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MCP23017 port expander
An example of wiring a single MCP23017 to add 16 additional GPIO pins. This example is wired to give the MCP23017 a device ID of 0.
MCP23S17 port expander
An example of wiring a single MCP23S17 to add 16 addition GPIO pins. This example is wired so that the MCP23S17 operates on SPI port 0 with a device ID of 0.
MCP23S17 Chaining
An example of wiring multiple MCP23S17 to add 32 addition GPIO pins. This example is wired so that 2 MCP23S17's operate on SPI port 0 with a device ID of 0 for the first chip and a device ID of 7 for the second chip
NOTE: It is possible to chain 8 MCP23S17's together for a total of 128 GPIO pins per SPI Select. Since the RaspberryPi has 2 SPI Selects you can theoretically use 16 MCP23S17's for a total of 256 channels.
Raspberry Pi P1 Header
Credits: wiringpi.com
Raspberry Pi P5 header (r 2.0)
Updated