From the AltaCore-ARINC manual (for ARINC Devices):
Single-Ended Discrete Status Register: 0x00000080
This word shows the input status of the bi-directional avionics discretes. A one for a given bit indicates that the discrete is in-active and a zero indicates the discrete is active. Bit0=Discrete1, Bit1= Discrete2, etc.
NOTE: The input voltage should not exceed 30 Volts. Exceeding this rating can damage the board.
Single-Ended Discrete Output Register: W: 0x00000084
This word is used to control the output state of the bi-directional avionics discretes. Discrete outputs can be driven low (Active) by setting the corresponding bit in this register to a one. Bit0=Discrete1, Bit1= Discrete2, etc.
NOTE: The bi-directional Avionics Discretes can sink up to 1 Amp when active. Exceeding this rating can damage the board.
If wired correctly, you would just use the Single-Ended Discrete Status Register to monitor which of the 8 available you have designated as inputs for TRUE/FALSE (SET/CLEARED) (0/1).
To control discrete outputs, you would perform a masked-write to the appropriate bits in the Single-Ended Discrete Output Register.
From the AltaCore-1553 manual (for 1553 Devices):
Single-Ended Discrete (SDISC) Status Register: 0x00000060
This word shows the input status of the bi-directional avionics discretes. A one for a given bit indicates that the discrete is in-active and a zero indicates the discrete is active. Bit0=Discrete1, Bit1= Discrete2, etc.
NOTE: The input voltage should not exceed 30 Volts. Exceeding this rating can damage the board.
NOTE: For MIL-STD-1760 External RT Address Discrete usage, one of the SDISC lines is reserved for “RT Enable.” Please see your hardware manual for pin-outs for 1760 discretes and this enable line. If this line is not pulled-down (active), then the PE will not automatically use the other 6 SDISC lines (RT Address and Parity) to obtain the RT Address line. Please see the External RT Address discussion in the AltaAPI manual (PDF Bookmark: Single RT and Multiple RT Configurations) and contact the factory for the application note discussing External RT Address and 1760 usage.
Single-Ended Discrete Output Register: W: 0x00000064
This word is used to control the output state of the bi-directional avionics discretes. Discrete outputs can be driven low (Active) by setting the corresponding bit in this register to a one. Bit0=Discrete1, Bit1= Discrete2, etc.
NOTE: The bi-directional Avionics Discretes can sink up to 1 Amp when active. Exceeding this rating can damage the board.
