EMI robustness in Anybus M40 with SPI interface

Are there any recommendations how to improve Anybus M40 module robustness against EMI? I use Anybus M40 for Modbus TCP/IP with SPI interface to an FPGA. In some high noise (common-mode) environments the M40 module hangs with both red status LEDs ON (unrecoverable error). I have reduced SPI clock speed, which improved things. I have added common-mode choke around the SPI signal lines, and I have improved grounding of the module. I have increased bypass capacitance. Are there any other things I should look into?

Hello Erik,

To check you signal quality, can you hook up an oscilloscope and check that the levels are correct or is the peak to peak voltage much shorter?

Also can you take this file and place it in the ABCC’s FTP-root and then browse to that page. This should list the fatal error that occurred.

AbccFatalLog40.shtm (3.9 KB)

This is the format for thelink you should use: http://<ip_address>/AbccFatalLog40.shtm

In general an corrupted SPI packet should not cause the device to go into an exception state. The ABCC and host communicate CRC’s to detect errors and request retransmission. Also in case of a poorly formatted on invalid message header, etc, the ABCC would deliberately corrupt the MISO packet to also notify the host of a problem

Thank you for your feedback and sorry late answer.

I found the answer: I had misunderstood in the HW design guide the word “functional earth”, and connected it to the same ground plane as “GND plane”. Splitting these two removed the problem completely, and we could remove the common-mode choke. When measuring the different ground points (“functional earth”, GND" and M12 spring contacts to chassis), it is still not 100% clear to me how these are connected internally in the M40. Is this documented in place I have not found?

Hi Eric,

I’ll check with some colleagues and see if we have some additional documentation on that.


Hi Erik,

Here’s what I heard back from my colleague:

Yes, tying GND and FE together can have those consequences. It may also work without problems though, and can in some cases make the situation better, it depends on the circumstances.

There is nothing here about what their present design looks like (I would need to see schematics, PCB layout and perhaps some photos to form a view about the ‘electromagnetic world’ in their design) so I can’t say if they are on the right track or not, nor what exact kind of reference information they are after. You might be able to use the reference schematics in the B40 as a guide though, they are based on the corresponding M40s.