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NM-251A-MUX
Dual Input NMEA Buffer Splitter With Multiplex Functions
User Guide
V1.01
Introduction
NM-251A-MUX is a two input channel
multiplexer with five output channels NMEA-0183
data multiplier. It enables combining and multiplying
of NM-0183 sources to all navigational instruments
through five talker ports and PC connection through
RS-232.
Operation
NM-251A-MUX acquires the NM-0183 signals
from the two optically isolated inputs, combines the
data and outputs them to the five general output
ports and the RS-232 output. The NM-251A-MUX has
three distinct functional modes that can be selected
from the user via dip-switches:
1. Mux mode: The combined output is routed to all five output ports and the RS-232 output. All
outputs can be configured to transmit at 4.800/9.600/19.200 or 38.400 baud.
2. Split/Mux mode: The combined output is routed to the fifth output and the RS-232 interface while
the first input is transmitted to outputs 1&2 and the second input to outputs 3&4. Combined output
only can be configured to transmit at 4.800/9.600/19.200 or 38.400 baud.
3. Fallback mode: NM-251A-MUX 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.
The NM-251A-MUX has also the following features:
1. Configurable output transmission speed: The output speed for the combined data stream is
user selectable via dip-switches and can be set to 4.800/9.600/19.200 or 38.400 baud.
2. Input Autosense: The NM-251A-MUX senses a correct NMEA signal application to any of the two
inputs while in mux or split/mux mode. The LED Ina and Inb flash at 1/2 Hz while no NMEA signal is
applied or the polarity is wrong. If at least one signal is correctly connected to one of the inputs it
starts functioning normally.
3. Data overflow indications: The NM-251A-MUX has different overflow indication for each input.
When an overflow occurs some of the input data are not transmitted. This is normal when
connecting to instruments transmitting great amount of data, and can be resolved if the output
transmission speed is increased.
4. Watchdog: The integrated watchdog reset feature will bring NM-251A-MUX back from any
unresponsive state into normal operation.
