| Pin | Description |
|---|---|
| Digital 0 | Serial RX |
| Digital 1 | Serial TX |
| Digital 2 | RFM12B Interrupt (INT 0) - if installed |
| Digital 3 | Interrupt / Wake Up Alarm Output from MCP7941x - if installed |
| Digital 4 | Slave Select for 23K256 SOIC and SD Card - if installed |
| Digital 5 | Green LED |
| Digital 6 | Red LED |
| Digital 7 | UNI/O bus: MAC Address (11AA02E48) - if installed |
| Digital 8 | Slave Select for ENC28J60 Ethernet |
| Digital 9 | Slave Select for 23K256 SRAM (DIP) - if installed |
| Digital 10 | Slave Select for RFM12B - if installed |
| Digital 11 | SPI bus: Shared MOSI (Master Output, Slave Input) |
| Digital 12 | SPI bus: Shared MISO (Master Input, Slave Output) |
| Digital 13 | SPI bus: Shared Serial Clock (output from master) |
| Analog 0 | Spare and available for use |
| Analog 1 | Spare and available for use |
| Analog 2 | Spare and available for use |
| Analog 3 | Spare and available for use |
| Analog 4 | I2C Bus: SDA (data) |
| Analog 5 | I2C Bus: SCL (clock) |