Genesis/Megadrive expansion port pinout

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.

pinNameDescription-pinNameDescription
A01GNDGround-B01GNDGround
A02A19Address 19-B02/DISK1)
A03/CAS0--B03A01Address 01
A04D00Data 00-B04A02Address 02
A05D01Data 01-B05A03Address 03
A06D02Data 02-B06A04Address 04
A07D03Data 03-B07A05Address 05
A08D04Data 04-B08A06Address 06
A09D05Data 05-B09A07Address 07
A10D06Data 06-B10A08Address 08
A11D07Data 07-B11A09Address 09
A12GNDGround-B12GNDGround
A13D08Data 08-B13A10Address 10
A14D09Data 09-B14A11Address 11
A15D10Data 10-B15A12Address 12
A16D11Data 11-B16A13Address 13
A17D12Data 12-B17A14Address 14
A18D13Data 13-B18A15Address 15
A19D14Data 14-B19A16Address 16
A20D15Data 15-B20A17Address 17
A21/ROM-B21/LWR(P-SRAM)
A22/ASEL(/LO_MEM)-B22/UWR(P-SRAM)
A23/RAS2--B23/CAS2-
A24/FDC--B24FDWR2)
A25FRES--B25A18-
A26+5v+5v DC-B26+5v+5v DC
A27SR3Audio Out (R)-B27SL3Audio Out (L)
A28VCC IN3)-B28VCC IN-
A29SR2Audio In (R)-B29SL2Audio In (L)
A30GNDGround-B30GNDGround
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.

1) Presumably signalling the presence of a CD?
2) Function unknown
3) VCC IN - It appears the SGCD is capable of powering the main console through these pins. Power from them is routed directly to the console power switch through a pair of diodes.
 
 schematics/megadriveexpport.txt · Last modified: 2015/11/20 08:59 by lawrence
 
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