11 | Data 11 | - | B16 | A13 | Address 13 |
| A17 | D12 | Data 12 | - | B17 | A14 | Address 14 |
| A18 | D13 | Data 13 | - | B18 | A15 | Address 15 |
| A19 | D14 | Data 14 | - | B19 | A16 | Address 16 |
| A20 | D15 | Data 15 | - | B20 | A17 | Address 17 |
| A21 | /ROM | - | B21 | /LWR | (P-SRAM) | |
| A22 | /ASEL | (/LO_MEM) | - | B22 | /UWR | (P-SRAM) |
| A23 | /RAS2 | - | - | B23 | /CAS2 | - |
| A24 | /FDC | - | - | B24 | FDWR | 2) |
| A25 | FRES | - | - | B25 | A18 | - |
| A26 | +5v | +5v DC | - | B26 | +5v | +5v DC |
| A27 | SR3 | Audio Out (R) | - | B27 | SL3 | Audio Out (L) |
| A28 | VCC OUT | 3) | - | B28 | VCC OUT | - |
| A29 | SR2 | Audio In (R) | - | B29 | SL2 | Audio In (L) |
| A30 | GND | Ground | - | B30 | GND | Ground |
| Label | Description |
|---|---|
| CAS | Column Address Strobe |
| RAS | Row Address Strobe |
Shamelessly cribbed from http://www.gamesx.com/cartouts/gennyport.htm and the missing bits filled in.