Raymarine NMEA Multiplexer Spezifikationen PDF herunterladen (Seite 16)

A quite different situation may arise with some fluxgates or gyrocompasses. These devices may

send their heading sentences with a speed up to 40 sentences per second! Instead of queuing a

burst of sentences every one or two seconds, the multiplexer must queue a constant stream of

sentences, possibly utilizing the maximum bandwidth of the multiplexer. Such a situation can lead

to a queue that is constantly filled up to its maximum capacity. This in turn results in heading data

that can be up to 20 seconds old when it is forwarded, which is totally unusable for any autopilot to

steer on. Enabling the Real Time option for this input can solve this specific problem. This option

bypasses the queue entirely. Only one sentence will be stored now and sent out when the time slot

for this input arrives. Outside its time slot, incoming sentences will be discarded. A lot of heading

sentences will be lost now but the ones that are passed, are passed almost immediately. Hence the

name ‘Real Time’.

When the red LED is blinking severely or stays on almost

continuously, it is advisable to investigate which instrument or

input leads to this overflow. The MPX-Config utility will show on

which input the overflow occurs by a blinking indicator in the

NMEA Input Settings section. Opening the Statistics window

from the Tools menu will give an insight about the amount of data

in a queue and whether it is filled constantly or occasionally.

Some general rules are applicable for reducing overflow situations.

A simple rule of thumb is that an overflow can never occur if the

speed of an output is equal or higher than the combined speeds of

all inputs that are routed to that output. For example: if the

multiplexer is in its default configuration and all four inputs are set

to 4800 Baud, the minimum output speed equals 4 x 4800 =

19200 Baud. This rule is only a hard rule when the input

bandwidth is fully utilized i.e. an instrument is sending data

continuously. This is hardly ever the case. As mentioned earlier,

NMEA data is often sent in bursts, resulting in a much lower overall

bandwidth. It could be perfectly feasible to have a system with

four instruments connected to the multiplexer, while running all in- and outputs on 4800 Baud

without a single overflow.

There are several ways to resolve overflow situations:

1. Configure the instruments on the listener ports to send less data or with greater intervals.

GPS receivers can sometimes be configured for this.

2. Use the sentence filter of the multiplexer to block unwanted sentences. Unwanted

sentences are discarded immediately and do not occupy queue space or bandwidth.

3. For sentences that should not be blocked, setting a divisor in the sentence filter may lower

their rate. A gyro may be ‘throttled down’ to 10 sentences per second or even less. From

the GPS output, the rate of the sentences containing satellite information could be lowered

to once every 10 seconds instead of being output every time a position is output by the

GPS.

4. Use the routing options to select which input is routed to an output or use the routing

options in the sentence filter to selectively route NMEA sentences to an output.

5. Increase the speed of the NMEA output that causes the bottleneck. This will only work

when the connected equipment also supports higher communication speeds.

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