SBX-4 Integrator Guide
Chapter 2-Integrating the SBX-4
Page 13 of 22
SBX-4 Dual Serial Port Overview
The SBX-4 features two full duplex serial ports (Port 0 and Port 1). Use of the second serial port
is an option as the SBX-4 supports identical functionality as its predecessor, the SBX-3 on Port 0.
The main purpose of the second serial port is to allow the continuous flow of RTCM data from
Port 0 to a GPS receiver while allowing monitoring of SBX-4 performance via NMEA response
messages on Port 1. Separating RTCM differential data from NMEA status data when output via
the same serial port poses software integration challenges and potential GPS RTCM reception
conflicts.
Single Serial Port Operation
Some integration designs using the SBX-4 beacon receiver include a micro-controller between
the beacon and GPS receiver to provide enhanced communication and data parsing capability.
Within this type of integration, it is possible for the micro-controller to command and query the
beacon receiver and to parse NMEA response strings from RTCM data without impeding the
continuous flow of RTCM data to the connected GPS sensor.
To separate NMEA messages from RTCM data output by Port 0 through a parsing routine,
without causing a parity failure of the RTCM, it is necessary to take advantage of the fact that “$”
and “<LF>” are not supported within the RTCM specification, provided there are no bit errors in
the RTCM stream to start with. Therefore, if a “$” is received, then it should be considered as the
beginning of a NMEA sentence and a “<LF>” should be expected within a window of certain
length. If the parsing routine does not receive one, due to an unlikely communication line error,
after a window of a particular number of bits has passed, re-synchronization to the RTCM data
must occur.
Note:
If port 1 is not being used, pin 18 should be connected to the input power supply through a
10 Kohm resistor.
Dual Serial Port Operation
Although an integration may be designed with a microprocessor between the SBX-4 and the GPS
receiver to tune and monitor SBX-4 status, the second serial port of the SBX-4 eliminates the
requirement to develop a NMEA message parsing algorithm.
With this type of architecture, the SBX-4’s main serial port may be interfaced to the GPS
receiver’s receive line to provide RTCM correction data. The secondary serial port interfaces to
the microprocessor for NMEA configuration and querying. This ensures that NMEA response
messages do not interrupt the continuous flow of RTCM data to the GPS receiver.
