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.

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.

Microcontrollers tend to consume other kinds of electronics. A project you might once have done with a 555 now probably has a cheap microcontroller in it. Music synthesizers? RC controllers? Most ...