Module driver 

Higher-level drivers built atop the codec and transport.

• acu — Access Control Unit (master).
• pd — Peripheral Device (slave).

Modules

acu

ACU (Access Control Unit) driver — issues commands to PDs and consumes their replies. Manages SQN cycling, REPLY_DELAY enforcement, retry, osdp_BUSY handling, and off-line detection.

pd

PD (Peripheral Device) driver — receives commands from the ACU and emits replies. A real PD also owns the SCS state on the slave side.