When the SeaTalk -> NMEA option is selected, NMEA In 4 becomes a SeaTalk^® compatible input, offering conversion of the most common SeaTalk datagrams on a RayMarine^® network into NMEA sentences.
 

Server: Data received on the NMEA inputs is sent to the computer. Data received from the computer is sent to a NMEA output.

Hub: All received data (on the NMEA inputs and from the computer) is sent to the NMEA output.

Auto: Automatic selection between Server and Hub mode. This is a useful feature for people who have a GPS and an autopilot connected to the multiplexer, and sometimes wish to sail without taking their laptop on board. When the laptop is not connected, the multiplexer is in Hub mode and crosstrack- and waypoint information from the GPS is directly sent to the autopilot. When the laptop is connected and sending data, the multiplexer automatically switches to Server mode and the autopilot will receive crosstrack- and waypoint data from the laptop instead.

The Auto mode also serves as an emergency backup: when the laptop or computer crashes, the multiplexer will fall back to Hub mode after a time-out of 10 seconds.

A flexible NMEA filter can be configured to pass or block specific sentences from each input channel. This greatly reduces the chance of an overflow and the resulting loss of data. Many GPS receivers for instance, transmit RMC, GSV, GSA, GLL and GGA sentences every second, accounting for 85% of the available bandwidth of the NMEA channel. By blocking unwanted or unnecessary sentences, bandwidth is preserved for other instruments.

Gyro- and fluxgate compasses produce NMEA sentences at a high rate (10 sentences/second or more). This can lead to a buffer overflow in the multiplexer. Although this overflow is handled properly without data corruption, it will lead to a delay of NMEA sentences of up to 20 seconds in extreme situations. This produces a severe problem for autopilots, which cannot make proper course corrections when their heading feedback is delayed for 20 seconds. The Real-Time option prevents this delay by bypassing the buffer of that specific channel and forwarding the data immediately to the multiplexers' NMEA output. As a result, the heading is never delayed more than 0.2 to 0.5 seconds, depending on the amount of other NMEA sentences passing through the multiplexer.

This option converts a Magnetic Heading sentence (HDG) into a True Heading sentence (HDT). If the originating magnetic heading sentence contains a magnetic variation, it is used to calculate the true heading before conversion. This feature is useful for certain equipment like VDRs which need a true heading input, while the only available heading source is a fluxgate, delivering a magnetic heading.

With Channel Priority enabled, similar NMEA sentences on different inputs are only passed from the input with the highest priority. The RS-232 port has the highest priority, followed by inputs 1 to 4, in descending order. When for instance two GPS receivers are connected to inputs 1 and 2, and both transmit the same type of NMEA sentences, only those received on input 1 are passed. A time-out function ensures that similar sentences from the GPS at input 2 are passed when the GPS at input 1 stops sending these sentences.
Another example is where a GPS provides an SOG to a windmeter, while the latter repeats the sentences from the GPS. When the GPS is connected to input 1 of the multiplexer (high priority) and the windmeter to input 2 (lower priority), the GPS sentences coming from windmeter will be blocked automatically while wind related sentences are passed.

When this feature is switched on, the multiplexer will transmit a proprietary NMEA sentence indicating on which input channel the following NMEA sentence is received.
VirtualPlex-1 uses this to route NMEA sentences from specific inputs to specific virtual COM ports (Channels option).

The Channel ID option changes the talker ID of every incoming NMEA sentence in "Mx" where 'x' is a number from 1 to 4, according to the input the sentence was received on. Example: an incoming GPRMC sentence on input 2 is changed in M2RMC. This is useful to distinguish between sentences from two similar instruments.

Talker ID substitution changes the talker ID of incoming sentences in a similar way as the Channel ID option does, but now the talker ID can be specified for each input channel. This option is useful for software or instruments that expect a specific talker ID.