Sending Float/Double Values Across CANbus

Kyle Muehlegg 0
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I am preparing a code that sends various electrical parameters across a CANbus (power, voltage, current, etc.). Precision is necessary for this circumstance.

There are two options I see:

  1. Send the value as an integer, but scale it on sending and receiver side (Sending x100, Receiving /100, for example). If I do this, then I can send 2.12V as 212 (or 0xD4).
  2. Send it as a float value, which would require 32 bits but no scaling.

My questions are as follows:

  1. Can float values be sent across CANbus?
  2. If yes to Q1, is that a common practice? Or do most CANbus communication programmers use scaled integers?

Thank you in advance.

can-bus
1 Answers

First of all, please read this answer since it answers all the floating point concerns:
Does the "Avoid using floating-point" rule of thumb apply to a microcontroller with a floating point unit (FPU)?

  1. Can float values be sent across CANbus?

Yes, though like any large number they depend on endianess, so you will have to specify a network endianess for floating point on your CAN application layer. Sending 4 or 8 bytes in a single package is obviously not a problem.

  1. If yes to Q1, is that a common practice? Or do most CANbus communication programmers use scaled integers?

It is not common practice, unscaled raw data is by far the most common and convenient. For example if you wish to express a voltage between 0 and 5V, you could for example send a raw data value between 0 and 65535 then scale it to mean Volt if you actually need to use that unit at some point (for example when showing voltage for humans on a display).

The raw data form also has the advantage of being technology forwards/backwards compatible. Suppose you read voltage with a 10 bit ADC. Your values will be in the range of 0 to 1023. You can express that as 16 bit raw data by multiplying with (65536/1024) = 64. Some years later you switch to a 12 bit ADC with values up to 4095. You can then keep the very same CAN protocol and format, just with increased resolution smaller "steps" of raw data.

Lundin0
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