Sifos Technologies PowerSync PSA100, Technical Reference Manual, Page: 13 / 147

PowerSync Analyzer Reference Manual
March 9, 2010 Sifos Technologies page 13
2.2.2. Connection Alternatives
PSE’s will generally be configured to source power on EITHER the data pairs (10/100BaseT) OR the unused pairs. For
any given PSE port implementation, this should be a design constant. Within the 802.3af specification, Mid-Span
PSE’s MUST utilize the unused 10/100BaseT pairs. The 802.3at specification allows for Mid-Span PSE’s that power
either pair and support 1000BaseT where all 4 pairs are used for data transmission.
PSEs can provide EITHER polarity (+48V or –48V) to which PDs must be insensitive. Some PSE’s may offer
programmability in configuring their ports for MDI or MDI-X which in turn may affect voltage polarity.
2.2.3. MPS Behaviors
Generally, a PSE will be designed to utilize either the AC or DC MPS method for determining a PD disconnect. Hence,
this is one fundamental characterization of each PSE port that affects both the types of tests and the method of
controlling a PSE port through PD emulation. Those ports supporting DC MPS need to see a low current condition
(below 10 mA) in order to remove power. Those ports supporting AC MPS need to see an effective load impedance in
excess of 1.98 M
Ω in order to remove power, with a PSE-dependent load threshold ranging from 27 K
Ω
to 1.98 M
Ω
!
These PSE’s will tolerate a PD operating at less than 0.1 watt indefinitely since a DC load current of 2 mA (assuming
port voltage of 48 VDC) translates to effective 24K
Ω
resistive load.
2.2.4. Legacy Modes and Proprietary Detection Schemes
Prior to 802.3af, a considerable amount of “legacy” PoE equipment including first-generation VoIP phones utilized
proprietary technology to generate and manage power over LAN. Typical legacy powered devices include a low
frequency coupling circuit between data transmission pairs and/or the existence of a large capacitance seen by common
mode measurements across either data or spare pairs. These features could be sensed by the power sourcing equipment
using proprietary techniques. Protocols for determining power requirements of the PD were also proprietary and
conducted at the MAC layer.
With the volume of legacy PD’s in the
marketplace, many new generation PSE’s seek to
utilize BOTH 802.3 signaling and legacy detection
methods simultaneously so that either type of PD
can be recognized and powered. This “hybrid”
detection behavior may violate 802.3 specifications
when detection signaling is compliant with
relevant 802.3af criteria including signal levels,
source impedance, and slew rates. However,
assuming this hybrid behavior does not damage
802.3 compliant PD’s, it is desirable to customers
who want maximum flexibility in their PSE.
Hybrid detection schemes may also include coarse
measurements to assess possible PD connections
prior to performing more refined 802.3 detection.
These proprietary techniques may help to prevent damage to ordinary Ethernet interfaces (non-PD) or they may be
beneficial in determining possible legacy PD connections. On the other hand, the added complexity complicates testing
by adding ambiguity as to exactly how a PSE is detecting either an 802.3 or other type of PD.
Figure 2.5
Hybrid Proprietary Detection Methods
Special Detection Schemes
10
50
Vo
lt
s
DC
Proprietary
Legacy
Detection
Pulses
802.3
Detection
Measurement
Proprietary
Course
Detection
Measurent
Special Detection Schemes
10
50
Vo
lt
s
DC
Proprietary
Legacy
Detection
Pulses
802.3
Detection
Measurement
Proprietary
Course
Detection
Measurent