Adcon A440, Техническое справочное руководство

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60
Software
• After the header, a synchronization character is used; this is a hex
0xAA
byte.
The implementation must ensure 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 in-
formation assembled is the destination (
DST ) address: this is in order for all
the receiving 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
information 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 in-
cluding) 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 soft-
ware. The data frames recognized by the A440 and their answers are detailed in
“Data Frames” on page 61.
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 acknowl-
edge, 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 head-
er frames. When a frame is sent by the master for the first time, it will be one with a
long header; all the stations on the path of the frame participating in a certain trans-
action will relay the frame with a long header. After relaying the frame, the stations
will remain active for a time calculated as (in seconds):
The above scheme assures that as long as another frame will follow during this time
interval (addressed to a station that is known to be also active), the header sent to
that station will be a short one.
Delay Hops 1 + =