Common Mode and
Differential Mode
Discovery Techniques
Rick Brooks
May, 2000
Discovery Process Goals
• identify appropriate power hungry devices
• identify cable and connection problems
• avoid powering legacy equipment
• minimize the probability of a false detection
• provide a robust system solution
• practicable at a relatively low cost
• allow transparent use of straight and crossover cables
• enable powering without the need for management
• allow for the use of multiple DTE power sources
— want to be independent of power sequencing
— discovery method needs to be independent of data transmission
— alternatives would be some form of management via data path
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
1
Basic Flow Diagram
•
Discovery waits to find a power hungry device before power is applied
•
Power maintenance is handled by the power source minimum load current
LOOP UNTIL
SUCCESSFUL
DICOVERY
POWER
OFF
Ramp
Down
DTE Power
Discovery
State
(DTE power off)
FAULT
Ramp
Up
DTE Power
DTE
Power On
(maintenance)
LOOP UNTIL
UNDER MIN LOAD
OR FAULT
OR POWER OFF
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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Differential Mode Discovery
•
several proposals have been presented
•
uses well known 10/100 types of signals
•
managed by the PHY
•
Loops a signal through one pair, down the cable, and returns a modified
signal on the other pair
•
Pros
— tests all four wires
— best chance of finding unwanted parallel connected wires, especially if long
— uses well known and characterized IEEE802.3 signals, low EMC risk
— tests on the same wires that will receive DTE power
— robust, digital signature can be used
•
Cons
— does not test the wires in the mode that will be excited by the power source
— may not be able to sense common mode shorts, opens, intermediate resistance
— depending on pair used, could require a few more parts than common mode
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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Differential Mode Discovery
•
- undiscovered cable fault
Since only the differential mode is excited, common mode shorts, or other
impedances cannot be detected
Data Switch
DC
current
flow
Cable, Patch Panel
DTE, load end
75
75
75
75
75
75
75
75
+ Isolated
-
DC
Source
Power
+
DC
Load
-
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
4
Common Mode Discovery
•
several proposals have been presented
•
Resistive, diode, AC coupled diode, and current reg diode identity networks
•
uses low frequency signals
•
can be managed by the PHY, or the power source
•
Pros
— tests on the same wires that will receive DTE power
— test the wires in the same mode that will be used by the power source
— low frequency signals are used; out of band for 10/100/1000
— robust, digital signature can be used
— inherently independent of data, allows for multiple DTE power sources
•
Cons
— does not test all four wires, unless more circuitry is added
— will probably not find parallel wires unless they are shorted
— can not sense if a single wire is open in one or both pairs
— resistive schemes could be fooled by leakage mechanisms
– organic fluxes, leaky electrolytic caps, etc….
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
5
Common Mode Discovery
- undiscovered cable fault
•
partial open circuit cable fault not found with most common mode discovery
techniques, unless more circuitry is used
•
at higher power, this could limit the voltage available at the load
Data Switch
DC
current
flow
Cable, Patch Panel
DTE, load end
75
75
75
75
75
75
75
75
+ Isolated
-
DC
Source
Power
+
DC
Load
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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Common Mode Discovery, transformer isolated version
•
transformer coupled common mode technique provides 2200 VDC isolation
•
uses an AC coupled diode network for polarity sensitive detection
•
pulses and synchronous detection are digitally controlled
1nF
75
75
1nF
10 to
100uF
Discovery
DRIVER
Discovery
RCVR
DC/DC
Converter
CAT5 CABLE
1nF
SWITCH/HUB
(DTE POWER SOURCE)
75
Power
Identity
75
coupled
Diode
Network
bridge
rectifier
LOAD
1nF
DATA TERMINAL EQUIPMENT
(power hungry)
POWER DELIVERY
PATH
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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Common Mode Discovery, opto-isolated version
•
Opto coupled discovery, opto-isolators provide the 2200 VDC isolation
— could be integrated into DC/DC power converter
•
similar features to transformer isolated version
•
A low voltage, low power aux DC power supply is needed
1nF
DC/DC
Converter
OPTO
SWITCH/HUB
(DTE POWER SOURCE)
1nF
DRIVER
LV Aux Power
OPTO
75
75
CAT5 CABLE
RCVR
1nF
75
Power
Identity
75
coupled
Diode
Network
bridge
rectifier
LOAD
1nF
DATA TERMINAL EQUIPMENT
(power hungry)
POWER DELIVERY
PATH
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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Common Mode Discovery, AC coupled diode network
•
Diode detection modified to allow either polarity of DTE power
•
allows for low voltage, polarity sensitive discovery using low duty cycle 5us pulses
•
with high duty cycle discovery pulses, can be made to look like an open circuit
— provides a higher level of discovery confidence
•
becomes high impedance at +/- 48 VDC
— resistors can be small size, or integrated
•
easily handles 48 volt transients (intermittent contacts, etc…)
silicon
or schottky
diode
0.3 to 1
uF
2k
AC coupled diode network
100k
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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Bench Results, AC coupled diode network , 120 meter CAT 5 cable
•
diode non-conducting direction (upper left)
shorted load (lower left)
diode conducting direction (upper right)
open load (lower right)
— vertical cursors show area of synchronous detection
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
10
Bench Results, AC coupled diode network , 1 meter CAT 5 cable
•
diode non-conducting direction (upper left)
shorted load (lower left)
diode conducting direction (upper right)
open load (lower right)
— vertical cursors show area of synchronous detection
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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Future simplifications, common mode discovery
•
Lower parts count achieved by using a single transformer
•
a bit more silicon area needed?
1nF
75
75
1nF
Driver/Receiver
DC/DC
Converter
Driver/Receiver
10 to
100uF
1nF
SWITCH/HUB
(DTE POWER SOURCE)
75
CAT5 CABLE
Power
Identity
75
coupled
Diode
Network
bridge
rectifier
LOAD
1nF
DATA TERMINAL EQUIPMENT
(power hungry)
POWER DELIVERY
PATH
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
12
Spice Simulations,
single transformer common mode discovery
•
Here is how detection can work with a single transformer for lower cost, less parts
•
10 meter, 40 meter, 80 meter, and 200 meter cables shown
•
transformer primary pins, diode conducting (lower left), non-conducting (lower right)
common mode loop
Single
Transformer
Version
Synchronous
Driver and
Detector
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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Summary
•
neither differential, nor common mode discovery methods find all possible cable
faults; no scheme is foolproof
— differential mode discovery does not find common shorts (but the power source will)
— common mode does not find opens on redundant wires, this is probably acceptable
— a combination of both discovery modes would provide higher coverage but with increased
complexity and cost; it is probably not worth it
— it’s probably unrealistic to make a “network analyzer” finding any possible cable issue
•
the common mode AC coupled diode satisfies the basic requirements for discovery
— easy to implement, various ways to implement the concept
— low cost
— gentle to legacy devices, low voltage, low power
— relatively robust, but not foolproof
— independent of data delivery, multiple DTE power sources possible
•
common mode discovery probes the loop using the same mode (common mode) as
the DTE power source will provide
•
acknowledgements:
— Robert Muir, Level One
— Dan Dove, HP
— Larry Miller, Nortel
Discovery Techniques, R. Brooks, IEEE 802.3, May 2000
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