By Melair in Electronics — Jan 17, 2019

Sega Megadrive/Genesis Cartridge Pinout

I’ve recently been getting into collecting for the Sega Megadrive/Genesis, it’s the one console I had most contact with as a kid. As part of that I’ve been looking at building a tool for verifying cartridges (and maybe making them available like the Retrocade).

As part of that I’ve needed to understand the cartridge, and this is a living summary of my investigations.

Physical

The cartridge interface is a 32x2 card edge connector, with a 2.54mm pitch. It’s sides are designated as A and B, of which B is closer to the front of the Megadrive when plugged in.

Additionally, when looking at the front of the megadrive and the cartridge, pin 32 is to the left and pin 1 to the right.

Pin Out

This data has been gather from many different websites, which will be referenced below. All directions are from the Consoles perspective, output is to the cartridge, input is from the cartridge. Output / input pins are tristate, and direction will change for write operations.

Side A

Pin	Direction	Designation	Description
A1	Output	GND
A2	Output	+5V
A3	Output	Address Bit 8
A4	Output	Address Bit 11
A5	Output	Address Bit 7
A6	Output	Address Bit 12
A7	Output	Address Bit 6
A8	Output	Address Bit 13
A9	Output	Address Bit 5
A10	Output	Address Bit 14
A11	Output	Address Bit 4
A12	Output	Address Bit 15
A13	Output	Address Bit 3
A14	Output	Address Bit 16
A15	Output	Address Bit 2
A16	Output	Address Bit 17
A17	Output	Address Bit 1	This is the first address bit, bit 0 is hardwired to low.
A18	Output	GND
A19	Output / Input	Data Bit 7
A20	Output / Input	Data Bit 0
A21	Output / Input	Data Bit 8
A22	Output / Input	Data Bit 6
A23	Output / Input	Data Bit 1
A24	Output / Input	Data Bit 9
A25	Output / Input	Data Bit 5
A26	Output / Input	Data Bit 2
A27	Output / Input	Data Bit 10
A28	Output / Input	Data Bit 4
A29	Output / Input	Data Bit 3
A30	Output / Input	Data Bit 11
A31	Output	+5V
A32	Output	GND

Side B

Pin	Direction	Designation	Description
B1	Input	Left Audio	Audio sourced from cartridge, left channel, mixed into output.
B2	Output	/MRES	Master reset, from system start up.
B3	Input	Right Audio	Audio sourced from cartridge, right channel, mixed into output.
B4	Output	Address Bit 9
B5	Output	Address Bit 10
B6	Output	Address Bit 18
B7	Output	Address Bit 19
B8	Output	Address Bit 20
B9	Output	Address Bit 21
B10	Output	Address Bit 22
B11	Output	Address Bit 23
B12	Output	/YS	VDP is currently drawing the backdrop colour.
B13	Output	/VSYNC	Vertical sync pulse.
B14	Output	/HSYNC	Horizonal sync pulse.
B15	Output	EDCLK	External Dot Clock (~13.4 or 10.7 MHz).
B16	Output	/CAS0	Read or Write on $000000-$DFFFFF region.
B17	Output	/CEO	Chip Enable for the cartridge. Normally low when accessing $000000-$3FFFFF region. When expension unit is present then low when accessing $400000-$7FFFFF.
B18	Output	/AS	Address strobe, address on bus is currently valid and not changing.
B19	Output	VCLK	68K Clock
B20	Input	/DTACK	Data acknowledge to 68K.
B21	Output	/CAS2	Read or Write on $E00000-$FFFFFF region, maybe (Upper 2MB).
B22	Output / Input	Data Bit 15
B23	Output / Input	Data Bit 14
B24	Output / Input	Data Bit 13
B25	Output / Input	Data Bit 12
B26	Output	/ASEL	Read or Write on $000000-$7FFFFF region
B27	Output	/VRES	System reset, from front panel switch.
B28	Output	/LWR	Lower byte WRite, the lower byte on the data lines should be written to the location in the address lines.
B29	Output	/UWR	Upper byte WRite, the upper byte on the data lines should be written to the location in the address lines.
B30	Input	/M3	Pulled high in console, cartridge shorts to ground to indicate console should run in Mark 3 (Master System) mode.
B31	Output	/TIME	Set for r/w at/to $A13000-$A130FF, given the name suggests it might be for a real time clock in the cartridge. This would enable usage as a chip enable on a RTC, using fewer address lines or logic to that RTC. Used in Sonic 3 for SRAM.
B32	Input	/CART	Pulled high in console, cartridge shorts to ground to indicate presence.