Filtering / Conversion of NMEA sentences
A separate description is available from Brookhouse and on the website.
The main application of this feature is converting or filtering NMEA output data from the C-series.
The format of NMEA sentences can be altered to suit NMEA listeners that require a different
version of NMEA or entire sentences can be filtered out to remove redundant data.
Multiplexer model AIS-C has two separate data streams: 1 . The combined data stream of AIS
plus other NMEA talkers connected to NMEA IN CH1 – CH3 and 2. The speed conversion data
stream (NMEA output from the C-series @ 38400 baud converted to 4800 bps and available at
grey connector).
Filtering / editing can be used for either of the 2 data streams, but not at the same time. The
channel number in the first directive of the filtering/editing script, uploaded to the multiplexer,
determines to which data stream the entire script applies.
•
If the channel number is 5, the script applies to the speed conversion data stream. All
other directives in the script have to apply to channel 5 as well. If not, they will be
ignored.
•
If the first channel number is not 5, the script applies to channels 1-4. The channel
number in each directive specifies to which channel the directive applies or channel
number 9 means that the directive applies to all channels (ch1 – ch 4).
If you need a script for a particular purpose, ask Brookhouse support. There are a number of
scripts available that can be made available at no charge.
Note:
Filtering (removal of NMEA sentences) on the speed conversion channel is usually not
neccessary, as the NMEA sentences that are required in the C-series output can be individually
specified in the C-series.
However, the filtering/editing feature is very useful for modification of NMEA sentences. The C-
series output NMEA version 3.01. The sentence format can be changed to an older version of
NMEA, e.g. to 2.1, which is required by some auto pilots. The NMEA format can also be made
compatible with some older repeaters and other instruments.
Important:
Only select the NMEA sentences (during C-series setup) for output by the C-series,
that are necessary as input for other equipment. Do not select them all. For example, if only DSC
radio(s) are connected (via the mux speed conversion to 4800), only the GGA or RMC sentence
is required. For auto pilots, only the APB or APA sentence is required.Too many redundant
NMEA sentences can cause degredation of the performance of the multiplexer. Keep in mind
that data received from the C-series NMEA Out port at 38400 bps is converted to 4800 bps. This
is a bandwidth reduction by a factor 1/8. The total length in bytes of the transmitted sentences
should not exceed 400.
Although the transmission of output sentences can be controlled from the C-series, filter-
directives may be loaded in the mux as a precaution that redundant data is not processed.
Compact LCD
For the compact LCD (repeater), the same script rules apply as for filtering/editing, i.e. data from
either the combined data stream or the speed conversion data stream can be displayed. The
channel number in the first display directive determines which one:
•
If the channel number is 5, the script applies to the speed conversion data stream. All
other directives in the script have to apply to channel 5 as well. If not, they will be
ignored.
•
If the first channel number is not 5, the script applies to channels 1-4. The channel
number in each directive specifies to which channel the directive applies or channel
number 9 means that the directive applies to all channels (ch1 – ch 4).
