[SOLVED] CAN Physical Layer -Recessive/Dominant Bus voltage Measurement

[Naveenk.wins]

Hi Friends,

I need to measure the voltage levels of Recessive Bit and Dominant Bit in CAN Bus.

Can any one help me how to measure the same. When i measure(voltage) i am always getting CAN_H -> 2.5605v and CAN_L-> 2.56v.

My doubts
1.How to send a frame which contains only - Recessive Bit.
2.How to send a frame which contains only - Dominant Bit.

Regards
S.Naveen Kumar

 

[btbass]

I don't think you can do that? If 5 consecutive bits with the same polarity are sent then bit stuffing happens to prevent this.

You will have to use a scope to measure the levels.

 

[microKernel]

Like btbas said you need an osiloscope to do measurements. A simple multimeter won't suffice.

Also remember CAN uses a differential scheme for signalling.
A dominant bit (usually 0) is the very little voltage differential between CAN_H and CAN_L. At that time both CAN_H and CAN_L will be around 2.5 (I don't remember the exact values, check the specs).
For recessive bit (usually 1) differential of CAN_H and CAN_L will be I think 2.2V min (check the specs again for exact values) but CAN_L would nor be 0. It woyld be around 1.2v.
Moreover take care of bus termination while doung the measurements.

 

[Naveenk.wins]

Hi,

This is how i measured the voltage across the bus CANH -CRO channel 1 and Connected the Ground to CANL - By this i got the differential voltage as 2.1V for Dominant Bit and <200millivolt for Recessive Bit.

You can individually measure the CANH Voltage and CANL voltage and subtract the same to get the differential voltage. Voltage >= 2 is recognized as LOGIC 0 and Voltage <500millivolt is recognized as LOGIC 1 in the BUS

Note: This voltage depends on the Standard used i am using J1939Standard.
Note: To measure the voltage Send an Identifier with lowest value i used 0x007.

Best regards
S.Naveen Kumar