[FUN] Communications Disruption during Device Connection

[Stephen Mitschke]

I raised an issue during one of the more lively discussions, regarding thepotential for communications disruption on a FF segment during:1. Live connection/disconnection of devices (hot swapping), due to surgecurrents (most likely device in-rush current during power-up) and,2. Inadequate device bus transceiver circuit protection in the event of anelectronic failure within the device (most likely scenario would be a shortcircuit of the bus segment).You asked me if I would follow this up with an email to you as a prompt toraise the matter with the FF EUC, with a view to (perhaps) making arecommendation that suitable safeguards be added to the FF specificationrequiring manufacturers to provide adequate safeguards in their circuitdesigns to make the outcome of hot-swapping and device failure currents morepredictable.From memory I think it was quoted during the Relcom paper that there is onlya 500 microsecond window of opportunity for devices to stabilise their powerconsumption before problems can occur. In any event I would imagine thatmany factors will influence the risks such as the number of devices on thesegment and the communications loading.Although spur guards address the issue to some extent they pose anadditional cost, and may limit the flexibility in some segment topologies.Then there is the need for the technician to confirm they are installed.I believe that with the chipsets commonly used in Profibus PA (e.g. theSPC-4 from Siemens), in-rush and short circuit current limiting features arebuilt into the design. Given the common physical interface between theprotocols (IEC 1158-2) would it not be wise to:1. Incorporate the same philosophy in FF as for Profibus PA (also refer toFISCO model), bya) Clearly specifying what is required,b) Putting the responsibility on manufacturers to ensure theprotection is built-in to their design,c) Add a test procedure to the FF device certification process to makesure new devices behave.What end users need with this type of technology is high degree of certaintyabout what they can and cannot do in maintenance situations withinreasonable bounds. This is especially so given that some devices apparentlygo off-line during a bus fault, and require re-initialising when they comeback on-line. That type of disruption to other functioning devices on asegment when one goes bad (or is connected/disconnected) is not acceptable.To conclude, it seems reasonable to me that putting the requirement onmanufacturers to ensure adequate built-in circuit protection to enable hotswapping, and short circuit protection is not technically, or economicallyunreasonable, and how to do it, and verify it should be laid down in the FFspecs.Wayne Gray-GarneyProcess Control SupportCHH Tasman Pulp

[Stephen Mitschke]

> This inrush current problem represents one of many deficiencies in the ISA> /> IEC Physical Layer specification. Fieldbus Foundation has passed devices> with high inrush current because there is no specification that forbids> high> inrush. Most vendors have restricted inrush current to practical levels> because they need for their devices to work in current limited (IS or> Non-Incendive) circuits.> > Users have this same need, and expect vendors to use sound design> practices.> However, you will find an occasional device that has an inrush current> several times their steady state current - and it meets all> specifications.> > This is an area where everyone would benefit from an upgrade of the specs.> By everyone I mean users, Fieldbus Foundation device and system suppliers,> Profibus PA suppliers (who use the same Physical Layer specifications),> and> even ISA and IEC.> > It is hard to imagine all of these people working together in a> cooperative> manner, but that is exactly what should happen. We could even get an end> user or 2 to work on the committee. It's just wishful thinking, but it> would be nice.> > Herman Storey> Equilon Enterprises LLC, WTC, Automation Engineering

[Stephen Mitschke]

> Ian,> > Here are a few comments on Wayne Gray-Garney's comments:> > Having the device circuitry limit the current drawn from the bus is not a> bad idea but it is incomplete. This capability does not solve the problem> of water in the conduit that shorts the drop cable or the instrument> technician who inadvertently shorts the drop cable while working on an> instrument. Current limiting in the junction box between the trunk cable> and the drop cable is the only way to deal with these shorts that would> otherwise take down the entire network.> > Regarding the inrush current when a new device is attached to a live> network: Depending on how one reads the Standard, a new device is allowed> to draw any amount of current for up to 500 uS. The implications is that> the existing devices on the network can ride out this power outage and do> not be reset. Surely, no one wants the network reset every time a new> device is plugged in. Some fieldbus devices do in fact draw a great deal> of> current when they are powered up but there is no Foundation test to> determine if devices are immune to the power outage. Perhaps it is too> late> to modify the Foundation's test suite but it would be very useful if> device> manufacturers specified their device power characteristics:> > -- The amount of current drawn by the device on power up and its> duration.> > -- The ability of the device to tolerate a 500 uS power outage without> resetting.> > -- The amount of current required by the device to power up if its inrush> current is limited by external means.> > If the third information item was available, then a current limiting> circuit> in the junction box could be made that minimizes disruptions on the trunk> cable caused by shorts on the drop cable or new, power-hungry devices> being> plugged on to the network.> > Incidentally, the above shortcomings are shared by both Foundation> Fieldbus> and Profibus because both use the same physical layer.> > Regards> > Maris Graube> Relcom

