We have a product that may be powered by a 24 VDC power brick or by a POE PSE. Our internal circuit ground is connected to case ground as is all of the internal powered circuitry. The POE ground is fully isolated from our internal and DC brick grounds; we use a flyback converter topology. The outputs of our POE switcher and main switcher (with 24V input) are connected together. If the brick power is present, we disable the POE switcher. We do this by detecting the presence of the brick power and holding the LM5072 Soft Start pin low by means of an optoisolator. We cannot use the RAUX or FAUX inputs on the 5072 because our reader ground is isolated from the POE ground.
Our reader has experienced issues with some PSE's when the PSE uses a POE port for Ethernet communication but does not intend to provide power to the reader (24VDC power brick is connected). The PSE apparently does not sense the appropriate signature to determine that it is acceptable to communicate via Ethernet to the reader (without power). I’m hoping you may have some knowledge of how we may be able to allow the PSE to use an auto detect mode and for our reader to present the signature it needs. I’m pasting an excerpt from a representative PSE manual below (in italics):
The Catalyst 4500 series switch has three PoE modes:
•auto—PoE interface. The supervisor engine directs the switching module to power up the interface only if the switching module discovers the phone and the switch has enough power. You can specify the maximum wattage that is allowed on the interface. If you do not specify a wattage, then the switch will deliver no more than the hardware-supported maximum value. This mode has no effect if the interface is not capable of providing PoE.
•static—High priority PoE interface. The supervisor engine preallocates power to the interface, even when nothing is connected, guaranteeing that there will be power for the interface. You can specify the maximum wattage that is allowed on the interface. If you do not specify a wattage, then the switch preallocates the hardware-supported maximum value. If the switch does not have enough power for the allocation, the command will fail. The supervisor engine directs the switching module to power up the interface only if the switching module discovers the powered device.
•never—Data interface only The supervisor engine never powers up the interface, even if an unpowered phone is connected. This mode is only needed when you want to make sure power is never applied to a PoE-capable interface.
The switch can measure the actual PoE consumption for an 802.3af-compliant PoE module, and displays this in the show power module command. However, it cannot display the consumption of an individual interface on an 802.3af-compliant PoE module.
PoE consumption cannot be measured on the WS-X4148-RJ45V PoE module. Therefore, for all PoE calculations, the PoE consumption on this module is presumed to be equal to its administrative PoE.
Customers prefer not to have to set the ports for never but that is the only mode which will work with our reader when it is DC brick powered. Must there may be a voltage on one of the 5072 AUX pins forour PD to present the required signature. If so, how can I accomplish this when the auxiliary source ground is isolated from POE?
Best regards,
Mike Thomas
