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

Status
Not open for further replies.
N

Naveenk.wins

Newbie level 4
Joined
Jun 2, 2011
Messages
7
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,334
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
 

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.
 

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.
 

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
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…