For the CAN bus, is it OK to swap CANH and CANL lines?

I couldn't find any reference that gave a definitive answer. But looking at a few datasheets, I don't think so. USB is looking at the presence or absence of a change in voltage. Whereas CANBus is looking at the voltage itself.

Here is an example of a USB transmission:

The ones and zeros are coded depending on whether or not there is a transition.

As opposed to CANBus which takes the difference in the voltage levels as seen in this app note:

If we look at a datasheet for a CANBus transceiver, for example the MCP2551, we see something along the lines of:

  Sym                  Characteristic                   Min     Max  Units
VDIFF(r)(i)    Recessive differential input voltage    -1.0    +0.5    V
VDIFF(d)(i)    Dominant differential input voltage      0.9     5.0    V

Since a negative voltage is mentioned, this leads me to believe that polarity is important and they are not taking the absolute value of the differential voltage.

So if we have:

$$CANH = 2.5V$$ $$CANL = 2.5V$$

Normally the transceiver would do:

$$CANH - CANL = 2.5V - 2.5V = 0.0V = Recessive$$

If you swapped the lines it would do:

$$CANL - CANH = 2.5V - 2.5V = 0.0V = Recessive$$

So far so good. The problem comes when we have:

$$CANH = 3.5V$$ $$CANL = 1.5V$$

Here, the transceiver would normally do:

$$CANH - CANL = 3.5V - 1.5V = 2.0V = Dominant$$

If you swapped the lines it would do:

$$CANL - CANH = 1.5V - 3.5V = -2.0V = Recessive (out\ of\ spec)$$

So you the receiving end would see nothing but recessive bits.

Swapping CAN-High and CAN-Low lines does not work. This is easy enough to establish empirically. I and my coworkers sometimes swap the lines by accident, and it is immediately apparent that it does not work.