Once I had the idea about connecting the LED's between the comparators, using NAND gates to decode valid 0 and 1 states faded away. I can draw up that schematic if there is wild and rampant demand.

The two trip points in post #17 are at 2.0 V and 3.0 V, so there is 0.5 V of margin in detecting CAN bus states and no dead band. Based on your experience, is this enough?

ak

 

Hi all,
It's been a while since I posted on here but I am back again looking for help and ideas.
My project is to build a quick and simple logic probe for CAN bus.
I have all the oscilloscopes, CAN interfaces etc. , but I often just want to do a quick test to see if the bus is running or not.

What I really want is to make a small battery powered logic probe with two flickering LED's that will tell me that data is running on both the CAN Low and CAN High lines of the bus.
I don't want to unduly load the bus so I need something with a fairly high impedance input, 10K or greater.
The two signal lines of the bus, CANH and CANL, in the quiescent recessive state, are passively biased to ≈ 2.5 V. The active state on the bus takes CANH ≈ 1V higher to ≈ 3.5V, and takes CANL ≈ 1V lower to ≈ 1.5V, creating a typical 2.0V differential signal.
I have looked at several DIY logic probe circuits and unfortunately 2.5V is smack in the middle of the area of uncertainty between high and low in most logic circuits and a standard logic probe circuit is unlikely to be reliable.

Why not use a proper CAN receiver or transceiver?