Communication

• Communication Timing
• Triggering and synchronization
• Internal clock accuracy
  • Clock of MTi’s without GNSS receiver
  • Clock of MTi-G-710
• Serial Connection Settings
  • Serial or USB communication
  • Transceiver voltage levels

Communication with the MTi is done via the binary Xbus communication protocol, except for data coming from the MTi device in ASCII format (NMEA). The communication protocol is extensively documented in the MT Low Level Communication Protocol Document.

Communication Timing

For many applications, it is crucial to know exactly the various delays and latencies in a system. A discussion on this can be found on BASE.

Triggering and synchronization

In case multiple systems are used during a measurement, it is important to have the measurement data synchronized between the systems. Processing synchronised data is much easier because there is no need to resample the data to compensate for timing inaccuracies like clock drift and clock deviations. Synchronization using multiple systems involves two important issues: starting the measurement at the same time and having a fixed time relationship of the sampling instances. Refer to BASE for more information on triggering and synchronization options: Synchronization with the MTi

Internal clock accuracy

Clock of MTi’s without GNSS receiver

The internal clock jitter of the MTi is less than 25 ns.

The internal clock of the MTi which generates the sample timing based on the set sample period is accurate to ±10 ppm with a maximum of ±15 ppm (this differs per MTi) over the temperature operating range. Using a typical MT (with an accuracy of 10 ppm), this means that the worst case deviation after a 1 hour log is ± 0.036 seconds (= 3600 s ∙ 10 ppm) or 15 sample counts in 1,440,000 at 400 Hz sample rate (± 25 ns/data packet @ 400 Hz).

Clock of MTi-G-710

In the event that the MTi-G-710 has a GNSS fix, the bias of the clock will be estimated and in the long term there will be no deviation from GPS time. On the short time scale, the clock jitter is the determining factor. Note that only GNSS time pulses (not other GNSS reference times) are used to determine the time reference. This clock bias estimation will improve the accuracy of the crystal used in the MTi-G-710, under normal operating conditions to <1 ppm.

The time pulse used to correct the clock of the MTi-G has minor inaccuracies, caused by the following:

• The delay caused by the distance between the antenna phase centre and the input pin of the GNSS receiver module in the MTi-G-710. The cable delay is 5.5 ns/m for PTFE, resulting in a 16.5 ns delay with the development kit antenna.
• Quantisation loss; a clock of 23.104 MHz results in a resolution of 43 ns.
• The rise time of the time pulse is 7-25 ns; best results when loaded with a high impedance.
• Software delay, for handling the time pulse interrupt clock_ticks/300 MHz.

Delay caused by the antenna cable length is compensated for in the GNSS receiver, but will vary with cable length.

The internal clock jitter of the MTi is less than 25 ns. The internal clock of the MTi which generates the sample timing based on the set sample period is accurate to ±10 ppm with a maximum of ±15 ppm (this differs per MTi) over the temperature operating range, if there is no availability of GNSS. Using a typical MT (with an accuracy of 10 ppm), this means that the worst case deviation after a 1 hour log is ± 0.036 seconds (= 3600 s ∙ 10 ppm) or 4 sample counts in 360,000 at 100 Hz sample rate (± 0.1 μs/sample @ 100 Hz).

Serial Connection Settings

Serial or USB communication

The MTi has a parallel serial (RS232/RS422/RS485 or alternative UART) and USB interface. However, it is not possible to have communication on both the serial and USB interfaces simultaneously. Therefore, the MTi wakes up as a serial device, unless USB is detected. When a USB interface is detected, the communication will be done via that USB interface.

Default settings for serial connection can be found in the MT Low Level Communication Protocol Document.

Transceiver voltage levels

The MTi’s RS232 transceiver has RS232 compliant output levels and is compatible with TIA/EIA-232-F specifications. The MTi's RS485/RS422 transceiver has RS485 compliant output levels and is compatible with TIA/EIA-485-A specifications. The differential lines of the RS422/RS485 communication are terminated with a 120Ω resistor.

For completeness, the input and output voltage levels are listed below:

Transceiver voltage levels