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(speaking NOT as chair, but as a participant)
I’m having trouble with this.
We don’t specify cables.
We don’t specify power sources alone in IEEE 802.3, except in how we specify the behavior or interoperable systems.
“Power classes” are a useful concept for interoperable systems – but, their main utility is in making plug-and-play systems. Power class is used in PoE and PoDL to allow PSEs to allocate power and PDs to deterimine what power they are going to get (if demoted).
In contrast, “Engineered systems” have their “power class” predetermined by an engineer. Unless we tightly constrain these (meaning very one, or very few classes) There seems no need for IEEE 802.3 to define these. We can always be put together with power sources. An engineer can, as you have ably demonstrated, calculate the ohmic losses and the load requirements for a multidrop network, given a voltage source.
If we do not define a detection (and/or classification) protocol, we need to be careful because there are other IEEE 802.3 single-pair standards which could be plugged into this power source.
We can’t get away from not-damaging existing BASE-T1 units by saying “multidrop networks” or “engineered systems”. To do that means a PSE is not only non-interoperable, but could damage other BASE-T1 devices. Remember that a point to point link segment IS a degenerate case of a point to point system. I would be willing to accept damage of a point-to-point unit (say 100BASE-T1 or 1000BASE-T1) being plugged into an already powered multidrop network as a 3rd (or higher) node – where it is clearly not a point-to-point application already (others may disagree here), BUT, I don’t want (and I believe others in 802.3 would share this opinion) to have IEEE 802.3 single pair systems blowing each other up. The existing systems have the following fault-tolerance requirement:
The wire pair of the MDI shall, under all operating conditions, withstand without damage the application of
short circuits of any wire to the other wire of the same pair or ground potential or positive voltages of up to
50 V dc with the source current limited to 150 mA, as per Table 96–6, for an indefinite period of time. Normala
operation shall resume after the short circuit(s) is(are) removed.
This would say that the source voltage is limited to 50 Vdc but it also limits the current, at least initially to 150mA. This suggests to me the need for a PSE detection protocol, before applying first power, which stays within that Voltage/current envelope. Achieving that (unless it is as defined in Clause 104 – which could be OK) could be a lot of work.
Additionally, what we seem to need to define to enable these engineered systems are PD characteristics. The PSE voltage/current capabilities is really for the engineer designing the system.
If we do this, what we end up with are:
I don’t think “power classes” adds to interoperability here, without the ability to auto-configure. If we auto-configure, power classes makes perfect sense, but, without them, we have to move towards
defining what the PD needs to tolerate – not what the PSE needs to put out.
George A. Zimmerman, Ph.D.
President & Principal Consultant
CME Consulting, Inc.
Experts in PHYsical Layer Communications
There is a discussion on what should be specified in the standard for multi-drop power in our adhoc meeting. I start this email to follow-up that discussion.
First I believe multi-drop power system is an “engineered” system, if anyone has any use case that is not “engineered”, please share with us if possible.
Multidrop Engineered Power System is different from PnP PoDL point-to-point power system: One PSE will power multiple PDs, and PSE cannot detect/classify how much power that the PDs demand during the operation.
With these differences in mind, initial thoughts to specify power class for multi-drop are shared as follows:
Multiple power classes could be specified for different use cases (industrial automation, automotive, … ).
Please share your thoughts on this matter if you have, thanks!
Dayin XU(徐达银), Sr. Research Engineer
Shanghai Research Center, Rockwell Automation