raoul@eplunix.UUCP (Nico Garcia) (05/28/91)
In article <8053@nst>, grayt@Software.Mitel.COM (Tom Gray) writes: > Draw less than 4mA from the loop and you will not have any > problems. Only works in the off-hook position, Tom. A standard phone is about 600 ohms, and has about 10 Volts across it off-hook. That's *about* 15-20 mA: the standards are deliberately extremely broad. It's not clear at all to me that 4 mA will not maintain the "off-hook" status in the phone company's circuits. However, the phone company does *not* want DC paths available for the on-hook state. In fact, let me go try this (putting a 2700 ohm resistor across my office phone line). Hmmm: goes off-hook as the voltage drops to 18 volts. Maybe my line is picky, but forcing it off-hook is not what *I* want from an in-use detector. -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu
stevem@specialix.co.uk (Steven Murray) (05/29/91)
raoul@eplunix.UUCP (Nico Garcia) writes: >In article <8053@nst>, grayt@Software.Mitel.COM (Tom Gray) writes: >> Draw less than 4mA from the loop and you will not have any >> problems. >Only works in the off-hook position, Tom. A standard phone is about 600 >ohms, and has about 10 Volts across it off-hook. That's *about* 15-20 mA: >the standards are deliberately extremely broad. It's not clear at all to me >that 4 mA will not maintain the "off-hook" status in the phone company's >circuits. However, the phone company does *not* want DC paths available for >the on-hook state. I'm sure it is true the phone company does not want the DC path - nevertheless they do allow for some leakage across the circuit. The original poster was about right I think. >In fact, let me go try this (putting a 2700 ohm resistor across my office >phone line). Hmmm: goes off-hook as the voltage drops to 18 volts. Maybe my >line is picky, but forcing it off-hook is not what *I* want from an >in-use detector. You used 2700 ohms - but the off-hook voltage was something like 18 volts - thats not 4 milliamps. Just leave the 4 milliamps bit for a moment, try a resistor of value 22k. Its still not 'proper' but I think you will find it works. I'm not suggesting you do this on anything but your own private telephone exchange :-) >-- > Nico Garcia > Designs by Geniuses for use by Idiots > eplunix!cirl!raoul@eddie.mit.edu -- Steven Murray uunet!slxsys!stevem stevem@specialix.co.uk I am speaking, but | If these are your opinions, then we are in agreement!! not for my employer.| Flames, spelling errors, complaints > /dev/null
raoul@eplunix.UUCP (Nico Garcia) (05/30/91)
In article <1991May29.084859.5324@specialix.co.uk>, stevem@specialix.co.uk (Steven Murray) writes: > You used 2700 ohms - but the off-hook voltage was something like 18 volts - > thats not 4 milliamps. Just leave the 4 milliamps bit for a moment, try > a resistor of value 22k. Its still not 'proper' but I think you will find > it works. I'm not suggesting you do this on anything but your own private > telephone exchange :-) Yeah, OK. I used 2700 ohms to mimic 10 Volts (what folks refer to as "off-hook" voltage, although this is not correct) delivering only 4 mA, and to see if it would force an off-hook condition. It did so. Voltage in the off-hook state depends on voltage drop from your telco, and their line voltage. 22 kohm might work at one home (2 mA at -48 Volts worst case in the States' phone system), but not in another, due to a lower impedance from the telco permitting more current. And what if you put one on every phone, and have ~5 phones? All of a sudden you`re drawing 10 mA. Enough for off-hook detection, I'm certain. Typical operating current, according to my copy of "Understanding Telephone Electronics", is 20 to 120 mA. That doesn not mean that 4 mA or less won't trigger off-hook or maintain it. Doesn't mean it will: I believe the original poster that it didn't happen for 4 mA, on *his* line. Also, the telco has to provide that current. I can only assume they won't notice half a dozen users drawing it. Widespread use, however, and I'll *bet* they notice such a constant drain of a few percent of the active load. And they could start pulling phone service because of it, until it stops. It's kind of like operating things beyond spec: I don't like relying on such behavior, or designing circuits that do so. It feels like really bad design to me. This is a common example of every new gadgetmeister solving the problem, and most of them doing it *badly* because of subtleties they don't think about. -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu
grayt@Software.Mitel.COM (Tom Gray) (05/30/91)
In article <1991May29.084859.5324@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes: >raoul@eplunix.UUCP (Nico Garcia) writes: > >>In article <8053@nst>, grayt@Software.Mitel.COM (Tom Gray) writes: >>> Draw less than 4mA from the loop and you will not have any >>> problems. > >>Only works in the off-hook position, Tom. A standard phone is about 600 >>ohms, and has about 10 Volts across it off-hook. That's *about* 15-20 mA: >>the standards are deliberately extremely broad. It's not clear at all to me >>that 4 mA will not maintain the "off-hook" status in the phone company's >>circuits. However, the phone company does *not* want DC paths available for >>the on-hook state. > Telephone line circuit switch hook detectors are designed to operate at currents ABOVE 12mA. They are also designed NOT TO operate at currents LESS than 7mA. No problem will result if you draw less than the 7mA from the loop. ================================================================= Tom Gray =====================================================================
