Another difference between the Rev3 and R4 WiFi is the number of their serial ports and I2C buses. The Rev3 has a single serial connection, shared by the USB port and the RX/TX pins.

Eight digital outputs act as the parallel interface. When a byte is received over serial, the individual bits are set on the individual digital lines connected to the printer’s parallel port.

The pcDuino3 nano is now available for $39. This tiny computer features an Allwinner A20 dual-core ARM Cortex-A7 processor with support for Android and Linux.