All the non-Veroboard Beasties have a common bus connector. Small peripherals can be connected directly to this, and more complicated expansion cards can be connected via a backplane. The backplane pin spacing is designed to allow both the backplane and expansion cards to be made on Veroboard, cutting production costs dramatically when only a few of each type of expansion card is required (as will typically occur in a small ubiquitous computing lab setup).
| 1 | PD0 (RXD) | 11 | PC0 (ADC0) |
| 2 | PD1 (TXD) | 12 | PB6 (CLK_IN) |
| 3 | PD2 (INT0) | 13 | !RESET (PC6) |
| 4 | PD6 (AIN0) | 14 | VCC |
| 5 | PD7 (AIN1) | 15 | GND |
| 6 | PC5 (ADC5/SCL) | 16 | —key— |
| 7 | PC4 (ADC4/SDA) | 17 | PB5 (SCK) |
| 8 | PC3 (ADC3) | 18 | PB4 (MISO) |
| 9 | PC2 (ADC2) | 19 | PB3 (MOSI) |
| 10 | PC1 (ADC1) | 20 | PB2 (!SS) |
When looking at a Beastie or expansion card PCB, the Beastie Bus should be on the right with pin 1 at the top. Pin 16 should be blocked off to act as a key.