Thanks for the reply, but though this is I2C, the PCA9615 converts it to a differential signal pair for each of the I2C lines. In that case, wouldn't we have to treat it as a differential pair?
Yes, over such lengths as matter, or other circumstances.
If you had 3000m of CAT5, you'd need it to be differential and impedance-controlled, as indeed it is; over 10m, it's not too important, the impedance at least.
Although being a multi-master sort of bus, you can't use very long lengths anyway, that would screw up arbitration. Bit times always need to be longer than the bus's electrical length.
And obviously, not like 3000m of CAT5 would carry the signal -- that's a lot of wire resistance!
What's more important, is keeping ambient noise out of the differential signal. Noise tends to be common mode, for example they mention ground loop voltage in the datasheet. This is rejected by the receiver (within its range), no problem.
Randomly routed wires would surely work for signal integrity purposes, but may pick up ambient noise as a differential signal. This is what you actually need to avoid. The impedance isn't very important, in DM or CM. Just ensuring that noise is shared equally by both lines.
These devices are very similar to CANbus, though it lacks the common mode range.
Take note of Fig2-3 (page 4):
https://www.nxp.com/docs/en/data-sheet/PCA9615.pdfDrive is unipolar, and this is how they make I2C work, seamlessly, over a differential pair. Instead of driving one line open-drain, two lines are driven open-drain
and complementary. So the common mode emissions are low. The receiver is differential, so the common mode rejection is good, at least within the operating range. Multiple devices can be attached to the line, and all will drive in the same direction, preserving the wired-OR arbitration mechanism that I2C is based on (and CAN as well).
Curiously, 7.2.2 (page 7) is in direct conflict with the 2nd-last paragraph page 10, and Vcm page 17.
And they do not give an absolute maximum / limiting value for the differential bus voltages.In short: when is differential not differential?
When it's not differential anymore.
That is, when the assumption that V(+) - V(-) = Vdiff fails.
For Vcm outside of the given range, the input receiver can't read it anymore, or the ESD diodes clamp the signal. Either way, data gets lost.
(RS-485 and CAN have an input range that exceeds the supply voltage, by using divider resistors to sense bus voltage -- this reduces signal gain, so the receiver needs to be more precisely balanced, but is also much more robust against common mode noise.)
Whether this is an issue, who knows. It might be worth using a common mode choke, so that CM noise can be filtered out. CMCs are perfectly acceptable here, because the impedance is well defined and stable (unlike some buses -- USB in particular). Maybe additional ESD protection as well, seeing as the internal protection is only rated for 2kV.
Tim