I inherited a design that uses inductors in-line with the SPI lines. Communication intermittently fails and I suspect these inductors are the cause; or at least they aren't helping. Can anyone comment on the use of inductors on SPI lines (or other communications for that matter)?

 

Looks like snubbers to quash overshoot and/or ringing on the bus. Long lines?

To discover the problem, scope the MISO, MOSI, and SCK to ensure:

a) MISO/MOSI are stable during the (rising/falling -- depending on mode) SCK transitions and
b) SCK transitions cleanly.

 

I think you need to look at the waveforms WRT to all Grounds, but particularly the ones that are at receiving ends of the signals. It is incorrect to conclude the she signals are actually isolated and who knows what ground reference is being used by the receiver. It looks more than a little bit dubious to me.

 

The SPI travels ~300mm off board so perhaps that's why they thought inductors were needed. I agree with your skepticism of 'isolation' @Papabravo, I'll be sure to take readings from the receiver's GND.

Before posting this I search around for a few hours and only found discussion of how to reduce inductance in signal busses, not increase it. I'll scope the lines and then remove the inductors to see how the signal changes.

I appreciate your input.

 

Those are not inductors, those are common mode chokes (https://www.we-online.com/web/en/el...pbcm/blog_detail-worldofelectronics_61439.php).

The circuit configuration here is essentially a low pass filter to help with potential EMI/EMC
emissions. Note the ISOGND vs GND that you have in your schematic.

 

One of the big unknowns here is how fast the designer expected SCLK to go. Some power users go for super high clock rates with peripherals that are millimeters apart. 300 mm is about 1 foot = 12". This would be borderline for maximum SCLK, but you could comfortably get by with a much lower clock rate say on the order of 125-375 KHz. What clock rate is your system running at?