The Expander Plug is a small board with 8 general-purpose digital I/O lines. Each of the lines can be configured individually as input (with optional 100 kΩ pull-ups) or as output. The 2×6-pin connection carries the 8 I/O signals, +3.3V and ground, as well as the raw PWR voltage and the IRQ line.
An I2C bus running at 3.3V is used as interconnect, with all the power and signal lines brought out to to both sides of the board to allow daisy-chaining with other I2C-conformant boards.
The two solder jumpers on the board allow up to four Expander Plugs to be used together on the same I2C bus.
Furthermore, a small “Breadboard Connector” is included with each plug, to make it easy to hook the Expander Plug up to a breadboard.
The Assembled Expander Plug has the SMD-sized MCP23008 expander chip and decoupling capacitor already soldered onto the board. Each plug comes with a Breadboard Connector and four 4-pin male headers. 6-pin headers for this and all other JeeLabs plugs can be found here (straight) and here (right angle).
The PCB Only version contains just the printed circuit board for the Expander Plug. No parts, no headers.
The documentation and full design details are freely available at JeeLabs.