rx2m-458-5tx2i-458-5 Radiometrix Ltd, rx2m-458-5tx2i-458-5 Datasheet - Page 9

• Manufacturer Part Number: rx2m-458-5tx2i-458-5
• Description: Uhf Narrow Band Fm Multi Channel Radio Modules
• Manufacturer: Radiometrix Ltd

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 TX2M 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 500 s (a long, or L)
and 250 s (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.
The advantage of using the above coding technique:
1.Ease of decoding:
2. Inherent error detection:
3. Comparable spectral efficiency:
Radiometrix Ltd
Operating principle of internal modem
Unlike biphase, where a bit is coded as either a long half cycle or a
short cycle, here all half cycles are detected separately.
Although only 14 sync/address 'bits' (halfcycles) are used, the
following 48 'bits' (halfcycles) only code 24 real data bits, leading to
enhanced noise/error discrimination (the equivalent of 5 more sync
bits).
For a maximum transmitted baseband frequency of 2kHz, a 3 byte
packet is sent in 22ms. An equivalent biphase coded packet
(comprising 19 sync + 24 data + 3 null flags) at 2kbits/sec takes
23ms
TX2M/RX2M Data Sheet
page 9