Joe, In your response you stated: " The second requirement is that the modem must not drop a connection in response to the application of tax pulses. As Duane Marcroft has pointed out, some fiddling with the modem control registers can sometimes allow this requirement to be met without a filter".
I didn't mean changing the modem control registers to bridge the tax pulse period. I don't think they are available for for that kind of adjustment in the German country code. What I was really meant was to modify the actual loss of carrier timer that disconnects the modem if carrier is lost for a short period. Modifying it this way prevents a user from modifying it later and causing problems that hard to solve. The adjustment resides in the Rockwell country code for Germany. Modifying the timer in this fashion insures loss of carrier due to tax pulses will always be successfully bridged. I also was impressed by David Drori's description of tax pulse filter issues and the problems encountered in their design. I've been there a number of times. I'm still haven't attained nirvana in my quest for Swiss tax pulse filters to fit inside PCMCIA cards. The inductors I've found have an interference fit with the inside of top cover. Inductor body is actually in contact with the cover. As a consequence a certain percentage will arc to the cover during 1500 volt isolation test. There's simply not enough space between inductors and inside of top cover to get two layers of insulation. It requires ramming, jamming and cramming the covers on. Which causes the top cover to bow and package is no longer within PCMCIA tolerances. Regards, Duane __________________________________________ On Fri, 28 Mar 1997 [email protected] wrote: > Fellow treggers: > > I have been away from my email for a few days, but I was pleased to return > and find such a lively exchange on tax pulses. I was particularly impressed > with David Drori's lucid description of the underlying issues and the > considerations that go into the filter design. I fully agree with David's > remarks. > > After reviewing what has already been said by all of the contributors on this > subject, I have only a few comments to add: > > 1) In Germany today (under BAPT 223 ZV5), there are only two requirements > related to tax pulses that modems must meet. The first is that the impedance > of the TE must exceed 220 ohms at 16 KHz. With some care in the design, this > requirement can be met without adding a filter. The second requirement is > that the modem must not drop a connection in response to the application of > tax pulses. As Duane Marcroft has pointed out, some fiddling with the modem > control registers can sometimes allow this requirement to be met without a > filter. > > 2) Tax pulses are not provided by default on all lines in Germany, but they > may appear on some lines when they have not been specifically requested. In > some central offices, if any one customer on a given channel bank has > requested tax pulses, all lines on that bank receive them. This is the > justification the BAPT used for imposing an immunity requirement on all TE. > > 3) In Switzerland, there is no requirement for modems to be immune to tax > pulses. However, there is a requirement for TE to present a minimum > impedance at 12 KHz. This requirement, which is misleadingly referred to as > an "insertion loss" requirement, can be found in section 4.1.2(CH)1 of NET 4. > The test signal for this requirement is +20 dBm. With a "dry" transformer > and a solid state DC hold circuit, it is extremely difficult (impossible?) to > meet this requirement without a series inductor. > > 4) While it may be possible to get products approved in Germany and > Switzerland with absent or inadequate tax pulse filters, there is a risk in > doing so. Tax pulses show up on a significant percentage of the phone lines > in these countries, and modems that are not immune to the tax pulses will > have problems. Unsophisticated users are not likely to recognize the cause > of the problem. All they will know is that the modem can not maintain a > connection on an outbound call, or that the throughput is quite low. This > may generate problems for the technical support staff, not to mention the > image of the vendor. > > 5) I recognize the design headache that the tax pulse filter creates for > designers of PCMCIA cards. Filters that perform well tend to require > inductors that do not fit. On the other hand, a mediocre filter is probably > better than no filter at all. I would be interested to hear from anyone who > knows of a good solution to this problem. > > > Joe Randolph > Telecom Design Consultant > Randolph Telecom, Inc. >
