Quick Summary This document describes the architecture of the CDC3 Host Interface, where the platform for execution is a Linux box. ‘Interface’ is a physical USB connection between the Linux box and the CDC3 Control Processor.

The Host Interface implements a fairly standard packet exchange mechanism, where data packets of size LESS THAN 256 are exchanged with the CDC3 Processor.

serial_recv Module

The serial_recv Module accepts input packets from the CDC3 Processor, and then routes them forward to other modules to do the actual work that is requested.

From a programming perspective, input coming from USB is a simple byte stream. serial_recv takes this byte input stream and makes data packets out of them. The incoming byte stream looks like this:

        0xAA       Start of Packet
        0x57       Start of Packet
        Cmd        Cmd Byte
        Len        Len of Data only, 254 bytes max
        tag        a sequnce number
        csum       checksum
        data[0]
        ..
        ..
        data[len-1]

serial_recv will very slightly modify this input packet to produce a packet that will be forwarded on. Here is the version of the packet that is forwarded on:

        Cmd        Cmd Byte
        Len        Len of Data only, 254 bytes max
        data[0]
        ..
        ..
        data[len-1]

There are 2 destinations for the packets which have been received. The Cmd field is used to determine the packet destination.

• 1. Packet_Handler: data log packets, request type packets
• 2. Response_Handler: response to host query