How do I use avionics level discretes?

Each of the CEI-420/CEI-420A/CEI-520/CEI-620/CEI-820 discrete inputs has a circuit as shown in this figure:

Discretes

The CEI-420 has eight single-ended, unipolar discrete inputs. The CEI-420A has 16 single-ended, unipolar discrete inputs, eight of which share the same lines as the eight discrete outputs. The CEI-520/CEI-620 has up to sixteen single-ended, unipolar discrete inputs. The CEI-820 has 16 single-ended, bi-directional, unipolar discretes. Since discretes on the CEI-820 are bi-directional, they may be used as inputs or outputs. For example, you may choose to use discretes 1-4 as inputs and 5-16 as outputs. Only the first 8 discretes on the CEI-820 are available on the front panel (P2) connector, while all 16 discretes are available on the mezzanine (P14) connector. The discrete input specifications are shown in the following table: 

CEI-420/CEI-520/CEI-620/CEI-820 Discrete Input Specifications

Input voltage range   -20 volts to +100 volts continuous
Input threshold       Approximately 3.3 V. Because of the 
                      4.7K pull up resistor, the discrete 
                      switches high as long as the 
                      discrete input source does not 
                      source current near the threshold 
                      voltage. This allows 
                      compatibility with TTL type signals.

Each of the CEI-220/420/420A/520/620/820 discrete outputs has a circuit as shown in this figure:

Discretes

You control the discrete outputs on the CEI-220/420/420A/520/620/820 by using the AR_SET_CONFIG API routine to write a value to their control register. Each discrete output consists of Open Drain configured MOSFET switch (Drain connected to DISOUTn) with the gate control by the Discrete Output Control Register.

When the control bit is set to an "0"