26
Data Throughput
A multiplexer is not the Holy Grail for connecting NMEA devices. It should be fairly obvious that if a
device combines data from four sources, the total amount of data that must be forwarded is the sum of
the amount of each source. Still, the NMEA standard specifically limits the communication speed to 4800
Baud or bits per second, which equals 480 characters per second.
So, when using a multiplexer, a situation could arise where more data is received than can be
transmitted because of the speed limitations of the NMEA outputs. Such a situation will lead to an
overflow of the input queues of the multiplexer.
When an NMEA sentence is being received while and a queue is filled up, this sentence is discarded
because the MiniPlex only forwards complete NMEA sentences. This event is indicated by a blink of the
red LED. The indicators in the “Input Overflow” section of MPX-Config will also show this event.
In its default factory setting, as shown in Figure 27, the data flow through the MiniPlex is very
straightforward: the data that is received on the NMEA inputs is always sent to the host interface(s).
Figure 27
In addition to this, the same data is also sent to Out2, as indicated by the checkboxes in the section
“Input
Settings”. The section “Output
Settings” shows that data from the computer (Host Data) is sent 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 at the same time 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 of Figure 27, this problem does not 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 is capable of.
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 like depth sounders or wind instruments only send
one sentence per second. A GPS however 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
