Output Formats
On Ethernet, RS232 and RS422 a proprietary binary output called NCOM. The NCOM format allows all the encoded measurements to be output over a 115,200 baud serial port at 100Hz. Software decoders (source code in C) are available to decode the NCOM format. Over CANbus the signals are encoded into CAN messages as defined by the CAN standards. CANdb (or CAN DBC) files are provided to help users configure their acquisition systems; this saves considerable time. On RS232 NMEA outputs are also possible, but these cannot be made at 100Hz because there is not sufficient bandwidth with this encoding.
Hardware Formats
The RT3000 family support a variety of hardware and electrical interfaces. These can be used to send the measurements from the unit to virtually any other system.
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RS232 This is the standard format used by serial ports on a PC. The User Cables have the same pin connections as a PC, so a Null Modem Serial cable can be used to make the connection. NCOM format is transmitted over RS232, normally at 100Hz and using 115,200 baud. NMEA outputs are also possible. RS422/RS485 It is possible to factory-select a four-wire RS422 standard instead of RS232. The same information is transmitted over RS422 compared to RS232, but the longer range that RS422 has makes it useful when the information has to be transmitted over a larger distance. |
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Ethernet
The RT3000 family is configured via Ethernet. An FTP server runs on the unit to allow stored data to be downloaded very quickly. The measurements are output using UDP broadcasts. |
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CAN bus An optional CAN bus module allows the RT3000 to output its measurements on a CAN bus. The RT3000 family supports CAN bus speeds up to 1MBit/s. All the normal measurements are provided. The connector on the normal RT3000 User Cables is the CIA (CAN-In-Automation) connector. |
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DGPS The DGPS input to the RT3000 family uses RS232. The unit can accept RTCM-104 messages, including the new RTCM messages that support L2 corrections. It can also accept RTCA and CMR corrections. The baud rate for the corrections is configurable. The DGPS port will also output the configuration to an RT-Beacon. The RT-Beacon needs to know which beacons to receive, or it can be set to an automatic mode. The RT-Base normally uses RTCA corrections, and this is the default setting for the RT3000 family. |
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1PPS
The 1PPS timing pulse from GPS is output by the RT3000 family. The output is normally high, with the falling edge representing the exact start of the second. The duration of the low pulse is 1 ms. The 1PPS output can be used to synchronise measurements from other sources. 100Hz IMU Sample The RT3000 family can output a 100Hz signal that corresponds to the sample time of the Inertial Sensors. The falling edge of the 100Hz signal denotes the sample moment for the sensors. The 100Hz IMU Sample has a 50% duty cycle (5 ms low and 5 ms high). |
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Event In The Event In can be used to capture external events, like the time the brake pedal is pressed or the time of the shutter for a camera, as shown here with a Kodak high-resolution camera. The Event input is timed very accurately (with nanosecond resolution). The input has a pull-up resistor and senses when the voltage falls. The input is unsuitable for high frequency inputs and only one event per second will be timed correctly. |
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Wheel Speed Out
The RT3000 family can simulate the pulse output of a 5th wheel or Piesler Wheel. It uses a complex algorithm for this to minimise the latency of the output and still maintain accurate speed and distance measurements. The algorithm is similar to a predictor-corrector algorithm used to solve differential equations. This gives an exceptionally accurate output for data acquisition systems to use. The output has TTL levels. Wheel Speed In The RT3000 family can accept TTL level input from a wheel pulse encoder. This type of input is especially useful in environments where GPS is poor. It allows the RT3000 to reduce the amount of position and velocity drift that would occur during long GPS blackouts. |
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Analogue The RT-ANA unit converts the low-latency signals on the CAN bus to analogue voltages. The RT-ANA has a galvanically isolated power supply to prevent ground loops in the analogue signals. The RT-ANA uses exceptionally high-grade 16-bit Digital to Analogue Converters (DACs) to faithfully represent the precision measurements from the RT3000. Up to 16 analogue channels are output. Around 40 channles are available to chose from. |
Signal and Encoding Formats
On RS232, Ethernet and CAN the signals are encoded. The RS232 and Ethernet share the same encoding, our NCOM format; RS232 also supports NMEA, though this cannot give the full functionality of the RT3000 family. Each of the formats is addressed here.
NCOM Format
The NCOM format contains the minimum outputs required to get the full functionality from the RT3000. Using the minimum set of outputs allows the NCOM output to fit on a 115,200 baud serial link. The same format is used over Ethernet even though Ethernet has a much higher bandwidth. The NCOM format includes the following measurements:
– Body Accelerations
– Body Angular Rates
– Navigation Status
– Latitude
– Longitude
– Altitude
– Velocity Vector (North, East, Down)
– Orientation (Heading, Pitch, Roll)
– Status Information
To derive additional measurements, like Slip Angle requires some mathematics to be performed by the decoder. The C code drivers provide all the matrix algebra required to generate outputs in different reference frames (like Forward, Lateral and Downward Accelerations). These drivers are provided free.
For example, the Lateral Acceleration measurement is not transmitted in NCOM. Instead, it is derived by rotating the Body Accelerations by the Pitch and Roll. Lateral Acceleration is a redundant measurement. Slip angle, Distance and several other measurements are also computed this way. These measurements are transmitted over the CANbus however, since it is usually quite hard to get program acquisition system software to perform 3D rotations.
NMEA Format
By default this format is not available. You will need special configuration files to obtain an NMEA output. The RT3000 can output the following NMEA messages:
– GGA
– VTG
– HDT
– ZDT
– GST
– PASHR
– PRDID
NMEA outputs cannot be transmitted at 100Hz because the bandwidth of RS232 is not high enough.
CANbus Outputs
Nearly all the available outptus are transmitted over the CANbus.
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500h to 5FFh |
Reserved for RT3000 Status Information |
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600h |
Date and Time |
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601h |
Latitude, Longitude |
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602h |
Altitude |
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603h |
Velocity (North East Down) |
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604h |
Velocity (Forward/Lateral) |
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605h |
Accelerations (body X, Y, Z) |
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606h |
Accelerations (Forward, Lateral, Down) |
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607h |
Heading, Pitch Roll |
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608h |
Angular Rates (body X, Y, Z) |
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609h |
Angular Rates (Forward, Pitch, Yaw) |
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60Ah |
Slip Angle, Track Angle |
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60Bh |
Distance |
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60Ch |
XY Position in Local Co-ordinates |
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60Dh |
XY Velocity, Yaw Angle, in Local Co-ordinates |
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60Eh |
Angular Acceleration (body X, Y, Z) |
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60Fh |
Angular Acceleration (Forward, Pitch, Yaw) |
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610h-614h |
Reserved for RT-ANA signals |
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More details on the messages and the signals are available in the RT3000 User Manual.
