PowerSync Analyzer
Application Note
PSE MPS Processes
PSE MPS Process Analysis
Under the IEEE 802.3af specification, power sourcing equipment (PSE) is obligated to detect the disconnection of a powered
device (PD) through one or both of two methods involving a “maintain power signature” (MPS) protocol:
DC MPS: Sensitivity to low current (low power) condition
AC MPS: Sensitivity to high load impedance condition
PSE’s are given full latitude to select the method and a significant degree of range on the disconnect criteria. Some key
specifications governing DC MPS method are provided in Table 1.
DC MPS essentially involves a
continuous sampling or monitoring
of the load current furnished by the
PSE port to the PD. While
relatively simple to implement, the
DC MPS method has the
disadvantage that PD’s must
either continuously or at least
periodically (3 times per sec) draw
10 mA, or .5 Watts of power or in
order to stay powered. For this
reason, some PSE’s favor the AC
MPS method.
Parameter
Description
Criteria
Imin1
The dc current level at which a PSE decides that the
PD has been disconnected. Hence, the “invalid”
signature threshold.
Imin2
The dc current level at which a PSE decides that the
PD is still connected. Hence, the “valid” signature
threshold.
Tmpdo
The required duration of an “invalid signature” before
power is removed from the port.
Tmps
The minimum time a “valid signature” must be
present in order to declare it “valid”.
Vopen_max The maximum port voltage during signaling following
DC disconnect power-down.
Table 1: DC MPS Specifications
0 to 5 mA
> 5 to 10 mA
300 to 400
msec
< 60 msec
30 Volts
AC MPS requires that the effective PD input impedance be monitored after the application of power. This is accomplished
by combining an AC signal with relatively high source impedance on the (nominal) 48V DC power rail, and then monitoring
the AC MPS signal amplitude to measure PD AC input impedance. This method has the advantage that the PD may consume
very low or zero DC power for extended periods of time and remain powered. The disadvantage is some added “noise” on
the DC power rail and some added complexity in the PSE design. Some key specifications governing AC MPS processing
are provided in Table 2 below.
Parameter
Description
Zac1
The “valid signature” impedance (must be resistive or
capacitive) where the PSE must maintain port power.
Zac2
The “invalid signature” impedance (must be resistive or
capacitive) where the PSE must remove port power.
V_open
The maximum peak-peak amplitude of the AC MPS signal
immediately following PD disconnect and prior to powerdown by the PSE port.
VportMax
The maximum voltage on the PSE port during AC MPS
disconnect detection.
V_close
The maximum peak-peak amplitude of the AC MPS signal
while the PD is connected to the PSE. (Note: This is
equivalent to “Vpp” specification for frequency < 500 Hz.
Fp
The frequency of the AC MPS signal.
SR
The maximum slew rate of the AC MPS signal.
R_rev
The effective source impedance of the PSE port when
performing AC MPS measurements.
I_sac
Maximum AC current flow while the PD is connected to the
PSE port.
Tmpdo
The time duration of an “invalid signature” before the PSE
removes power from the port.
Vopen_1
The peak voltage on the port following an AC disconnect
induced power down event. (Note: This is equivalent to
Voc.)
Table 2: AC MPS Specifications
June 5, 2008
Copyright 2007 Sifos Tehnologies,Inc.
Criteria
< 27 KΩ
> 1.98 MΩ
< 10% of Vport
(e.g. < 4.8 Vpp for
Vport = 48V)
< 60 Volts
< 500 mVpp
< 500 Hz
< .1 V/µsec
> 45 KΩ
< 5 mA
300 to 400 msec
30 Volts
Page 1
PowerSync Analyzer
Application Note
PSE MPS Processes
AC MPS Specification Issues
Despite the obvious advantage of supporting low DC power consumption by the PD, the AC MPS method is too vaguely
described within in the IEEE 802.3af specification to be 100% foolproof and interoperable method of PD disconnect
detection. Some key areas where specification issues exist are:
•
•
•
•
•
Wide Discretion Range: PSE’s have full discretion on the interpretation of “signature loads” in the range of 27
KΩ to 1.98 MΩ . A PD with a connection signature of 50 KΩ or even 100 KΩ could probably survive a
tremendous amount of PSE-PD interoperation testing with no evident problems. PSE’s that use higher MPS
frequencies and look for 1.98 MΩ signature impedance to remove power may be vulnerable to high impedance
shunt conductance paths that remain after a PD is disconnected. Example: High humidity environments.
