Raymarine NMEA Multiplexer Spezifikationen PDF herunterladen (Seite 15)

Data Throughput

Depending on the configuration of the multiplexer, NMEA sentences are sent to one or more

outputs with different communication speeds. A situation could arise where more data is received

than can be transmitted because of the different speed settings of the inputs and outputs. Such a

situation leads to an overflow of the input queues of the multiplexer. When an NMEA sentence is

being received while a queue is full, this sentence is discarded. Only complete NMEA sentences are

stored and forwarded. This event is indicated by the red LED, which will blink briefly when a

sentence is discarded and the indicators in the Input Overflow section on the MPX-Config screen.

In its default factory setting, as shown below, the data flow through the MiniPlex is very

straightforward. First of all, the data that is received on the NMEA inputs is always sent to the

computer via the host interface(s).

In addition to this, the same data is also routed to Out2, as indicated by the checkboxes in the

section Input Settings. The section Output Settings shows that data from the computer (Host)

is routed to Out1 only.

This basic configuration allows you to combine data from navigation instruments and send it to a

computer using the Host port(s) and to a chart plotter connected to Out2 while the computer can

control an autopilot connected to Out1.

The default speed setting of the inputs and outputs ensures that there will be no queue overflows.

Please note that the default communication speed of Out2 is 38400 baud. Most chart plotters

support this high speed for reception of AIS data. However, if you want to connect this output to a

repeater display or a VHF radio, the speed of Out2 must be lowered to 4800 baud. If this leads to

excessive overflows, it is necessary to reduce the amount of data sent to this output. This can be

achieved by changing the default routing settings (disable certain inputs for Out2) or by using the

Sentence Filtering & Routing facility of the MiniPlex.

Now back to data throughput. The standard speed of an NMEA port is 4800 Baud or bits/second,

representing 480 characters per second. It is quite obvious that when four inputs receive data at

480 char/s and all of this data must be sent out again on an output with the same bandwidth of

480 char/s, a huge problem will arise. In the default configuration this problem does nor occur

because the speed of Out2 is set to 38400 Baud (3840 char/s). When Out2 is not used, set it to

the highest speed or disable routing of any of the inputs to Out2. When Out2 is connected to a

chart plotter, try to choose the highest speed the plotter will accept.

If data from more than one input needs to be routed to an output operating at 4800 Baud,

problems with data throughput can arise when too much data is sent to this output.

NMEA data tends to be sent in bursts. Some devices line depth sounders or wind instruments only

send one sentence per second while a GPS may send bursts of up to 13 sentences every two

seconds. While the average throughput over time may be low, an input might fill its queue quite

rapidly when a burst of sentences is received. The queues in the MiniPlex are quite large and may

contain up to 30 sentences of GPS data.

A couple of occasional blinks of the red LED over a period of a few seconds means that large bursts

of sentences are received and a queue is hitting its limit. Some sentences are discarded but most

of them will be passed without problems. Such a situation is totally acceptable and would mean

that for instance one depth, wind or position update is missed every few seconds.

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Raymarine NMEA Multiplexer Spezifikationen Seite 15