The NM-251D-DTM acquires the NMEA-0183 sentences from the first input port only. For every four completed NMEA-0183 strings that receives the NM-251D-DTM produces and transmits the following sentence:
The $GPDTM indicates to the connected systems such as ECDIS and VDR systems that the datum used is the WGS84. All the inputted strings and the generated Datum sentence are then transmitted to the five outputs and the RS232 port (pass thru function).
Input Ports (listeners)
The NM-251D-DTM uses one of the two input ports. The input port Inb is not available at the NM-251D-DTM 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-DTM 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.
Output Ports (Talkers)
Each of the four general purpose talker ports can output NMEA sentences in both RS422 and TTL signal levels, depending on the connection topology chosen (see figure 1, 2 and 3), and can fan out one instrument. Current drawn from all port can be up to 40mA (maximum rating 60mA @ 24VDC power supply) simultaneously, efficient enough to drive any NMEA compatible instrument.
The RS-232 port can deliver NMEA sentences to any modern computer running the appropriate software on Windows 98 and above. The RS-232 port is not optically isolated and should be used for testing purposes only to avoid DC leakage. It can be used though to interface a computer system if proper isolation is achieved or if the computer is supplied from an isolated power source.
The Out5 port has two functionality modes that can be selected via jumper J6 located inside NM-251D-DTM (see figure 4). By shorting poles 1-2 on J6, the Out5 converts signals received from computer to RS-422 signal level. This functionality is independent from the rest of the NM-251D-DTM circuits. By shorting poles 1-3 on J6, Out5 is configured as a fifth output of the NM-251D-DTM. Further details on how to configure the functions of the NM-251D-HDT are shown on table 1.
The NM-251D-DTM can be connected as shown in figures 1,2 and 3 using all RS-422 outputs (figure 1), all TTL outputs (figure 2) or using either the RS-422 or TTL output of each port taking care not to connect the same port for both signal level outputting (figure 3).
Installation and Operation manual