CPU pinout

Pin out

        .--\/--.
 AD1 <- |01  40| -- +5V
 AD2 <- |02  39| -> OUT0
/RST -> |03  38| -> OUT1
 A00 <- |04  37| -> OUT2
 A01 <- |05  36| -> /OE1
 A02 <- |06  35| -> /OE2
 A03 <- |07  34| -> R/W
 A04 <- |08  33| <- /NMI
 A05 <- |09  32| <- /IRQ
 A06 <- |10  31| -> M2
 A07 <- |11  30| <- TST (usually GND)
 A08 <- |12  29| <- CLK
 A09 <- |13  28| <> D0
 A10 <- |14  27| <> D1
 A11 <- |15  26| <> D2
 A12 <- |16  25| <> D3
 A13 <- |17  24| <> D4
 A14 <- |18  23| <> D5
 A15 <- |19  22| <> D6
 GND -- |20  21| <> D7
        `------'

Signal description

Active-Low signals are indicated by a "/".

• CLK is the 21.47727 MHz (NTSC) or 26.6017 MHz (PAL) clock input. Internally, the real clock (φ0) is derived by dividing the input clock frequency by 12 (NTSC 2A03) or 16 (PAL 2A07).
• AD1 (both pulse waves) and AD2 (triangle, noise, and DPCM) are the Audio Out pins.
• Axx is the address bus and Dx the data bus.
• OUT0-OUT2 are output pins used by the controllers ($4016 output latch bits 0-2). These 3 pins are connected to either the NES's or Famicom's expansion port, and OUT0 is additionally used as the "strobe" signal (OUT) on both controller ports.
• /OE1 and /OE2 also go to the controller ports, and each enable the output of their respective controller, if present.
• R/W is the read/write signal, which is used to indicate operations of the same names. Low is write.
• /NMI and /IRQ are the two interrupt pins. See the 6502 manual for more detailed explanation.
• M2 can be considered as a "signals ready" pin. It is a modified version the 6502's φ2 that allow for slower ROMs.
• TST (tentative name) (pin 30) is special: normally it is grounded in the NES, Famicom, PC10/VS. NES and other Nintendo Arcade Boards (Popeye and Donkey Kong 3). But if it is pulled high, extra diagnostic registers to test the sound hardware are enabled from $4018 through $401A, and the joystick ports $4016 and $4017 become open bus.