32
Care should be taken when selecting other speeds than 4800 Baud with respect to possible queue
overflows. See the paragraph “Data Throughput” on page 26 for more information.
Talker ID
The multiplexer can change the Talker ID of incoming sentences. The first two characters of an NMEA
sentence represent the Talker ID, indicating which instrument (talker) has sent this sentence. Normally
you don’t need to change this and these fields can be left blank (“--”).
Changing the Talker ID of a sentence might be necessary for instance when using a GPS compass. Such a
compass often outputs a true heading sentence starting with $GPHDT. Many instruments or navigation
programs however require a true heading that starts with $HE, which is the standard Talker ID for true
heading devices. By entering HE in the Talker ID field of the input connected to the GPS compass, the
incoming $GPHDT sentence will be changed into $HEHDT.
Setting a Talker ID can also be useful when similar instruments are connected to the multiplexer while
the navigation software must be able to distinguish between the data from these instruments. A
catamaran for instance could have a depth sounder in each hull, sending similar sentences to the
navigation software. Setting the Talker ID for those inputs to H1 and H2 respectively allows the software
to recognise both depth readings separately.
Proprietary sentences do not have a Talker ID and are thus unaffected by this option.
Enter the Talker ID in the edit box of the desired input and press the Enter to send the setting to the
multiplexer. A Talker ID may consist only of upper case characters and numbers. See the Technical
Reference section for an overview of the most commonly used Talker ID’s.
When the Talker ID is changed, the background turns yellow to indicate that de modified Talker ID has
not yet been sent to the multiplexer. Pressing Enter will send all Talker ID’s to the multiplexer and the
background colour will return to white again. It is possible to change all Talker ID’s first and press Enter
after changing the last one. Starting at the Talker ID field of input 1, pressing the Tab key will jump to
the Talker ID of the next input.
Clicking the “Read
Configuration” button will read the Talker ID settings from the multiplexer. Any
modified Talker ID’s that have not been sent to the multiplexer (yellow background) will revert to their
original values. To clear a Talker ID, simply clear the edit box(es) by selecting them with the cursor and
press the Del or Backspace key on your keyboard and finally press Enter. An empty Talker ID setting will
show two dashes.
Real-Time
The “Real-Time” option bypasses the queue of an input. All incoming NMEA sentences are stored in a
queue - one for each input. Each queue can store several NMEA sentences, waiting to be forwarded. A
GPS for instance can send up to 13 sentences at once every one or two seconds, and a queue can store
these sentences until they are processed and forwarded.
An electronic compass however, only sends one heading sentence each time but at a rate of up to 20
sentences per second. This high rate will fill up a queue rapidly with a lot of heading sentences. When
these sentences are retrieved from the queue to be forwarded, they may contain a heading that is
lagging several seconds behind the current heading – too much for a reliable indication or for an autopilot
to steer on. This situation will also be indicated by the overflow LED on the multiplexer and the indicators
in the “Input
Overflow” section of MPX-Config.
When the “Real-Time” option is enabled, the queue for that input is bypassed. Only one single sentence
will be stored, waiting to be forwarded almost immediately. The result is that some heading sentences
will be discarded but the ones that are forwarded have no significant delay.
This option should be used only for instruments that output one single sentence like an electronic
compass. If this option is enabled for a GPS, only the first sentence of an entire group will be passed, the
rest will be discarded.
To Output 1 & 2
These options allow you to set the default route from the NMEA inputs to the NMEA outputs. The default
route as shown in Figure 34 routes all inputs to Out2 and none to Out1. By default, the NMEA inputs are
always routed to the host ports.
The default route is applied to an input when no specific route for that input exists in the sentence filter.
A match in the sentence filter always takes precedence over the default route.
