How can I make a PoE circuit that is compatible with 24V passive PoE and 802.3at active PoE?
802.3 PD devices are required to support mode A (1,2/3,6) or mode B (4,5/7,8).  Passive POE is pretty much always provided on the spare pairs (4,5/7,8) for 10/100.  The easiest solution to supporting both is to use the mode A pairs for 802.3 (1,2/3,6) and use the "spare" pairs for passive POE (4,5/7,8).  That way no synchronization or control coupling is required between the two systems.