Detection Back Off
One question remains: After an “unsuccessful” detection attempt (resistor was not 25 k, i.e., an invalid signature), how quickly can another detection attempt be carried out? The answer to that depends on whether the PSE is delivering power over the data pairs (Alt-A) or over the spare pairs (Alt-B). For Alt-A, there is no back off requirement. But since most PSE chips will carry out port initialization/pre-detection/open-circuit detection all over again, the next detection will naturally take some time. But a typical PSE design will try to complete a second detection within about a second after the end of the preceding failed detection. Section 33.2.4.1 of the AT standard has this suggestion:
If a PSE performing detection ...
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