Actisense NMEA 0183 User Manual download pdf (Page 20)

Port Conguration tab

The Port conguration tab of the main window allows the user to select the priority and Baud rates of the NMEA and

USB / RS232 input ports of the NDC-4 hardware, plus other useful features.

Figure 12 – Port Conguration tab

Why is there a need for Port Priorities?

Each of the ve NMEA input ports can transfer anything

up to 480 bytes of data per second, making a total of 1920

bytes/second. The data combined output of the NDC is

also forced (by the NMEA 0183 standard) to send a

maximum of 480 bytes/second. Therefore, there exists the

distinct possibility that there could be more data to send

out than is possible to achieve. If this situation occurs the

port priority system comes into operation.

This system stipulates that the highest priority input port

will have the highest number of time slots in the combined

output. The next lowest priority port has less time slots,

down to the lowest priority input port that has only a few

time slots. Each time slot allows one NMEA 0183 sentence

to be sent until either no data remains (the usual case),

or the transmit capacity has reached its maximum of 480

bytes/second.

If the rate of data in to the NDC hardware continues at

this very high level, then eventually some sentences (from

the lowest priority channel rst) will have to be dropped.

If however, the rate drops below the maximum, then the

sentence data will be sent at the next opportunity.

How to change port priorities and Baud rates

The port conguration tab consists of a vertical port

priority list, a Baud rate pull-down list for each NMEA /

USB / RS232 port, the P-code enable toggle control, and

Get from Hardware, Reset to defaults and Send to

Hardware buttons (refer to Figure 12).

Input Port Priority list:

Click on two inputs in turn to change their positions in the

priority list. The topmost item has the highest priority and

the item at the bottom, the lowest.

RS232 or IN/OUT 0 or USB Baud rate lists:

Select the required Baud rate for the appropriate port(s).

Please note that the NMEA 0183 specication restricts

the Baud rate for NMEA data to 4800 Baud. Therefore,

any input port that has an NMEA device attached to it that

cannot modify its Baud rate must remain at 4800 Baud

(otherwise no data will be received). However, some

NMEA devices (e.g. GPS units) do allow for the Baud

rate to be increased to 9600 Baud, or even 38400 Baud

(Hi-Speed NMEA). Only in these cases should the Baud

rate of the appropriate port be changed from 4800 Baud to

match that of the attached device.

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Actisense NMEA 0183 User Manual Page 20

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