Behavior Given Small DC Loads: Removal of an AC MPS signature load may or may not be sensed as a cause to
power down at the PSE depending upon other characteristics of the PD. A PD that initiates a power down by
setting current to less than 5 mA and disconnecting it’s AC MPS signature load might remain powered in many
cases because whatever load is drawing the low DC current will still in all likelihood appear as a valid AC MPS
load signature to the PSE. The threshold for cut-off is 1.98 MΩ and there is no lower bound on AC MPS
frequency. A PD must effectively draw zero DC current in order to assure power-down across all PSE’s regardless
of the specified requirements for DC MPS!
Lack of a Minimum AC MPS Frequency: Capacitive AC MPS signatures in a PD are only meaningful given a
known frequency of the AC MPS signal. Looking at the 802.3af specification for PD’s, there is an AC MPS
signature requirement consisting of no higher than 26.25 KΩ resistance in parallel with no less than .05 µF
capacitance. This resistance component forces all signatures to be valid regardless of AC MPS frequency,
however it therefore precludes purely capacitive AC MPS signatures that would draw no DC power. PD
developers may attempt to sidestep the associated DC power drain associated with the MPS resistive load
signature.
Undefined Waveform: The specification in no way attempts to describe the shape of the AC MPS waveform – it
could be a continuous (sine wave) or it could be discrete pulses, or anything in between. This too will affect how
PD MPS signature loads appear – narrow pulses might be more affected by a capacitive signature than a sinusoidal
counterpart waveform.
AC MPS Signaling After Power-Down: There is no explicit indication of what happens to the AC MPS signal
following the power-down event nor any description of how that signal behaves in the presence of detection load
signatures. An inference might be made that the AC MPS should be disabled and should not interfere with the
detection signaling and detection measurement processes. However actual devices are not all implemented with
capability to control AC MPS per port meaning the signal may or may not interfere with other processes.
Implication to the PD
Interestingly, the IEEE 802.3af specification allows for the combination of AC and DC MPS techniques. PD’s are required
to present a 26KΩ or smaller AC MPS signature when powered and are also required to maintain the minimum load current
requirements of DC MPS method. As a practical reality however, the DC Method effectively dictates what a PD must do to
assure it will stay powered since a PD cannot know whether the PSE is using the DC MPS or AC MPS method. A PD must
therefore draw the maximum Imin2 current level of 10mA to stay powered regardless of the presence of the AC MPS
signature resistance thus meaning that to a PSE using the AC MPS method, the effective signature resistance in that PD will
always be satisfied by the 10mA current flow (effectively a 5KΩ signature). The one case where this is not true is a PD that
modulates it’s steady state DC Load such that it meets the required 20% duty cycle for valid (greater than 10mA) load
current. (see Tmps) . In this isolated case, the AC MPS PSE could possibly rely upon the presence of the MPS signature
load to keep power alive to the PD.
DC MPS Measurements
The PowerSync Analyzer’s PSE Conformance Test Suite offers two tests, mps_dc_valid and mps_dc_pwrdn to assess DC
MPS behaviors at the PSE. These tests generate all four parameters described earlier for the DC MPS method.
Imin1 – (Maximum) Invalid DC Signature Threshold
The mps_dc_pwrdn test measures Imin1 by stepping the DC load current from a valid value (> 10 mA) to a sequence of
“invalid” values starting at 0 mA and verifying that a power down occurs within one second of the load step. Imin1 is then
reported as the highest value of DC load current that successfully forces the port shutdown.
June 5, 2008
Copyright 2007 Sifos Tehnologies,Inc.
Page 2
PowerSync Analyzer
Application Note
PSE MPS Processes
Tmpdo – Shutdown Timing
The mps_dc_pwrdn test measures Tmpdo as the time interval from a
DC current load step between a “valid” current level and a known
“invalid” level until a power-down transition is observed. Ideally, the
test is constructed using the invalid load and/or a load transient to
actively pull charge from the PSE port in order to accurately capture
the power-down transition. The test requires accurate timing
alignment to the load current step transition.
Imin2 – (Minimum) Valid DC Signature Threshold
The Imin2 measurement within the mps_dc_valid is essentially the
inverse of the Imin1 measurement. The test measures Imin2 by
Figure 1: Shutdown Timing
sequentially launching 500 msec second load transients looking for
the highest level of DC load current that does not result in a port
power-down event. Port voltage is sampled over the duration of
the low-load transients to find any transitions below the
Transition
minimum powered port voltage level.
To Imin1
Tmps – (Minimum) Valid DC Signature Timing
Tmps represents the minimum time (or duty cycle) over which a
valid DC signature (load current level) must be maintained in
order to negate (or “cancel”) a disconnect power-down event.
