Raymarine RayTech RNS 5.0 Getting Started page 26

22
Basic NMEA and RS-232 cabling principles
This section explains some of the basic principles involved with NMEA and RS-232
electrical connections.
Knowledge of this information is not required to connect t RNS to your peripheral
instrumentation: however, it is provided for the advanced user as a pertinent technical
background.
NMEA basics
Most marine electronic devices that output data do so over NMEA ports. These ports
are known as a 'balanced pair', which means that the data signal is carried over two
wires (via an RS-422 electrical layer, in computer/datacom terminology). The signal
level is determined by calculating the voltage difference between the two wires, hence
the NMEA output signal is called a Differential Data Signal (DDS). This DDS should not
be confused with Differential Global Positioning System (DGPS), which is a GPS system
error correction method.
DDS are designed to be error-resistant by keeping the positive and negative leads close
together for the entire cable run. This means that they both pick up the same levels of
noise, which is ultimately subtracted from the signal, preserving the signal's data
integrity.
NMEA ports typically have four connections (two for each lead); Transmit (positive/
negative) and Receive (positive/negative). Some devices may use a single-direction
port, meaning that they can transmit or receive only.
To connect one NMEA device to another:
1. Connect device A's positive Transmit (Tx+) lead to device B's positive Receive (Rx+)
lead.
2. Connect device A's negative Transmit (Tx-) lead to device B's negative Receive
(Rx-) lead.
IMPORTANT: When performing the above connection, make sure that you connect
the devices directly; DO NOT use the boat's DC grounding circuit to carry the Tx- signal.
You may encounter a device that does not properly implement the NMEA specification,
and shares it's Tx- lead with the device DC power ground circuit. In this case, you must
still run a cable directly between the Tx- and Rx- leads as previously stated. DO NOT
share the DC power ground circuit.
RS-232 basics
Most desktop and laptop computers have RS-232 ports. These ports use a 3 wire
interface, in which the transmit and receive leads reference the same signal ground. It
is important not to confuse the RS-232 cable's ground with the boat's power ground
RayTech RNS - Getting Started