The NM-251D-VV is not a Speed Log unit and is not replacing a speed log if it is required by the class.
For Vessels that the speed log is not a requirement but is critical for the correct operation of systems like radars, ECS or any other application, the NM-251D-VV uses the SOG value and transmits data like a real Speed Log unit.
The NM-251D-VV is a member of the NM-251 Series family and is programmed to function as a speed over ground converter.
It converts the speed over ground data received from the GPS to the $VWVHW format.
The NM-251D-VV acquires the NMEA-0183 strings from the Ina port provided that they follow the 4.800/8/n/1 serial format. The NM-251D-VV will detect if the input string is the $GPVTG or $GPRMC, it will extract the speed over ground data value and will create and output to all five ports and the RS232 the $VWVHW completed string:
x.x: Speed in Knots
y.y: Speed in Km
The selection of the input string to be processed ($GPVTG and/or $GPRMC) is selected with the configuration of the SW-1 and SW-2 in the dip switch array as shown in the table 2 bellow.
The NM-251D-VV can relay or filter out all input NMEA-0183 strings with the configuration of the SW-3 and SW-4 in the dip switch array as shown in table 2 bellow.
Input Ports (listeners)
The NM-251D-VV uses one of the two input ports. The input port Inb is not available at the NM-251D-VV 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-VV 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-VV (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-VV circuits. By shorting poles 1-3 on J6, Out5 is configured as a fifth output of the NM-251D-VV.
The NM-251D-VV 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.
Installation & Operation manual