The mps_dc_valid test measures Tmps using timed DC current
load transitions. After powering the PSE port, a transition to zero
mA DC load current is followed by a timed “valid” DC load
Figure 2: Tmps Load Waveform
current pulse, then a second interval of 0 mA over a 500 msec
window. Valid load current is restored approximately 450 msec
following the initial transition to zero load current. Port voltage is also measured over this exact same time window.
The duration of the of the timed “valid” DC pulse is successively
stepped starting at 100 msec down to 10 msec to recover the first
duration where a port shutdown occurs during the 450 msec time
window thus suggesting Tmpdo expiration. Tmps becomes the
minimum current pulse duration required to keep power alive
throughout the full sequence. Another view of Tmps is the
minimum duty cycle of valid load current required to keep power
alive. In this view, the total period is the combination of smallest
valid Tmpdo (300 msec) and largest valid Tmps (60msec) thus
creating a 16.7% duty cycle (60 msec / 360 msec).
Figure 2 demonstrates a power-down decision where a 60 msec
load pulse led to proper decision to maintain power. Figure 3 on
the other hand demonstrates a Tmps violation since the PSE killed
power given a 75 msec load pulse over 430 msec (> 17 % duty
cycle).
Unfortunately, the IEEE 802.3af specification is worded poorly in
its description of Tmps – if read literally, it says that Tmps (< 60
msec) is the minimum time required in order to interpret valid DC
current as “valid”. In fact, from the PSE’s perspective, it is the
maximum allowed time criteria for interpreting valid current as
“valid”.
Figure 3: Tmps Violation Power-Down despite >60
msec load pulse.
Vopen_max – This is the peak signaling voltage detected in the interval immediately following DC disconnect power-down.
This parameter is bound by the maximum open circuit signaling voltage of 30Volts.
June 5, 2008
Copyright 2007 Sifos Tehnologies,Inc.
Page 3
PowerSync Analyzer
Application Note
PSE MPS Processes
AC MPS Measurements
The PowerSync Analyzer’s PSE Conformance Test Suite provides three tests, mps_ac_pwrdn, mps_ac_vf, and
mps_ac_voff to evaluate AC MPS behaviors at the PSE. The parameters and test methods will be presented for mps_ac_vf,
mps_ac_pwrdn, then mps_ac_voff.
V_open: Peak-to-Peak Amplitude of AC MPS Signal with Open Circuit Load
The V_open measurement is derived from a waveform capture timed to the removal of the AC MPS signature load given that
there are no other loads that would appear as valid signatures. The peak-to-peak amplitude is assessed as the difference
between waveform maximum and minimum values over the 200 msec duration of the sample. Sampling must be finalized
before PSE power removal at 300 msec (or later). The measurement method is fully insensitive to waveform shape.
Fp: AC MPS Waveform Frequency
The same waveform captured for V_open analysis is also analyzed
to determine signal periodicity by assessing an effective mid-level
voltage, then counting mid-level transitions over the duration of
the sample and calculating signal frequency in Hz.
Resolution of this measurement is limited to the fact that sample
time duration is restricted by the inevitable power-down event.
SR: AC MPS Maximum Slew Rate
While the waveform shape is not defined within the specification,
a maximum slew rate is defined that is equivalent to maximum
slew rates elsewhere for detection signaling and power-up edge.
Figure 4: AC MPS Power-Down
Slew rate requires a time interval measurement in order to
resolve to microseconds, especially for small amplitude signals.
The mps_ac_vf test carefully evaluates the waveform captured
to converge on optimal trigger levels for a subsequent slew rate
(risetime) measurement.
Isac: AC MPS Detection Current Level
This parameter places an upper bound of 5 mA on AC current
flow that is associated with the AC MPS detection process.
This represents a compliance limit that could only come into
effect in the event of a PD that had a relatively low AC input
impedance on the order of 100 Ω (e.g. > 80 µF at 20 Hz) and the
Figure 5: AC MPS Waveform
PSE had a sufficiently stiff source. In reality, since the MPS
cutoff signature can range up to 1.98 MΩ , there is no reason for the PSE to furnish this type of AC sourcing capability.
The mps_ac_vf test does make a measurement of “steady state” AC MPS current measured while the PSE is powered and
furnishing a small DC load. The measurement is performed by capturing the AC MPS voltage magnitude with an AC peak
measurement and then resolving AC current using known AC input impedance characteristics of the PSA test port. The AC
load presented by the PSA test port will be between 10 KΩ and 20 KΩ depending upon AC MPS frequency.
