Command Handling The DPU accepts commands via two interfaces – from the Spacecraft via the MIL-STD-1553 interface, and from the ICU via the SSI interface.  Any of the DPU commands can be sent from the SCU or the ICU.  The source for a command from the DPU is one of the following:
* ICU – command is generated and sent by the ICU via the SSI.
* SCU – command is generated and sent by the SCU via the 1553.
* G-ICU – command is generated on the ground and sent to the DPU via the ICU/SSI.
* G-SCU – command is generated on the ground and sent to the DPU via the SCU/1553.