4
For Australian version the following channel assignment is supplied:
Channel
Frequency (MHz)
150.825
150.850
150.875
150.900
150.925
150.950
150.975
151.000
151.025
151.050
151.075
151.100
151.125
151.150
151.175
151.200
151.225
N vlaue
0
1
2
3
4
5
6
7
8
/LOAD 00 6033
/LOAD 01 6034
/LOAD 02 6035
/LOAD 036036
/LOAD 04 6037
/LOAD 05 6038
/LOAD 06 6039
/LOAD 07 6040
/LOAD 08 6041
/LOAD 09 6042
/LOAD 10 6043
/LOAD 11 6044
/LOAD 12 6045
/LOAD 13 6046
/LOAD 14 6047
/LOAD 15 6048
/LOADTB 6049
9
10
11
12
13
14
15
16
17 - 65
in sequence starting at CH 17 (151.250)
Operating principle of internal modem
This device is a simplex link handling a 1200 baud asynchronous datastream (1 start, 8data, 1stop).
Buffering in the transmit end handles TX startup timing requirements, while the presence of sync
codes in the transmitted bursts prevents the receiver from outputting garbage in the presence of noise.
No error correction or retransmission of lost packets is supported. To operate with proper +/-12V RS232
levels an external buffer (such as a MAX232) will be needed
Physically the modem code resides in the PIC microcontroller which controls the radio functions. The
modem uses the internal hardware serial port of the processor.
Operation: Serial interface
Both transmit and receive modems implement a 32 byte software FIFO. At the transmit end this is
used to allow for the TX1M transmitter start up time (about 32mS), while on receive it buffers arriving
packets to the constant output data rate. All timing and data formatting tasks are handled by the
software. The user need not worry about keying the transmitter before sending data: the link is entirely
transparent.
For transmission across the link data is formatted into packets, each comprising 3 bytes of data and a
sync code. If less than 3 bytes are in the transmit end FIFO then a packet is still sent, but idle state
replaces the unused bytes. When the transmit end FIFO is completely emptied, then the transmitter is
keyed off .
Operation: Radio interface
Raw data is not fed to the radios. A coding operation in the transmit software, and decoding in the
receiver, isolate the AC coupled, potentially noisy baseband radio environment from the datastream.
The radio link is fed a continuous tone by the modem. As in biphase codes, information is coded by
varying the duration consecutive half-cycles of this tone. In our case half cycles of 500µs (a long, or L)
and 250µs (a short, or S) are used. In idle (or 'preamble') state, a sequence of Ls is sent (resembling a
1kHz tone).
A packet comprises the Synchronising (or address) part : LSSSSSSSSSSSSL followed by the Data part,
made up of twelve Groups (of four half cycles duration). Each Group encodes 2 data bits, so one byte is
encoded by 4 Groups. Although there are 16 possible states for a four half cycle group (from SSSS to
LLLL), only four of the possible states are used for valid data:
All other possible group combinations (except LLLL) are void, and result in the entire packet being
rejected by the receiver software as a noise artifact. Idle state tone (LLLL ....) is decoded as null data,
but does not void the packet, as a packet containing less than 3 bytes still needs to be decoded.
Radiometrix Ltd
TX1M/RX1M Data Sheet
page 8
RADIOMETRIX [ RADIOMETRIX LTD ]
