Hi Yair,
You missed my point. In your response you state that MPS threshold for disconnect is 4 mA. Please point me to the text in the standard where the PD designer
learns about this 4 mA threshold. I don’t see the 4 mA threshold called out anywhere in the PD sections of the standard.
Thanks,
Chris
From: Yair Darshan <YDarshan@xxxxxxxxxxxxx>
Sent: Thursday, June 14, 2018 7:58 AM
To: Chris Bullock (bullock) <bullock@xxxxxxxxx>; STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: RE: [802.3_4PPOE] MPS issue due to the allowance for reflected voltage in 3-pair mode
Hi Chris,
Thanks.
See below.
Yair
EXTERNAL EMAIL
Hi Yair,
Yes, I agree with everything that you stated below, but where did you come up with MPS=3mA that you used in your example?
Yair: The 3mA is an example of a case that a PD vendor decide that now he wants to be disconnected. Because in 2-pair and 3-pair mode when class is 4, the
MPS threshold for disconnect is 4mA, then 3mA will guarantee disconnection. Actually, any number below 4mA will gurantee disconnection with D3.4. But now with D3.5 it may not happen due to the Irev. If PD wants it to happen, now he need to set the MPS to
avalue less than 2.7mA (4mA-1.3mA). But the PD vendor don’t know about Irev that is why we need to tell him that there is Irev too that he needs to account for
when he set MPS.
Why not use MPS=8mA for your example, given your logic that PD designer does not read the PSE section?
Yair: Because the use case that I am addressing now is to guarantee disconnect and not maintain power. If PD wants to maintain power then he will set the MPS
to 8mA but I am talking about the opposite case.
If I’m a PD designer, and I need to violate Iport_MPS, all I need to do is draw less than 10 mA, correct?
Yair: Incorrect. Below 10mA still doesn’t guarantee anything.
Between 4 to <10mA it is may or may not disconnect. My use case is about the case that a PD wants not to receive power. That is why he need to set its MPS to less than 4mA in 2-pair or 3-pair mode class 4 per D3.4 and less than 2.7mA per D3.5. I hope I understand
you correctly.
The actual parameters that the PD designer needs to be aware of to implement the feature that you describe are Ihold_min and Irev, both of which are clearly
defined in the PSE section.
Yair: The problem is that the PD vendor reads the PD section and may not read the PSE section. This is the normal case. There are big companies that do PSEs
and PDs. Most of the companies are PSEs or PDs not both. This is why we have two independent sections for PSE and PD. Each section need to be clear and sufficient to design its device. This was always the concept. The fact that Irev is specified in the PSE
spec and PD spec doesn’t have any clue about it IS THE PROBLEM that I am trying to resolve. We had many such cases (many of Lennart’s
comment) and we resolve it in that way (adding links to the other sections or adding similar parameters to the other section).
In this particular feature that you are describing, the argument that the PD designer does not read the PSE specification does not hold because the PD
is manipulating a parameter of the PI in order to get the PSE to respond in a particular way. I don’t understand how a PD designer could believe that he/she could implement such a feature without thoroughly reading and understanding the PSE requirements.
Yair: This is the concept of how we build our spec. In order to build PD you need to read only the PD section. It is better to read the PSE section as well
to fully understand everything but not always it is easy to make the connections and the cross effects. Irev is a perfect example. How the reader, even if he reads the PSE spec can understand that Irev will affect MPS if he has backffed issue. How?
It is simpler whenever one of us believes that it is not clear and straightforward, to make this clear by adding a link or something.
Thanks,
Chris
Hi Chris,
Your input is not clear to me.
Are you saying that if a PD wants to be disconnected and present e.g. MPS=3mA, PSE may not disconnect him? How it is possible if for class 1-4 the PSE is required to disconnect at
<4mA?
Now because of Irev that PD was not aware of it ,PD may not disconnect since now the current will be 4.3mA >4mA.
If PD was aware of Irev, than he could guarantee to be disconnected by setting Iport_MPS to <(4mA-1.3mA)=2.7mA (or other means).
Do you agree?
If Yes, my latest proposal for the added text to the PD MPS section: "IPort_MPS may be affected by Irev. See 145.2.10.4, 145.3.8.8.”
will solve the problem.
Do you agree?
Yair
EXTERNAL EMAIL
Hi Yair,
The MPS thresholds for a PD are different than the MPS thresholds for a PSE. The PD cannot guarantee that the PSE will remove power simply by violating
the PD MPS thresholds.
There is no way that a PD designer can create the feature that you are talking about without reading the PSE section.
Thanks,
Chris
George makes a good point, 'implementation hints' are tricky to include in the standard - especially at this point.
Yair, we are updating the EA whitepaper on BT anyway to include the changes since D3.2. A big new section will be
explaining reflected voltage / reverse current.
Would it be acceptable if we explain the MPS issue there (rather than in the spec)?
