Radio interface
The raison d'être of the M48 device is to insulate the user from the peculiarities and limitations of the
radio module's audio path. While a detailed analysis of the over-the-link signals is not intended, a basic
overview may be useful:
Three signals connect the modem chip to the radio: tones_out, tones_in and n_txe.
All are digital levels (so connection must be made to the digital baseband pins of the radio, not the low
level analogue connections).
At the bit-level the radio link signal generated by the M48 is a proprietary biphase (synchronous)
bitstream with a maximum frequency component of 3.2KHz. There is a pseudo-random data whitening
function applied to the data, but the code still does not have a perfect 50:50 mark space balance, so
should be considered a DC coupled signal. At the receiver end a conventional averaging data extractor
will function but a quasi-dc coupled (peak detecting) type, such as the Radiometrix QR96, will give
optimum performance.
The data stream is further formatted into packets. Each packet contains a framing sequence, address
and status information, a CRC and 1-16 data payload bytes. Transmitter on-time delay is allowed for by
the addition of a preamble sequence, while during continuous data transmission the packets are
transmitted end-to-end.
[ Unless otherwise programmed, all units send on address 1 and receive on address 0 (broadcast) and
address 1 ]
M48 also has an auxiliary serial output (only) pin, which is used to allow the user to send serial programming
strings to the associated radio module through the device.
Programming the M48A
In the simplest case an M48 can be used 'as supplied' as a straight forward transparent data link. There are,
however, a number of further options and features which require the user to program parameters into the
device.
Normal transparent data mode is referred to as 'data' mode. To program the device, it is necessary for it to
be in 'setup' mode.
The device is programmed through the RS232 port (without needing extra 'pgm mode' switching pins or
extra hardware). The command set uses simple readable ASCII words, and decimal or hex numbers. All
commands are in upper case.
When in setup mode the usual radio data communication functions are suspended, but limited one-packet-
at-a-time non-transparent operation is possible, via special commands.
The setup mode is activated by means of a 'break / respond' protocol
entering setup :
user generates a 'break' (long 'space' condition)
M48 sends two characters, an @ and a random printable ASCII character
user sends back the random character
M48 replies with a login message “M48 SETUP” and enters setup mode
There is a 5 second timeout following the break within which the response must be sent , or the device
reverts to data mode. An ASCII 1 (ctrl A) resets the timeout and generates a new @ and random character
sequence.
[ 'Break' conditions can be generated in several ways. Some terminal emulators have alt or ctrl key
sequence. Dedicated hardware can be used to force a long (50-100mS) logic zero onto the TXD pin. Or an
ASCII zero can be sent at a much lower baud-rate ]
Once in setup mode, input bytes are stored in a 32 byte (maximum) command buffer and echoed back to the
serial output. Space characters are echoed back, but are not loaded into the buffer.
Commands are either short words or single command characters. (For example, a <cr> character is the
evaluate/process command). After successful execution of a command the device will reply with an 'OK'
message. Commands which program a value into the internal non-volatile E2PROM take around 50mS for
the write cycle,
Radiometrix Ltd, M48A Apps board Manual
page 5
RADIOMETRIX [ RADIOMETRIX LTD ]
