Commercial In Confidence
38
with other radios. The Communique CMD400 does not set these parameters independently,
with one exception the radio signal format in this radio is set to be the same as that of the
serial port even though the baud rates can be different. The exception is mode C where the
radio signal format did not include parity, if compatibility with this radio is required parity
must be disabled in the radio signal regardless of the serial port configuration. Later versions
of the CMD400 had an additional mode entitled “mode C plus parity” in which parity was
included, use of this mode did not give rise to the exception.
3.16.6
SYNCHRONOUS/ASYNCHRONOUS FORMAT
The radio signal format can be programmed for asynchronous or synchronous operation at
baud rates up to 1200. At baud rates of 2400 or more operation may only be synchronous.
In synchronous mode inverted NRZI encoding is used where a one is represented by a
transition in the binary data, every transmitted bit fits into a time slot defined by the baud
rate, this allows a phase locked loop to lock on to the data stream to give better performance
in noisy conditions, the inverted NRZI encoding allows this to continue even when the signal
is idling sending stop bits. The inverted NRZI encoding gives a further advantage with
GMSK signalling since the polarity of the signal is unimportant.
In asynchronous mode NRZ encoding is used where a “one” tone represents a binary one,
and a “zero” tone a binary zero, whilst each character consists of bits of equal duration
defined by the baud rate, the time between the end of a stop bit and a following start bit may
be arbitrary. This prevents the implementation of a phase locked loop to improve signal to
noise performance but does allow use within older systems that do not implement
synchronous transmission or NRZI encoding.
3.17
ERROR REPORTS
The modem reports errors in two ways, firstly the BUSY led will come on and the SYS led
will flash a number of times, the BUSY led will then go out again and if the fault persists the
procedure will be repeated. An error number can be determined by counting the number of
times the SYS led flashes while the BUSY led is on. Alternatively the error can be read by
monitoring the serial port using a PC comms program running at 9600 baud, 8 data bits, 1
stop bit and no parity. An "E" is output followed by the error number. Error numbers for both
modes are as follows;
ERROR No
FAULT
1
The position of the channel switches has changed.
2
A channel has been loaded that has no RX frequency
programmed.
3
Transmission has been attempted on a channel that has
no TX frequency programmed.
4
The receiver synthesiser phase locked loop has failed to
lock due to bad channel data or programming of an out
range
frequency.
5
The transmitter synthesiser phase locked loop has failed to
lock due to bad channel data or programming of an out
range
frequency.
6
The contents of the microprocessor's EEPROM are
