nuova marea ltd
marine electronics & design
Address: 26 Mirtidiotissis str, Kastella, 18533 Piraeus, Greece
Tel.: +30.210.4134628, +30.210.4134698, Fax: +30.210.4134814
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NM-251A
NMEA-0183 Multiplier
User Guide
V1.03
Introduction
The NM-251A is a two channel NMEA-0183
input / four or five channels NMEA-0183 output
multiplier. It enables data distribution from two
NM-0183 sources (primary and secondary with
priority encoding) to all NMEA-0183 instruments
through four or five talker ports and PC connection
through RS-232. Data from PC’s charting system
can be delivered back to the autopilot from the
same RS-232 port which acts bidirectional (talker
and listener).
Operation
The NM-251A acquires NMEA-0183 sentences from the most significant input and immediately sends
them to the general purpose outputs and the RS-232 output. A watchdog timer routine supervises if the NMEA
signals are correctly applied in the primary input and switches to the secondary input whenever there is no
NMEA sentence for at least eight seconds. In this case the secondary input starts receiving data if there is an
instrument attached. If not, the device automatically returns to the primary input after eight seconds. When the
secondary input is already in receiving mode and an NMEA signal is applied to the primary port, the device
immediately switches to the primary input. In the case that no signal is applied to any input, the device “circles”
around sampling the two listener ports every eight seconds until an NMEA-0183 signal appears to any of the two
listener ports.
In addition the NM-251A can deliver at the same time different NMEA signals connected to Ina and Inb
to output ports Out1/2 and Out3/4 respectively. To select this function user should turn the SW-1 to ON position
at the dip switch array SW1 (see figure 4) located inside the NM-251A. In this mode the user can select from
which input port signals will be delivered to RS232 interface by setting SW-2 to “OFF” position for Ina or “ON”
for Inb.
Input Ports (listeners)
The NM-251A has two input ports that can be connected to any instrument indented for marine use that
can output NMEA-0183 signals. Each one of the two listener ports is optoisolated as specified in NMEA-0183
protocol, thus data(-) pole should never been connected to NM-251A ground. If the instrument’s talker port is
single ended, connect that terminal to the Ina(+) or Inb(+) input and tide the Ina(-) or Inb(-) to instrument’s
ground. If a RS-232 signal level is connected the ground pole (GND) should be connected to Ina(+) or Inb(+)
and the data pole (TXD) to Ina(-) or Inb(-) respectively.
The two listener ports can acquire NMEA sentences carried in TTL, RS-232 and RS-485/422 signal levels.
