Serial interface – modem operation
To connect to a true RS232 device, inverting RS232-CMOS level shifters must be used. Maxim MAX232
or equivalent are ideal, but simple NPN transistor switches with pull-ups often suffice. With typical
microcontrollers and UARTs, direct connection is possible.
The Radio / data stream interface
A 32 byte software FIFO is implemented in both the transmit and receive sub-routine. At the
transmitting end this is used to allow for the transmitter start up time (about 3mS), while on receiving
end it buffers arriving packets to the constant output data rate. All timing and data formatting tasks
are handled by the internal firmware. The user need not worry about keying the transmitter before
sending data as the link is entirely transparent.
For transmission across the radio 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
codes replaces the unused bytes. When the transmit end FIFO is completely emptied, then the
transmitter is keyed off. . A link latency ( first byte in to tx to first byte out of rx) of 14mS is thus so
achieved.
Raw data is not fed to the radios. A coding operation in the transmit sub-routine, 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 device. As in bi-phase codes, information is coded by
varying the duration consecutive half-cycles of this tone. In our case half cycles of 62.5us and 31.25us
are used. In idle (or 'preamble') state, a sequence of the longer cycles is sent (resembling an 8KHz tone).
A packet comprises the Synchronising (or address) part, 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.
Figure 3: i9600 transmitting and receiving
The oscilloscope screen captures show a single byte being transmitted by a TDL2A module, which
incorporates an i9600 type modem controller. A BiM2-433-64 transceiver is used to capture the
transmitted data. The character appears on the serial data output (RXD) pin of the other i9600 after
about 12.5ms. Busy (STATUS) pin is momentarily set high to indicate the presence of a valid data in
the receive buffer of the i9600.
It can be clearly seen that unlike raw radio modules, i9600 does not output any noise when there is not
any transmission. Data fed into the TXD input of a i9600 appears at the RXD output of another i9600
within radio range in the original form it was fed.
Radiometrix Ltd
i9600 Data Sheet
page 3
RADIOMETRIX [ RADIOMETRIX LTD ]
