Some thoughts about differential buses RS 485, CAN, USB

[ikalogic]

Hello,

For a next project, i am trying to sniff with the same IC a number of differential busses: RS485, CAN and hopefully USB.

I am not interrested in the higher layers of the communication. I just want to convert the differntial signaling to logic levels that can be interpreted by an FPGA.

I was thinking of using this IC : https://www.electro-tech-online.com/custompdfs/2012/09/am26ls32a.pdf (I like this IC because it can take +/- 25V on differential input pins, making is suitable for work in hazardous situations...)

Now, here is my question:
It seems to me as if RS485, CAN and USB use very similar signaling. At least, from the receiver point of view. Is that correct? Am i missing something..?

Thank you very much!

 

[ikalogic]

Have been making some reading here and there. As Wiki says (https://en.wikipedia.org/wiki/CAN_bus), the driving of CAN is quite different than RS485; I am aware of that, but still, from the receiver point of view, i can't seem to find any difference...

Can any one confirm..?

I will be doing some tesing monday at the office and will let you know if you are interrested

 

[dougy83]

CAN, RS485 and USB are all fully differential when transmitting data. USB has an additional state SE0 (both lines low).

 

[ikalogic]

Thanks Dougy83.

Ok let's drop USB, what about just CAN and RS485. Can i assume from your comment that they are compatible in terms of reception..?

 

[Mikebits]

The CAN bus seems to be more a protocol layer definition. It seems the physical layer is somewhat weakly defined and each car maker has their own way of defining the connector and hardware. From what I did find on the 9 pin definition, the voltages between the CAN and 485 seem to be slightly different.

 

[dougy83]

A CAN receiver IC, e.g. https://www.electro-tech-online.com/custompdfs/2012/09/AMIS-30663-DPDF.pdf should be able to handle reception of both RS485/422 and CAN. The CAN transceiver shown has a very large permissible input voltage range, which RS485/422 receivers may not possess.

Both protocols are differential, RS485 has a minimum differential threshold of 200mV (I think... from memory) while the CAN IC shown requires ~0.7V difference. RS485 will be well above this level unless severely attenuated