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.