The NM-251D-HDT is a member of the NM-251 Series family of interfaces and is programmed to function as a heading data signal format converter. It enables conversion of the NMEA-0183 input sentence containing the heading data in a different format (e.g. $xxHDG, $xxHDM, $xxVHW etc.) to the following true heading format:
In addition it filters out all input NMEA-0183 strings except for the $xxROT and transmits the $HEHDT to all five RS-422 signal level outputs and the RS-232 port.
The NM-251D-HDT acquires the NMEA-0183 strings from the Ina port. Depending on the type and the format and the input signal the NM-251D-HDT will function as described below:
1. If the incoming string matches the configured via the dip-switches input format (see table 2 bellow), the NM-251D-HDT will extract the heading information contained and will retransmit it in a new heading true formatted string ($HEHDT,xxx.x,T*[CS]). In this manner the $xxHDG, $xxHDM, $xxVHW and the $xxRMC may be converted to the $HEHDT.
2. If the incoming string is the $xxHDT, it will override any configuration and transmit it unchanged.
3. If the incoming string is the $xxROT, it will transmit it unchanged.
4. If the incoming string does not contain heading data (e.g. $GPGGA, $IIMTW etc.), it will ignore it and will instead transmit the latest processed value of the heading data in a heading true formatted string. In this way the refresh rate of the output ports is increased.
Input Ports (listeners)
The NM-251D-HDT uses only one of the two input ports. The input port Inb is not available in the NM-251D-HDT custom interface and should not be connected. The listener port Ina is optically isolated as specified in NMEA-0183 protocol, thus data(-) pole should never been connected to NM-251D-HDT ground. If the instrument’s talker port is single ended, connect that terminal to the Ina(+) input and tide the Ina(-) to instrument’s ground. If an RS-232 level signal is connected, the ground pole (GND) should be connected to Ina(+) and the data pole (TXD) to Ina(-) respectively. The listener port Ina can acquire NMEA sentences carried in TTL, RS-232 and RS-485/422 signal levels.
Installation and Operation Manual