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SerialDataFormat / MaximumMessageRate

Maximum message rate

A relatively common question from users is how much data can the serial data link support. At the moment (March 2008) all hardware is set to a default of 115200 bits per second. With stop and start bits you need approximately 11 bits to transmit a single byte, and a typical RT message required 5 bytes, so we have:

115200 bites per second / 11 bits per byte / 5bytes per message = about 2100 messages per second.

  • At 1Hz update rate (typical of what is quired for temperatures etc), that is about 2000 messages
  • So at 5Hz update rate (typical of what is required for display updates) that is about 400 channels of data
  • At 10Hz update rate (typical of what is required for throttle, RPM type channels that are changing very quickly) that is about 200 channels of data
  • At 100Hz (typical of what is required for damper position monitoring) that is about 20 channels of data

Obviously in real channels there are a range of channels that require different speeds, however the above example shows how important it is to select appropriate update rates on channels.

There is another consideration in that all hardware is “intelligent enough” to only transmit updates on channels when they are required, i.e. when there is a change. So for example is an analogue channel is set up for an update rate of 100Hz but it is measuring a temperature that is only changing slowly, then the actual transmit frequency will be far slower – maybe only a few 10’s of Hertz.

The new hardware (at this time this includes the Mk2 SPEEBBOX, Mk2 IMU, DASH3, VIDEO4) can support higher bitrates of up to 921kbaud. Once all hardware is able to support the high data rate then the “default” data speed will be increased.

Page last modified on March 26, 2009, at 06:00 PM