Programmable Peripherals
Programmable peripherals are microcontrollers used as I/O processors.
Some Microchip PIC microcontrollers have a Parallel Slave Port (PSP)
that can be designed into a more complex computer system as a
memory mapped peripheral.
The first example of this is a UART and A/D for a MC68EC000 single board computer.
I'm working on this:
The second example using the PSP is a SPI interface and A/D for a Rabbit 2000
system.
You can also think of devices connected via standard serial interfaces (RS232, RS422, etc) as peripherals. In this case there is a protocol and command set to communicate with the device.
An example of an RS232 peripheral is a MODEM.
Another example based on a PIC microcontroller provides:
RS232 interfaced 1-Wire & A/D & R/C Servo
4 R/C servos
Dallas Semiconductor 1-Wire interface
4 A/D channels.