NM-251B
Speed Log pulses to NMEA-0183 Converter (User Guide) | 
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Introduction
NM-251B is a Speed Log ppm output to NMEA-0183 converter that enables connection of every speed log that uses the “closing” contact interface with digital systems and instruments that comply with the NMEA-0183 standards. The NM-251B is capable of converting the speed log contact closures to a standard NMEA $VWVHW string with checksum error control and output at TTL, RS-422 and RS-232 signal levels.
Input
NM-251B has one input port for connecting to the speed log that can be configured to interface both “closing” contact and optocoupler output type Speed Logs. Further details on how to configure the NM-251B are shown on table Configuration shall be done prior to connecting to the Speed Log or else damage may occur to the instrument.
General Purpose Output port
NM-251B has five general purpose output ports that are able of transmitting NMEA sentences in both RS422 and TTL signal levels, depending on the connection topology and each one can fan out one instrument. Current drawn from each output port can be up to 20mA, efficient enough to drive any NMEA compatible instrument.
RS-232 interface
The RS-232 port can deliver NMEA sentences to any modern computer running the appropriate software on Windows 98 and above provided that serial communication follows the 4.800/8/N/1 standard. This port is not optically isolated and shall not be used for other than testing purposes.
Connections
The NM-251B can be connected as shown in figures 1, 2 and 3 using RS-422, TTL or both signal level type outputs.
Figure 1
Connection diagram for NM-251B using only RS-422 outputs | 
Figure 2
NM-251B using only TTL outputs | 
Figure 3
Connection diagram for NM-251B using RS-422 and TTL outputs |
