Technical Manual

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Wireless Sensor Interface A720 (addIT™)
4.5.3. Generic Format of a Radio Frame
The standard frame format used by the A730 family (and therefore by the A720) is as
follows:
• The frame starts with a header of zeroes; there are two header types: long and
short. The long headers are used to wake-up a remote station and must be 140
bytes long, while the short headers are only 16 bytes long.
• After the header, a synchronization character is used; this is a hex 0xAA byte.
The implementation must assure that a 16-bit sync character is checked, i.e.
0x00AA, and not only an 8-bit 0xAA character.
• Following the synchronization pattern, the bytes are assembled by shifting the
bits one by one: each 8 contiguous bits will be “cut” into a byte. First informa-
tion assembled is the destination (DST) address: this is in order for all the re-
ceiving stations to know if they must assemble the whole frame, or they can
go back to sleep. Only the addressed station will remain active after this infor-
mation was decoded.
Note: The byte ordering convention used on the network is “big endian”, i.e. the MSB is sent first
and the LSB last.
• Next are the source (SRC) address and the length of the data field. The data
field follows, and the frame is ended by a 16-bit CRC field. The CRC is com-
puted starting with the first byte after the SYNC character until (but not includ-
ing) the CRC bytes.
The data field can transport various type of data frames. After being successfully de-
coded and checked, these data frames are passed to the upper layer of the software.
The data frames recognized by the A720 and their answers are detailed in the next
chapter.
A device will answer to the requesting device with the answer frame. The answering
device will poll the radio channel for an acknowledge; the acknowledge may be ei-
ther the same frame send further up the network (if the communication has hops, i.e.
routing stations in between the master and remote device), or an acknowledge send
by the master – the master sends only a short radio frame containing the SRC and DST,
both being its own Id. If the answering device does not receive the acknowledge, it
will repeat the frame after a one second delay (only once).
Another notable feature of the system is the way it handles the long and short header
frames. When a frame is send by the master for the first time, it will be a long header
one; all the stations on the path of the frame, participating in a certain transaction,
will relay the frame with a long header. After relaying the frame, the stations will re-
main active for a time calculated as (in seconds):
00 00
.......
00 0xAA DST-H DST-L SRC-H SRC-L DLEN
.......
DATA1 DATA2 DATAn CRC-H CRC-L
Data Frame
Delay Hops 1+=