WiringPi includes a command-line utility gpio which can be used to program and setup the GPIO pins. The I2C, SPI and UART interfaces can also be used as general purpose I/O pins when not being used in their bus modes, giving a grand total of 8 + 2 + 5 + 2 = 17 I/O pins on the P1 connector (plus 4 more on the P5 connector on a Revision 2 Pi) and 28 I/O pins on the B+ and version 2 and 3 boards (Although 2 are reserved for the HAT I2C interface, but can be used as normal GPIOs if not using a HAT board) The model Zero-W is adds on-board Wi-Fi, Bluetooth and the Pi camera connector to the existing model Zero board.The model B v3 features a quad-core Arm A8 processor (64-bits) with the same RAM and GPIO as the model 2, however it also features on-board Wi-Fi and Bluetooth.40-pin GPIO connector and very little else. The model Zero is a souped-up (1GHz) cut down Pi A+.

The model B v2 features a quad-core Arm A7 processor with 1GB of RAM.

The model A+ and B+ Raspberry Pi’s represents 2 years of research, development and testing and now features a single 40-pin GPIO connector with 28 usable GPIO pins and 4 USB sockets.

The model A which is essentially the same as the model B v1.1 but without the USB hub and ethernet connector.

The model B, Revision 1.1 Raspberry Pi has an additional 4 GPIO lines on a separate connector which you have to solder onto the board.

The original model B with the 26-pin GPIO connector.

Over the years there have been some updates: Additionally there is a 2-wire I2C interface and a 4-wire SPI interface (with a 2nd select line, making it 5 pins in total) and the serial UART with a further 2 pins.

Two of these pins (on 40-pin Pi’s, just one on 26-pin Pi’s) can be designated for hardware PWM output too. There are 8 general purpose digital I/O pins – these can be programmed as either digital outputs or inputs.

The original Raspberry Pi Model A and B version B1 was a $35 single board computer with a 26-pin General Purpose Input/Output (GPIO) connector and this carries a set of signals and buses.

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