Pre-Standard/Legacy Detection Schemes
These come in several types:
1.  In Fig. 3.1 we have shown the key non-IEEE detection schemes. The top half shows the principle behind the well-known Cisco Inline Power (“ILP”) detection. A low-frequency “ring signal” is injected on one pair and this gets looped back by a valid (ILP-based) PD. The ring signal returns to the PSE on the other pair, which then declares it to be a valid (ILP-based) PD and powers it on. This may need additional hardware, not usually part of most PSEs and PDs. For example, a ring generator is needed on the PSE side, and a low-pass filter, or normally closed relay contacts, on the PD side. When power is sent down the cable, it will be used to actively disconnect circuitry/relays ...

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