Modulation Technique Used (Low Speed Modem); Generic Format Of A Radio Frame - Adcon A440 Technical Reference Manual

to sleep. The destination ID is positioned very early in the frame header (see also
"Generic Format of a Radio Frame" on page 59).
From the above it becomes clear that in order to initiate a communication, a re-
quester must send first a header which is at least 0.5 seconds long: these are called
long header frames. Of course, after the communication is established the headers
are short, of only 16 bytes (i.e. 8 msec. – called short header frames). If a timeout
occurs, the system will restart by sending long header frames.
4.4.2.

Modulation Technique Used (low speed modem)

The communication via radio is made by using a special MSK (Minimum Shift Keying)
scheme; both the encoding and the decoding of the MSK frames is made in soft-
ware—there is virtually no hardware modem. A zero bit is transmitted as a sequence
of 250 µs one level followed by a 250 µs zero level, while a one bit is transmitted as
a 500 µs one level followed by a 500 µs zero level. A complete sequence of one and
zero level forms a bit cell.
This modulation scheme is self-clocked. The transmission speed is content-depen-
dent, varying from 1 kbps (when sending only ones) to 2 kbps (when sending only
zeros). On the average, an 1500 bps transmission speed is reached.
The data interchange between stations is made by means of frames. The frames
have a header, a 16 bit-sync character, a data block, and a 16-bit CRC number. The
bytes forming the frame are send synchronously, with no start and/or stop bits. The
data block is assembled after the sync character was detected. The LSB of a byte is
sent first and the MSB is sent last.
4.4.3.

Generic Format of a Radio Frame

The standard frame format used by the A730 family is as follows:
00
0 0
00 00 0xAA DST-H
.......
• The frame starts with a header of zeros and 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.
Modulation Technique Used (low speed modem)
1
1
DST-L SRC-H SRC-L DLEN DATA1 DATA2
1
0
DATAn CRC-H CRC-L
.......
Data Frame
59