Commodore 64 / 128 Cartridge Slot

44 pin female edge at the computer. The pins function as described below:

1 GND System ground. All four ground lines are usually tied together.
2,3 +5VDC 5-volt power supply to the cartridge. (Total user port and cartridge devices can draw no more than 450 mA.)
4 /IRQ IRQ. As long as this is low, it requests an interrupt.
5 R/W Read/write line. Reads when low, writes when high.
6 DOT CLOCK 8.18 MHz video dot clock input, for your own video control.
7 /I/O1 I/O 1 goes low when 64 detects use of $DE00-$DEFF; can be used with CP/M.
8 /GAME GAME replaces BASIC ROM with external cartridge ROM when grounded.
9 /EXROM EXROM replaces RAM $8000-$9FFF with cartridge ROM when grounded.
10 /IO2 I/O 2 goes low when 64 detects use of $DF00-DFFF.
11 /ROML ROML chip enable selects ROM $8000-$9FFF when EXROM is low; needs address bits A0-A12.
12 BA Bus available signal from the VIC-II chip. To use, pull DMA low. An external device can control the 64 while BA is high.
13 /DMA Direct memory access request line.
14-21 D7-0 D7 through D0. The data bus carries eight bits of data.
22 GND System ground
A GND System ground
B /ROMH ROMH selects external ROM at $A000-$BFFF (or $E000-$EFFF, for MAX) when GAME or EXROM is low; needs address bits A0-A12.
C /RESET RESET detects a positive voltage, resetting when rising from ground to +5 volts.
D /NMI NMI connects to 6510 Non-Maskable Interrupt line. It is spike sensitive - needs a pulse in either direction. Normally high, so many devices can signal NMI.
E S02 Phase 2 system clock. Essential for I/O timing, but not necessary for external ROM.
F-Y A15-0 Address bus (A15 - A0). The full 16 address lines are necessary for DMA.
Z GND System ground

Source: Commodore 64 User Manual, Programming the Commodore 64, Commodore 64 Programmer's Reference Guide