Pin usage for unit testing (GPIO layout):

ADC = MCP3008 Analog to Digital Converter
DAC = MCP4922 Digital to Analog Converter
SR  = 74HC595 Shift Register

18  - NC (used for base pin and interrupt tests)
9   - ADC 12 
10  - ADC 11, DAC 5
11  - ADC 13, DAC 4
12  - DAC 3 (CS)
16  - SR 12 (LATCH)
20  - SR 11 (CLK)
21  - SR 14 (DATA)
26  - ADC 10 (CS)
27  - PUD/Interrupt testing 