[Stephen Mitschke]

Please respond to this offer from Dick Caro to start making a difference onmaking the fieldbus standard(s) stronger. Thanks Dick and others.IanSyncrude Canada Ltd.- - - - - > Would anyone care to write the modification to S50.02 Part 2 for> "Limitation of Inrush Current"? There is a meeting of SP50 at ISA with> nothing of substance currently on the agenda. If there is some text, it> can be submitted to committee and will work itself into both the ISA> standard and the IEC standard. Send any text to me and I will forward> it to the committee(s).> > Dick Caro> ============================================> Richard H. Caro, Vice President> ARC Advisory Group

[Stephen Mitschke]

> Thanks Maris. Your wisdom and experience is valuable.> As many of you know, IEEE 802.3af committee is preparing specifications> for power on Ethernet that will certainly be useful for field> deployments of IEC 61158 - Type 5, otherwise known as Foundation> Fieldbus HSE. In addition, the EIA/TIA TR42.9 committee is preparing> specifications for Ethernet UTP wiring in industrial applications.> EIA/TIA is responsible for the Category 3, 5, 6, and 7 wiring and has> also assumed responsibility for connectors used in this environment. I> seems that inrush current limiting will also be a useful concept in this> area as well. It would be nice if the same specifications were used for> both FFB H1 and FFB HSE, which will only be possible if the solution for> the same problem is presented to both SP50 and IEEE 802.3af.> > Dick Caro> ARC Advisory Group

[Stephen Mitschke]

> I have seen some discussions related to power-on current draw. I am not> certain> why a FF representative has not commented on this issue since I believe a> specification is in place. In the registration process, FF requires that> hardware is tested to the Physical Layer Conformance Test Specification,> FF-830.> In this specification, the maximum current drawing for a power-on> condition is> 20mA during the period of 500 uS to 20 mS after power is applied. The> only> exception is the first 500 uS "for charging of RFI filters and other> device> capacitance." Since many of these standards were not ratified and> possibly> unavailable to some Manufacturers for product development activities> during FF> early infancy, I would hope those "out of spec." instruments would have> been the> result of releasing equipment before the rules were completely set. Since> buses> must work together on both a hardware and software (i.e., firmware) level,> a> true guarantee of interoperability would include testing of both layers.> At the> moment, only one layer is tested by FF. I would find it interesting to> know how> well suppliers are doing on hardware conformance from the FF> organization's> point of view and if they are thinking of policing or testing such design> aspects. Since we are a supplier, working on a level playing field is> very> important to us for many reasons.> > Stewart Thoeni> ABB Inc.

[Stephen Mitschke]

> I believe that Maris' comments are useful and valid. However, he is> offering 2 approaches to the problem and I believe that one is much> preferable. I will summarize my understanding of the 2 approaches and> state> my preference.> > 1. One approach is to make field devices and systems withstand the 500> microsecond caused by one of the devices being connected to a segment.> This> approach leads to a never ending set of design compromises that are> undesirable or unacceptable.> 2. A second approach is to limit peak device current draw to something> like> twice its steady state current draw. This is a proposal - I would like to> hear from device vendors if this is reasonable for them. This is the> approach I favor.> > The second approach will allow design of segments with all of the> necessary> current limiting to allow safe work practices on energized circuits. This> approach will also allow design of segments with higher power devices that> require an EXP switch to disconnect power before working devices hot. The> higher power Fieldbus devices are much preferable to running external> power> to devices that need a few watts of power (10 or so).> > If everyone could agree to limiting peak inrush on field devices and> publishing the results, it would seem that the rewrite of the physical> layer> would be fairly simple. I would propose that an open forum such as this> is> a good way to see if we can achieve some consensus before we do the> re-write.> > Herman Storey> Equilon Enterprises LLC, WTC, Automation Engineering