On Tue, 2018-06-12 at 18:56 +0000, George Zimmerman wrote:
Yair – all of these values are tutorial in nature. There are no requirements in your proposed text, therefore they are completely advisory.
We write standards – the important thing to capture is what is required. The information may be valuable to implementers, I don’t argue that. However, publish that elsewhere (for
example, in an EA whitepaper) if you so desire.
George A. Zimmerman, Ph.D.
President & Principal Consultant
CME Consulting, Inc.
Experts in PHYsical Layer Communications
1-310-920-3860
george@xxxxxxxxxxxxxxxxxxxx
Hi George,
The value of the proposed text is much higher that you have described see why:
The issue is:
-PD operates in 3-pair mode with ideal diode bridge that has the backfeed issue. This means that whatever load is in the unpowered PSE alternative, it is reflected to the PD load
and added up.
-PD wants to be powered off. As a result it sets its MPS to 3.9mA which means must be OFF. PD vendor may not be aware of the Irev spec since it is in the PSE spec.
-Now in this condition Irev=1.3mA will be added from the PSE to the PD MPS i.e. 3.9mA+1.3mA=5.2mA and PD will not be disconnected.
This is the problem.
The solution is that the PD vendor will take the 1.3mA in account of the MPS setting.
The problem is that the PD vendor is not aware of Irev since this is the PSE spec and many times as a response to comment we agree to add text to PD section to tell PD about parameters
that affects the PD but appear in the PSE i.e. we said that the PD vendor get a PD spec and start to design without looking on PSE spec or aware of it which I believe it is a correct scenario.
By the way, during last meeting we change the PSE and PD spec to address similar issues concerning to the effect of Irev on other spec items and this is one of them.
I can never agree to a situation that you have a clear requirement in a PD for what to do regarding MPS (or other spec items) without telling the PD vendor that there is something
waiting for him in the corner… in the PSE spec that is not mentioned in the PD spec that can make his PD uncompliant. I don’t see a reason to
hide such critical information.
Yair
EXTERNAL EMAIL
Yair - the proposed text is purely advisory and informative. At this stage we should only be concerned with missing or incorrect requirements or correcting (preferably deleting) incorrect information. I just don’t see the point of adding more informative advice
to the standard.
George A. Zimmerman, Ph.D.
Experts in PHYsical Layer Communications
Hi Lennart,
I agree that it doesn’t affect the case when the PD is disconnected from the cable.
I was referring to the case that the PD is connected and wants power removal. The question if it is rare or not, is irrelevant since it is already in the spec and we need to address
it somehow and meet it.
I agree that the solution can be that PDs that do want to have power removed can set their Iport_mps value for power removal to be lower than (4mA-1.3mA)=2.7mA. However, in order
to make this clear to the PD vendor (since Irev is in the PSE section) I believe that we need to add text to the PD MPS section as follows (or equivalent):
Proposed remedy:
Add the following text in clause 145.3.9, page 222 text after line 49:
"When a PD is operating under 3-pair mode conditions, the value of IPort_MPS as seen by the PSE over the powered pair may increase by Irev (see See 145.2.10.4, 145.3.8.8 ). As a result,
the PD may need to set IPort_MPS to alower value than IPort_MPS min to ensure power removal."
Yair
EXTERNAL EMAIL
Your analysis is correct, the reverse current is added to the PDs own current.
I don't consider this an issue we need to do anything about however:
- it does not impair the primary function of MPS in any way (to remove power when the PD is disconnected)
- it only affects PDs that use the method of removing MPS in order to have the PSE remove power, I would say this is pretty rare;
- PDs that do want to have power removed can accommodate for the maximum 1.3mA of reverse current (draw less than 2.7mA of their own)
Note that reverse current only happens under 3-pair conditions, and then the 'must disconnect' current level is 4mA for PSEs.
On Mon, 2018-06-11 at 12:03 +0000, Yair Darshan wrote:
Hi all,
I found new problem that we need to discuss how to handle it.
In 3-pair mode during power on state, when a PD don’t want to be powered, it generates e.g. MPS=1.9mA which means PSE must
disconnect, but due to the PD that doesn’t meet the backfeed on the unpowered pair, the unpowered pair consumes additional 1.3mA and this is added to the MPS. Under these conditions, the PD will not
be disconnected.
Moreover, in general, a constant error of additional MPS current is added by the PSE…..instead of PD only should control the
MPS current.
Let’s start to discuss this.
Yair
Darshan Yair
Chief R&D Engineer
Analog Mixed Signal Group
Microsemi Corporation
1 Hanagar St., P.O. Box 7220
Neve Ne'eman Industrial Zone
Hod Hasharon 45421, Israel
Tel: +972-9-775-5100, EXT 210.
Cell: +972-54-4893019
Fax: +972-9-775-5111
E-mail: <mailto:ydarshan@xxxxxxxxxxxxx>.
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