WHAT IS DETECTION AND WHY DO WE NEED IT?

Before a compliant PSE provides power to a powered device (PD), it has to identify it, i.e. to determine that on the other side of the cable there is a device capable of accepting power. This process is called detection. For safety reason we obviously don’t want to apply 50V to a cable without knowing what kind of remote equipment is connected there (if anything).

DETECTION CRITERIA

The PSE detects presence of a compliant PD by periodically probing the condition of the line via its so-called Power Interface (PI). Power interface is just a fancy name for the connector RJ45. During detection, PSE is sending a series of low-voltage pulses of different levels within the so-called detection range, which is between 2.70 V and 10.1 V. If the PSE is designed to provide power over four pairs in the cable, it has to do detection on both pairsets. It should then use the observed Vport/Iport curve to calculate certain characteristics of the device impedance, primarily effective differential resistance "R_DET" and offset voltage (see the picture below). PSE needs to use at least two measurements with different pulse levels. It may do it two ways: by sourcing voltage pulses and measuring current, or by sourcing current pulses and measuring the voltage.



When a compliant PD device observes voltage at its input RJ45 connector in the detection range, it has to present a valid detection signature. The following table provides required characteristics of the PD detection signature.

CHARACTERISTICS OF A VALID PD DETECTION SIGNATURE
PARAMETER	CONDITIONS	MINIMUM	MAXIMUM
Differential Resistance RDET	2.70 V to 10.1 V	23.7 kΩ	26.3 kΩ
V offset	-	0	1.9 V
Voltage at PI	IPORT = 124 μA	2.7	-
Input capacitance	2.70 V to 10.1 V	0.050 μF	0.120 μF
Series input inductance	2.70 V to 10.1 V	-	100 μH

Since a PSE can only observe voltage at its own connector (PI), which may be 100 meter away from a PD, the acceptance range for PSE is set slightly wider than the required valid signature for PD devices. According to IEEE802.3 standards, a PSE shall accept as a valid signature a device with all of the characteristics specified in Table below (as measured at the PSE PI). It does not meant that a PSE will necessarily reject a device whose signature is outside of the valid range, as we will see below.

PSE ACCEPTANCE RANGE
PARAMETER	MINIMUM	MAXIMUM
Accept signature resistance RGOOD	19 kΩ	26.5 kΩ
Accept signature capacitance CGOOD	0	0.150 μF
Signature offset voltage tolerance	0	2.0 V
Signature offset current tolerance	0	12 μA

If PSE detector found a device signature outside of the acceptance range, it may still power this device if its signature is not in the "invalid" range (see Table below). There is a gray area between acceptance range and rejection range, in which PSE may accept or reject depending on its design algorithm.

INVALID DETECTION SIGNATURE
PARAMETER	MINIMUM	MAXIMUM
Reject signature resistance RBAD	15 kΩ	33 kΩ
Reject signature capacitance CBAD	10 μF	-
Open circuit resistance	0.5 MΩ	-

In practice, as far as I know, most (if not all) PSE calculates only differential resistance and do not care much about other characteristics. Inductance is not even included in non-valid signature characteristics. So, the designers primarily need to assure that their PD presents R_DET within the required voltage range in order to be accepted by PSE's detector. A good practice also is to have some C_IN to filter out noise. The simplest way of implementing a valid signature is to connect a 25k resistor and 0.1 μF capacitor across the output of the rectifier bridges (see PoE block diagram). Some controllers (like Microchip series PD70xx1) dynamically connect this resistor across the line when they observe detection pulses, and then disconnect it when voltage raises above the detection range. This way the power dissipation in R_DET in steady state is eliminated.

The main reason why a system may fail detection is when additional components (besides R_DET, C_IN, protection devices and EMI filters) are connected across PoE line. If PSE or PD has high leakage current to earth ground it may also fail detection.

When PSE accepts PD detection signature, it can start the next process called classification. Some PSE also do connection check (either prior or after the detection) to determine the system configuration: see PD configurations.