I have been reading about CAN and DeviceNet recently and was wondering why DeviceNet uses 24VDC instead of 5VDC?

 

24VDC is pretty much the standard in industrial control systems so it's an obvious choice for systems in that environment.

 

@nsaspook gives a reasonable answer. The truth is more subtle. DeviceNet supports a cable system that can be 500 meters long at 125 kbits/second. Question: if the power supply of 24 Volts is located at one end, how much voltage is left at the other end given the standard IR drop for an AWG 15 ga. power pair used in the "thick cable". Give me your answer and I'll tell you what the designers decided was the worst case they would allow for.

Don't know if you've found the following, but it might be useful.
https://literature.rockwellautomation.com/idc/groups/literature/documents/um/dnet-um072_-en-p.pdf

PS -- I was on the design team.

 

One more thing. The DeviceNet power supply was intended to supply "Network Power" for the CAN transceivers and opto-isolators. It was never meant to supply I/O Power.