Marine VHF Audio and Signalling Processor
CMX885
The CMX885 generates its own internal data clock and converts the binary data into the appropriately
phased frequencies. The binary data is taken from Tx DATA 1 register ($CA), most significant bit first.
In Tx Raw mode (En_RAW=1) the host must supply all data to be transmitted by the modem via the
TxData register ($CA) in the correct order and format to conform to the DSC standards.
Setting the Modem Enable bit $C1:b2, will enable the DSC modem, which will then transmit the data
supplied by the host through the Tx Data 1 register, $CA. The Modem Configuration register $C7:b10
should also be set to enable/disable Raw data mode, as appropriate.
In Tx Formatted mode (En_RAW=0), the modem will automatically transmit the dotting pattern followed by
the phasing sequence and then encode the data presented at the Tx Data 1 register ($CA) with the
correct checksum bits and then transmit it in the correct position with automatic repetition to conform to
the time diversity requirements of the standard. Data in the Tx Data 1 register is presented as two 7-bit
characters, in the case where an odd number of characters needs to be sent, then the un-used character
should be set to 0000000 and b7 set to 1. The final characters sent by the host should be a valid “End-of-
Sequence” character followed by the “Error Check” character.
Valid DSC characters should be supplied by the host through the Tx Data 1 register, $CA. Two characters
can be loaded in the same C-BUS write operation. The modem will begin transmitting the dotting
sequence followed by the phasing sequence as soon as the Modem Enable bit $C1:b2 is set. The format
of the data supplied by the host is similar to the Rx format:
A A B1 B2 B3 B4 B5 C D1 D2 D3 D4 D5 E1 E2 F1 F2 F3 G1 G2 G3 H I
There is no facility for automatically generating continuous dotting (preamble), Y data (all 1’s), B data (all
0’s) or phasing (synchronisation) sequences internally, however, the modem will continue to transmit the
last data loaded into the TxData1 register, so only a single data load is required.
7.8
NOAA/NWR SAME and WAT Decoding
A data decoder and tone detector suitable for use with the NOAA’s NWR (NOAA Weather Radio) system
is provided in the device. Full details of the system are publicly available from the NOAA website at
http://www.nws.noaa.gov/nwr/ and the CMX885 provides support for the WAT detection and SAME
decoding. It is possible to route the signal input to either Input 1 or Input 2 so that NWR monitoring can be
performed in parallel with existing radio operations (subject to suitable RF sections being provided
externally).
See:
o
o
o
o
o
NWR Status and Data – $BB read
Mode Control – $C1 write
Status – $C6 read
Modem Configuration – $C7 write
Interrupt Mask – $CE
The NWR SAME data message consists of six possible elements in the following sequence:
1
2
3
4
5
6
Preamble
Header Code
Warning Alarm Tone/Attention Signal
Voice Message
Preamble
End Of Message (EOM)
Elements 1, 2, 5 and 6 will always be transmitted in an NWR SAME message and repeated three times.
Elements 3 and 4 may or may not be transmitted depending on the specific type of message or its
application.
The coded message is transmitted, using Frequency Shift Keying (FSK), in the NWR audio channel. In
this application and the implementation currently used by the FCC EAS, it is more accurate to refer to the
code format as Audio Frequency Shift Keying (AFSK). It is transmitted at no less than 80% modulation
(± 4.0kHz deviation minimum, ± 5.0kHz deviation maximum).
© 2010 CML Microsystems Plc
27
D/885/3
CMLMICRO [ CML MICROCIRCUITS ]
