Famicom - 60 pins
NES - 72 pins, Lockout chip in all licensed carts and lockout defeaters in all unlicensed carts
The Famicom and NES cartridge connectors provide the same signals with some exceptions. The Famicom has an audio input pin and an audio output pin, see the next section.
The NES front loader has ten pins that run from the cartridge to the expansion port. It uses four pins for the lockout chip and there is a pin that carries the 21MHz master clock to the cartridge.
The NES top loader does not connect the lockout chip pins and does not physically have pins for what would be pins 18, 19, 54 and 55. The Everdrive N8 and NES PowerPak use pin 54. One of the remaining pins (pin 51 has been suggested) can be connected with a 1.2K resistor and the audio output point.
The Game Genie uses a cartridge connector that is slightly thicker than a regular NES cart. This was advantageous in a Front Loader because the pins on the NES connector could be bent back by the pushing down motion required to get a game PCB's contacts to connect with both sides of the cartridge connector. In a Top Loader, it is not, and the Game Genie's board requires tremendous force to insert in a Top Loader. There was an adapter made for the Game Genie that would allow it to fit in a Top Loader without trouble, but they are extremely rare.
Cartridge Design and Boards
Famicom - Nintendo's own cartridge shells come in a variety of colors and are typically shorter than NES cartridge shells. They are held together by internal plastic tabs. Some official third party companies (Konami, Sunsoft, Namco, Taito, Irem, Jaleco and Bandai) had the right to manufacture their own cartridges and used their own boards and shell designs.
NES - All licensed cartridges come in dark gray and are held together by five screws (early releases) or three screws and two tabs on the top. Nintendo always manufactured the boards and the chips, but occasionally made exceptions in the US (Konami, Sunsoft, Virgin Games, Acclaim) The screws were originally regular slotted screws, but Nintendo changed to using security screws. Unlicensed cartridges used their own boards and cartridge shell designs.
Famicom - Expansion audio supported
NES - Expansion audio not supported (mod required)
The Famicom routes its internal audio to the cartridge slot on pin 45. Most cartridges do not generate sound, so they simply send it back to the system by connecting pin 45 to 46, where it is sent straight to the output circuitry in the console. Cartridges that do generate sound, like Akumajou Densetsu and the Famicom Disk System RAM Adapter, mix their audio with the Famicom's audio, then send it back to the system.
The NES routes its internal audio to the output circuitry, there is no audio output and input pins on its cartridge connector. The cartridge connector does have extra pins that could be used for this purpose, but the audio will be routed to the expansion port on the front loader, where it can be mixed with the internal NES audio. There is no expansion port on the top loader and some of the expansion port pins are unconnected, requiring a different mod.
Famicom, Famicom AV & NES Top Loader - Top Loading (Famicom has eject lever)
NES Front Loader - Front Loading
The front loading mechanism was probably the second biggest mistake Nintendo ever made (the Virtual Boy being the first), as its spring loaded mechanism and design that required the cartridge to bend pins would cause the connector to eventually become unreliable.
Famicom - 10VDC, 850mA, negative tip (Famicom AV didn't come with an adapter).
NES - 9VAC, 1.3A, polarity immaterial (DC conversion occurs inside system)
A Famicom, Sega Master System or Sega Genesis/Mega Drive Model 1 power adapter will work in any NES, the plug is the right shape and DC is unaffected by a DC converter. The NES power adapter must only be used with a NES, even if it will fit other systems.
Famicom, NES Top Loader - None
NES Front Loader - Lockout chip
Unlicensed cartridges for Famicoms require nothing more than a licensed cartridge. For the NES, all licensed carts included a lockout chip that communicated with the chip in the console. If the connection was broken, then the game would constantly reset. All unlicensed NES carts either cloned the lockout chip (Tengen) or used circuitry to try to defeat the lockout chip.
Camerica cartridges have a switch on them to enable the lockout defeating mechanism. Position A enables the lockout defeat, position B disables it. You should always set the switch to Position B when using Camerica cartridges on a Top Loader.
Video and audio output
Famicom - RF (Japanese Channel 1-2)
NES Front Loader - RF (US Channel 3-4) & RCA AV Composite
NES Top Loader - RF (US Channel 3-4) (and AV, but extremely rare)
Famicom AV - Nintendo Multi-out Composite Video & Audio (can be used with Japanese or US RF adapter)
The US RF modulators broadcast on Channel 3 (60-66Mhz) and Channel 4 (66-72MHz). Japanese RF switches broadcast on Channel 1 (90-96MHz) and Channel 2 (96-102MHz). The US reserves those frequencies for FM radio stations (frequency spectrum 88-108MHz). The old TVs with dial controls, which were still used quite frequently in the 1980s, went from 2-13 VHF and 14-83 (UHF). However, in more modern TVs, some will support the Japanese channels on Channel 95 or 96 and can add channels from 84-158, covering the whole UHF frequency range and more of the VHF frequency range.
The Famicom RF switch-box is not auto-switching, it has a TV-GAME switch on the back of the console which functions like the switch on an Atari-style switchbox. The NES RF switch-box does not have a TV-GAME switch, its switchbox will automatically change the input when the console is turned on or off.
Famicom - Hard-wired to console, 3' cord, microphone on controller 2
NES Front Loader - Rectangular-shaped, start and select on controller 2, 6' cord, detachable
NES Top Loader - Dogbone-shaped, start and select on controller 2, 6' cord, detachable
Famicom AV - Dogbone-shaped, start and select on controller 2, 3' cord, detachable
Japanese households, especially in the cities, had substantially less square footage than US households. Nintendo did not believe the cables needed to be long for Japan, but changed their minds for the US.
Famicom - 15-pin Expansion Port
NES - Controller Port 2 (also Famicom AV if modded)
NES Front Loader - 48-pin Expansion Port, unused
On the Famicom, any peripheral connected to the expansion port unless it plugged into the cartridge port. This includes the Famicom Light Gun (Zapper), Arkanoid VAUS controller, the Bandai Family Trainer (Power Pad), two extra Controllers for four-player games, the Famicom Keyboard and Data Recorder. The Expansion port only brought most of the inputs, the three output lines and the two controller enables, audio input and IRQ, it did not serve as a general purpose expansion port.
The NES expansion port has all the functionality of the Famicom Expansion port and much more. It brings the data lines, the upper address line, audio in and out, video out, all the input and output lines, the two controller enables, the IRQ, NMI and the lockout chip clock signal. It also has 10 lines that go directly to the cartridge. With an appropriate RAM cart, this could have been used to implement a NES version of the Famicom Disk System. Unfortunately it was never used in products that saw a release (and neither were the SNES and N64 expansion ports outside of Japan). The expansion port can be used to mix audio from Famicom cartridges with the internal NES sound by connecting two pins (pin 3 and typically pin 9) with a 47K resistor. The expansion pin must also be connected on the cartridge converter for real Famicom cartridges.
The NES controller ports bring three input lines for controllers to use. By convention, all controllers (Zapper, Arkanoid VAUS, Power Pad) that require more than the controller input line use controller port 2. Both ports have all three lines. The Famicom AV, which has the same ports, does not bring the two extra lines to the controller ports, but by soldering two wires from the 15-pin expansion port, the correct inputs can be brought to controller port 2 (but NOT controller port 1).