Raymarine NMEA Multiplexer - Manual (Page 8)

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NMEA inputs
Each NMEA in (listener) port is a differential pair across
terminals a and b, and can receive NMEA data from one device.
The speed of each listener port is fixed at 4800 baud.
NMEA data on listener ports 1 and 2 is multiplexed in real time
(i.e. unbuffered).
NMEA data on listener ports NMEA In 3 and 4 is fed via a
discrete buffer for each port. Each buffer holds approximately
2 seconds of NMEA data.
Input priorities
Data at the listener ports is prioritized, so that each NMEA in
port is read twice as often as the next one. Thus for example, data
at input
1 is read in twice as often as data at input 2, four times as
often as input 3 and eight times more often than input 4.
NMEA outputs
The NMEA Out (talker) port is a differential pair across terminals
a and b, and feeds multiplexed NMEA data from all the NMEA
inputs, to up to four products.