The Slave Plug is a small board containing a dedicated ATtiny85 micro-controller, which can be programmed through SPI. This chip is more limited than an ATmega, but it does have differential ADC with an optional 20x gain stage, making it possible to take around 55 mV full-scale ADC samples.
Two I/O pins of the ATtiny are connected to AIO and DIO, with a daisy-chain option. The rest are brought out to the side with a 6-pin header. By setting an appropriate jumper, the header becomes compatible with JeeNode port headers, meaning that the Slave Plug can act as a true slave processor and "off-load" some tasks while interfacing to various plugs.
The Slave Plug is aimed at advanced developers & makers - keep in mind that programming an ATtiny85 is more involved than working with an Arduino or JeeNode. See the Arduino Tiny library and this code for additional information.
The Kit version comes with the ATtiny85 chip, IC socket, and the other required components. All parts are through-hole — making assembly even easier than for a JeeNode kit. Note that no 6-pin headers are included.
The PCB only variant contains just the board for the Slave Plug. No parts, no headers.
As connectors you can use a Plug Headers set with 10 male and 10 female 6-pin headers, which makes it easy to connect multiple plugs and JeeNodes in a “standard” way. Or you can solder the board directly in place for more permanent use.
The documentation and full design details are freely available.