[Stephen Mitschke]

> Ian, I forwarded your request to our R&D Director in Germany, Mr Michael> Kessler and following is his response.> > Manny Romero> Pepperl + Fuchs> _____ > > Hello Manny,> > For us the inrush current is sufficiently described in IEC1158-2.> PROFIBUS-PA uses this definition and all devices conform to it. FF> unfortunately has relaxed this spec in FF-830-1.0 (Test case 5.1)and there> are (eg FR) devices that even do not conform to this relaxed spec. The> problem is not the lack of standards but the fact that some vendors do not> care about the standards. Therefore it is hard for me to understand this> request.> > Best regards> Michael Kessler - P+F Mannheim

[Stephen Mitschke]

We're back. Let's keep this excellent thread going folks.IanSyncrude Canada Ltd.-----------> Mr. Romero,> > There are two points to this communication that need clarification.> > 1. The power up test case (Test case 5.2) states that when a new device> is> attached to the segment "no more than 20mA above quiescent current draw> from> 500uS to 20mS" is allowed. The time from 0uS to 500uS is specifically> excluded from this requirement. There is NO current limit specified for> the> first 500uS. This is per IEC 1158.2 as referenced in FF-830 FS 1.0. The> suggestion that the IEC and ISA may wish to modify the specifications to> limit current during this time merits consideration. The proper path to a> change is to work with the IEC and ISA to modify the standard. Please> note> that other protocols also use this inrush standard, and would therefore be> effected.> > 2. F-R devices DO conform to the standard. Conformance to IEC1158-2 is> required to obtain FF registration. The fieldbus Foundation is diligent> in> testing to the specifications.> > Regards,> > Tom Wallace> Emerson Process Management

[Stephen Mitschke]

> Hi Stewart,> > I have been aware of this issue for some months now. Since the> specification (and the FF PL test) allows unlimited current draw in the> first 200 or 500uS (depends on the which version of the spec you want to> follow, we allow 500uS), there is not much the FF can do except> encourage change of the PL spec. The FF would rather have ISA/IEC> address this issue. There are other protocols that also use this spec.> > To address the issue, what Maris and I did was to inform developers of> this issue and hope that some would try to get their current draw at> power up within reasonable levels (ideally I would like to see a bus> powered device draw no more than 59mA or less on power up). > > There are some major design contraints that would need to be met on> device developers for certain types of devices. Do you use slow charge> capacitors (etc.) to limit current draw, but then it takes the device a> few seconds to power up (slow power up response time) or do you use a> fast charge that may use a lot of current, but powers up and responds> quickly?> > -Kurt Zech> Fieldbus Foundation

[Stephen Mitschke]

> Hi Manny,> > Please forward this on the Dr. Kessler.> > Be very careful here, all FF registered devices CONFORM to the IEC1158-2> spec. This is NOT the question here. The question is that the IEC1158-2> spec ALLOWS unlimited current draw in the first 200-500uS (depending on> version) after power up. > > It is test case 5.2 that is in question here (not 5.1), and PER IEC> 1158-2, "The first 500uS is excluded (from the "no more than 20mA above> quiescent current draw from 500uS to 20mS") to allow for charging of RFI> filters and other device capacitance". The fact that some devices may> draw more than 20mA in the first 499.9999uS is 100% allowable per the> version of IEC 1158.2 referenced in FF-830 FS 1.0.> > Unless Profibus-PA has changed the specification, some of their devices> are probably in violation of this also. > > Also, if someone knows of devices that are in violation of 5.1 (no more> than 20mA in the first 500uS to 20mS), please let me know. I need proof> of the violation of course. I can then remove the registration of such> devices.> > I haven't heard about devices being in violation of 5.1 or 5.2. I have> heard of some older _non-registered_ devices that were in violation of> the 9.0V lift-off voltage, but none that violated 5.1 or 5.2. > > I know of no current FF registered devices that are in violation of any> physical layer test. If anyone has _evidence_ to the contrary, please> let me know privately.> > Thanks,> Kurt Zech> Fieldbus Foundation