Jack J1 : Analog Trig IN (connect to trig out of a vintage drum machine)
Jack J2 : Analog Clock IN (24ppqn, SYNC24)
The CV trigger section allows you to connect the TRIG OUT of a vintage drum machine in order to control the arpeggiator clock. You access the trigger on the front panel through Jacks J1 & J2. Then a circuit made of Zener diodes limits voltage to +5.1V to protect the inputs of the arduino board.
The trigger must be an electrical short pulse signal between +5V and +15VDC (typical output of a TR808 or TR707). You can try with analogue audio but the envelop is certainly too long (even on rimshot or clave) ; you need a dirac.
On PCB prior to rev1.4 you need to use Dupont wires to connect the trigger circuit to the arduino.
With PCB rev1.4 you simply set jumpers on CV1 and CV2 to connect the Jacks J1 & J2 to the arduino (faster and easier).