The mps_ac_pwrdn test returns the following parameters:
Tmpdo: Shutdown Timing
The mps_ac_pwrdn test measures Tmpdo as the time interval from the removal of a valid AC MPS signature load until the
actual power-down event is observed. The test deploys a very short load transient to actively pull charge from the PSE port
at the end of the Tmpdo time window order to assure the power-down transition is captured in the event the PSE does not
actively discharge the port.
Imin1: DC Load Current Required for AC MPS Shutdown
This measurement fulfills two purposes. The first is to validate that an effective open circuit load does in fact cause a powerdown from the PSE. The second purpose is to resolve the maximum DC load current configuration that is interpreted by the
PSE port as a valid AC MPS signature. This measurement therefore demonstrates AC MPS “sensitivity” to the
characteristics of the PSA test port’s active load circuit. This parameter should not be confused with the specified Imin1
parameter developed when testing DC MPS PSE’s.
The mps_ac_voff test returns two additional parameters:
V_open1: The PSE Output Voltage Following Disconnect Decision
June 5, 2008
Copyright 2007 Sifos Tehnologies,Inc.
Page 4
PowerSync Analyzer
Application Note
PSE MPS Processes
This measurement allows for an initial power-down transition, then recovers peak port voltage in
the time interval immediately following this shut-down event. As with open circuit detection signaling, this parameter is
limited to 30 volts. The test will adapt in the event that an initial voltage transition following the disconnect decision is
positive rather than negative (declining) – a condition that will be deemed non-compliant.
Vport(max): The Maximum PSE Port Voltage While Determining Disconnect
This test assesses the peak voltage measured prior to port power removal by the PSE. Analysis of power-down waveform is
analyzed to resolve most likely transition point between disconnect measurement and power-down. Peak voltage in the time
interval preceding this transition point is then evaluated.
Sample PSE Conformance Test Reports (DC and AC MPS)
Figure 6: DC MPS Test Results from PSE Conformance Test Suite
Figure 7: AC MPS Test Results from PSE Conformance Test Suite
Test Configuration
PowerSync Analyzer
Sifos
OUT1
POWER
Sifos
OUT1
P S A -1 2 0 0
Sifos
OUT1
Sifos
OUT1
Sifos
OUT1
Sifos
Sifos
OUT1
OUT1
Sifos
OUT1
Sifos
DET
DET
DET
DET
DET
DET
DET
DET
PWR
PWR
PWR
PWR
PW
PWR
PWR
PWR
PWR
ARM
ARM
ARM
ARM
AUX
PS1
IN
OUT2
ARM
AUX
PS1
IN
OUT2
ARM
AUX
PS1
IN
OUT2
ARM
AUX
PS1
IN
OUT2
ARM
AUX
PS1
IN
OUT2
ARM
AUX
PS1
IN
OUT2
ARM
AUX
PS1
IN
OUT2
ARM
AUX
PS1
IN
OUT2
AUX
PS1
IN
OUT2
DET
DET
DET
DET
DET
DET
DET
DET
DET
DET
DET
DET
PWR
ARM
PWR
ARM
PWR
ARM
PWR
ARM
PWR
ARM
PW
ARM
PWR
ARM
PWR
ARM
PWR
ARM
PWR
ARM
PWR
ARM
PWR
ARM
AUX
AUX
AUX
AUX
AUX
AU
AUX
AUX
AUX
AUX
AUX
AUX
PS2
PS2
PS2
PS2
PS2
PS2 In
PS2 In
PS2 In
PS2
PS2
P S E A F -2
P S E A F -2
P S E A F -2
P S E A F -2
P S E A F -2
P S E A F -2
P S E A F -2
P S E A F -2
P S E A F -2
P S E A F -2
P S E A F -2
PS2 In
P S E A F -2
PSE
For more information on the Sifos Technologies’Power over
Ethernet test & measurement solutions look us up at:
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Sifos Technologies, Inc.
1061 East Street
Tewksbury, MA 01876
sales@sifos.com
Publication #1000-0003
June 5, 2008
Copyright 2007 Sifos Tehnologies,Inc.
PWR
ARM
AUX
PS1
IN
OUT2
PS1
IN
OUT2
PS2
DET
AUX
AUX
PS1
IN
OUT2
Sifos
OUT1
OUT1
DET
PWR
PS1
IN
OUT2
TRIG
Sifos
OUT1
DET
PW R
AUX
RS -232
Sifos
OUT1
DET
PWR
ARM
10/100BT
Sifos
OUT1
DET
Page 5
DET
PWR
ARM
AUX
PS2
P S E A F -2