Ever found yourself in the situation where you have a Power over Ethernet (PoE) Powered Device (PD) device and Power Sourcing Equipment (PSE) but the two don't seem to be working together? Well you may have fallen down the not-so-well-documented rabbit hole of Mode A vs. Mode B.
Taking a closer look 802.3af PoE, you'll find two different sub-standards of 802.3af associated with 10/100 TX Ethernet, namely Mode A and Mode B. The main difference is best illustrated with the following diagrams.
The diagram above shows a PSE supplying power on top of the 10/100 TX data on pins 1,2,3,and 6. PSE providing "phantom" power on the data pairs is known as Mode A, or Endspan.
This diagram above shows a PSE supplying power on pins 4,5, 7, and 8, the "spare" pairs in an Ethernet cable running 10/100 TX. PSE providing power on the spare pairs is known as Mode B, or Midspan. Most "injector" devices (as opposed to full switches) are Mode B.
And here come the "Gotchas." While PDs must support BOTH Mode A and Mode B to be compliant with the 802.3af standard, there is no such requirement for PSEs. There are a few different ways this can lead to mistakes.
1) You might assume your device is 802.3af compliant because it is listed as PoE-capable, but this is not always the case. A recent customer had a PoE-capable Ethernet/IP Rotary Shaft Encoder, but closer datasheet inspection revealed it was not 802.3af compliant. As it turned out, this was only a 4-wire device and therefore incompatible with the customer's existing Mode B PSE. We supplied a Mode A PSE and resolved the issue.
2) In addition to no requirement for PSEs to perform BOTH Mode A and Mode B, many vendors' documentation doesn't state which mode their device utilizes.This makes it challenging to locate the proper equipment for scenarios like the one above.
3) You must confirm specific model numbers and operation. We came across one vendor's PoE switch whose generic datasheet listed Mode A and Mode B and offered two part numbers ending in A and B with PoE Mode as the only difference between the two. Only upon close inspection did we notice the A part number utilized Mode B and the B part number utilized Mode A. Crazy enough?
As we receive confirmation from our vendors, we will be maintaining this list of PSE equipment we offer and the associated operational mode.