Joe, Thank you for the quick reply and your insights into NET 4, CTR 21, NTR 3 and prTBR 21. I like your advise to design the holding circuit with 1 watt of dissipation sans current limiting. I have a holding circuit design that limits the current between 50 to 60 mA and abruptly brickwalls the current to 4 mA. Once the current drops to 4 mA the DAA performance is so poor the modem disconnects. I don't know if the PTT's will buy it.
I'm glad to see someone is ranting about the blatant disregard for the physical limitations of a PCMCIA Card DAA design. I've already designed a French DAA holding circuit for the same customer in a PCMCIA card that passed. I designed for the lowest dissipation possible and cross my fingers hoping it would survive the current limiting test. Fortunately it survived testing and V.34 receiver performance didn't have to pass any compliance testing. I was told the metal case of card was so hot after testing it could not be held. With temperatures this high I'm sure the receiver performance was so bad no one would be willing the purchase it. It's a sad commentary on designing the best modem possible only to have the PTT compliance requirements destroy the performance. The V.34 is leading edge technology pushing the physical limits of the PSTN. While the French PTT compliance requirements are at least 8 to 10 years behind the times. I tested the V.34 card in question under the 60 mA current limiting situation and the results were staggering. While DAA was sinking 20 mA of line current the connect rate at 28.8 bps was 78%. While DAA was limiting the current to 60 mA the 28.8 bps connect rate was 42%. The only thing the French insistence will get them is a V.34 with the lowest throughput in the world. I guess we can only hope that the remainder of Europe will see the problem and fix CTR 21. Hopefully it's not too late. Regards, Duane Marcroft Telecom Consultant On Sun, 25 Aug 1996 [email protected] wrote: > In a message dated 96-08-23, Duane Marcroft writes: > > >I'm having difficulty designing a UK holding circuit on a PC card. > >The difficulty is caused by two I-V templates. One is from NET 4 > >and the other from TBR 21. The holding circuit for each template > >is different. > > > >The template from NET 4, 4.5 DC characteristics (Figure 1) has soft > >current limiting, linearly dropping from 60 mA at 40V to 125 mA at 0V. > > > >The TBR 21 template Figure 4: (TE voltage/current characteristics) > >has 60 mA brick wall current limiting. It drops straight down from > >36.2V to 0V. > > > >To meet NET 4 template causes the holding circuit darlington > >transistor to generate a large amount of heat. I don't think a PC > >card will not survive long under those conditions. > > Duane: > > I'm out of my office this week, so I can't double-check the documents you > referenced. However, if memory serves, I can offer the following: > > The current UK template, as it appears in the UK specification NTR 3 (and > probably NET 4 as well), does not impose any current limiting requirement on > the DAA. The downward sloping line that you refer to is simply the load line > of the C.O. feed circuit (50 volts, 400 ohms, I think). Thus, it is not > possible for the DAA to draw "too much current" under the NTR 3 requirement. > > In NTR 3, there is a dotted line at 60 mA that references a footnote. The > footnote says that it is recommended that TE not draw more than 60 mA, > because such a requirement may appear in a future harmonized European > specification. This is an indirect reference to CTR 21. > > In all of the draft editions of prTBR 21 that I have seen, there has been a > requirement for a 60 mA current limit. This requirement derives from the > national requirements in France, where a 60 mA current limit has been > required for many years. To my knowledge, France is the only country in the > world with such a requirement. Apparently, the French have insisted on > including this requirement in TBR 21. > > I had hoped that the 60 mA requirement would be dropped from prTBR 21 by now. > However, I think that this requirement is still in the version that is > presently out for national voting. If so, we may find that the price we pay > for a harmonized specification is that we must comply with the onerous > requirement for current limiting in the TE. > > Perhaps most members of the TE 5 committee do not recognize the consequences > that this requirement imposes for TE, especially for space-constrained TE > such as PCMCIA cards. In prTBR 21, the 60 mA limit must be met for a feeding > condition of 50 volts through 230 ohms. This translates to a power > dissipation in the DAA of almost 2 watts! > > It is extremely difficult to dissipate this amount of heat in a PCMCIA card, > or any other space-constrained DAA. It is even more difficult to maintain > the DAA's V.34 performance characteristics while dissipating 2 watts. I have > managed to do this on a few non-PCMCIA designs, but so far (thankfully), I > have not been asked to do so inside a PCMCIA card. > > To add insult to injury, I believe that there are few, if any, PSTN lines in > France that still require current limiting in the DAA. Most modern central > offices use solid state SLIC circuits that contain their own current limiting > for self-protection and for battery conservation. My guess is that the only > PSTN lines that still rely on current limiting in the TE are old, > electro-mechanical offices, with transformer feed circuits. I don't think > there are many such offices left in France. > > For your UK design, my suggestion is to ignore the recommendation for 60 mA > current limiting, and design a "conventional" DAA that never has to dissipate > more than about 1 watt. > > Today, the only country where the 60 mA limit is required is France. If CTR > 21 gets issued with this requirement included, we will have to contend with > the requirement throughout Europe. This would make CTR 21 a bittersweet > victory for modem designers: we get a simple, harmonized specification that > is generally easy to meet, except for one ridiculous requirement. Oh well. > > If anyone out there on treg has some insight into this current limiting > issue, I would like to hear from you. Specifically, I have the following > questions: > > 1) Is the requirement for current limiting still contained in the version of > prTBR 21 that is presently out for national voting? > > 2) If so, did the TE 5 committee that assembled prTBR 21 consider the > effects of having to dissipate 2 watts in the DAA of PCMCIA card, > sub-notebook computer, or hand-held TE? > > 3) How many PSTN lines in France still depend on current limiting in the TE > in order to function properly? > > 4) Is there any hope that the requirement for current limiting will > eventually be withdrawn? > > > Well Duane, I suppose I've ranted about this long enough. I guess your > question sort of hit a nerve. I think you will be fine with your UK design, > since current limiting is not mandatory. Just hope that your client doesn't > come back and ask you to do France, too! > > > Joe Randolph > Telecom Design Consultant > Randolph Telecom, Inc. > >
