Question:
I know that Expanded Device Type Code is LS 14 bit in Data Link Layer Specification (SPEC-018), 15th bit is Field Device Burst Mode bit, and 16th bit is Master Address bit.
But when the Expanded Device Type code is converted to binary, we must use 16 bits to represent this code. 15th and 16 bits are always 1...?!?
Please explain how to use the Expanded Device Type Code.
Answer:
See Subsection 5.3.2 of the Token-Passing Data Link Layer Specification (HCF_SPEC-081).
Unique ID is used by hosts to track the device through its life cycle. The Unique ID is composed of the Expanded Device Type (16 bits) and the Device ID (24 bits), a total of 5 bytes.
When a Master and device are communicating, they must mask the first two MSBs, insert the Master Address bit and Burst Mode bit into those two bits, to create the long frame address from the Unique ID. A device will recognize the message is for itself based on the lower 38 bits of the address. The Master Address bit is not changed by the device when it sends the response, therefore a Primary Master request and response both begin with "1" and a Secondary Master request and response both begin with "0". A device in Burst Mode must set the second bit to "1" in the response to a Master.
