Bit calculation for CAN Bus Control in Node-Red

General
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Issues with Bit Manipulation for CAN Bus Control in Node-Red

Hi everyone,

I'm struggling with bit manipulation for CAN bus commands in Node-Red and hope someone has a smart solution. I am using red-contrib-socketcan on a Raspberry Pi with a CAN Adapter.

:white_check_mark: What’s Already Working?

Reading and wiriting / sending CAN commands is working perfectly via Node-RED.

:dart: What Am I Trying to Achieve?

I want to send data to the CAN bus to control, for example, the interior light in my Van (VW Grand California) which is equipped with several CAN busses. The only bus I want to use is for comfort functions.
This already works with the following command:

cansend can0 12345678#00.02.00.00.00.00.00.00

:point_right: Here, bit 1 in byte 1 is set, resulting in 0x02 at the second position of the string.

To turn the light off again, I reset bit 1 to 0:

cansend can0 12345678#00.00.00.00.00.00.00.00

:warning: The Actual Problem

The same identifier (12345678) is also used to control other devices, such as the satellite receiver.
This device is switched on by setting bit 4 in byte 1 (0x10):

cansend can0 12345678#00.10.00.00.00.00.00.00

:warning: Problem: Sending this command accidentally turns off the interior light!

:hammer_and_wrench: Solution Approach

Before modifying a bit, I need to:
:one: Read the current value of byte 1 from 12345678.
:two: Set or clear the required bit.
:three: Recalculate the new byte value.
:four: Send the updated CAN command.

Example:

  • The interior light is already on (0x02 in byte 1).
  • Now, I also want to turn on the satellite receiver (set bit 4).
  • The new byte value should be 0x12 (0x02 + 0x10 = 0x12).

Correct command:

cansend can0 18EF70C9#00.12.00.00.00.00.00.00

:point_right: Now the satellite receiver is turned on, and the interior light stays on!

To turn off the receiver, I need to remove bit 4 what leads to:

cansend can0 18EF70C9#00.02.00.00.00.00.00.00

:thinking: My Challenge in Node-Red

Right now, I'm struggling to properly implement this bit manipulation in Node-Red. :confused:

Does anyone have an idea how to read the current value, modify the required bit, and generate the correct CAN command?

Any help would be greatly appreciated! :pray:

Best regards,
Stefan

2

SOLVED - can be closed

3

You could add how you resolved this so future reads can find a solution to a similar issue. The ethos of the forum is to help and share with others.

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