stevem@specialix.co.uk (Steven Murray) (05/31/91)
rpw3@rigden.wpd.sgi.com (Rob Warnock) writes: >In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes: >+--------------- >| Telephone line circuit switch hook detectors are designed to operate >| at currents ABOVE 12mA. They are also designed NOT TO operate at >| currents LESS than 7mA. >| >| No problem will result if you draw less than the 7mA from the loop. >+--------------- >No *immediate* operational problem... However, in the nightly automatic test >scans [forgot the Telco acronym], your line will be flagged as "leaky", and >may eventually spit out a trouble ticket. When they figure out the "leak" is >inside your house, they'll send you the "repair bill" for a "false service >call due to customer premises equipment". Hey! Thats neat! Do you know what current the test flags as a leak? -- Steven Murray uunet!slxsys!stevem stevem@specialix.co.uk I am speaking, but | If these are your opinions, then we are in agreement!! not for my employer.| Flames, spelling errors, complaints > /dev/null
gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Hardware CONTRACTOR) (06/01/91)
>>In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes: >>| No problem will result if you draw less than the 7mA from the loop. > >>No *immediate* operational problem... However, in the nightly automatic test >>scans [forgot the Telco acronym], your line will be flagged as "leaky", and >>may eventually spit out a trouble ticket. When they figure out the "leak" is >>inside your house, they'll send you the "repair bill" for a "false service >>call due to customer premises equipment". Everyone interested in this thread should look at (Usenet) Comp.dcom.telecom, which (among other things) discusses the nuts and bolts of telephony. (foggy memory alert...) I think the leakage resistance of a (legal) on-hook unit with a REN (ringer equivalence) of 1.0 is 22 Megohms. That means a leakage current of only a couple of >micro< amperes. There's an FCC spec & everything. This requirement makes a line-powered off-hook light much more interesting. I have seen a phone alleged to have been FCC registered which has a line powered off-hook light; it seems that they cheat a little. When another instrument goes off-hook, what looks like a CMOS comparator turns on the light. At this point the lighted instrument is off-hook. The tricky part is shutdown when all the other instruments go on-hook. geoff steckel (gwes@wjh12.harvard.EDU) (...!husc6!wjh12!omnivore!gws) Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line. This posting is entirely the author's responsibility.
jon_sree@world.std.com (Jon Sreekanth) (06/01/91)
In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes:
Telephone line circuit switch hook detectors are designed to operate
at currents ABOVE 12mA. They are also designed NOT TO operate at
currents LESS than 7mA.
No problem will result if you draw less than the 7mA from the loop.
Even if the hook detector is not confused by < 7 mA, won't that
trigger some automated test equipment that's supposedly looking for
leaky (hence faulty) lines ?
/ Jon Sreekanth
Assabet Valley Microsystems Fax and PC products
346 Lincoln St #722, Marlboro, MA 01752 508-562-0722
jon_sree@world.std.comgbell@sdcc13.ucsd.edu (Greg Bell) (06/02/91)
In article <1991May29.084859.5324@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes: > >You used 2700 ohms - but the off-hook voltage was something like 18 volts - >thats not 4 milliamps. Just leave the 4 milliamps bit for a moment, try >a resistor of value 22k. Its still not 'proper' but I think you will find >it works. I'm not suggesting you do this on anything but your own private >telephone exchange :-) > > Never mind the fact that if you design something that places a resistive load across the phone line, and later your phone rings, the current you're drawing jumps because the phone line ring voltage is higher than the on-hook voltage. So, your resistor answers the phone on the first sign of a ring, then drops it... -- ----------------------------------------------------------------------------- Who: Greg Bell Address: gbell@ucsd.edu What: EE hobbyist and major Where: UC San Diego -----------------------------------------------------------------------------
wolff@duteca (Roger Wolff) (06/02/91)
jon_sree@world.std.com (Jon Sreekanth) writes: >Even if the hook detector is not confused by < 7 mA, won't that >trigger some automated test equipment that's supposedly looking for >leaky (hence faulty) lines ? I don't think so. We had a full "a" to "ground" short that they didn't come and fix before we called and told them about it. Guess what? They didn't beleive that it was their cable! Anyway, I'm glad that it finally was their cable, because we didn't have to pay a thing! they have been digging a few days for us! Roger. -- EMail: wolff@duteca.et.tudelft.nl * There used to be text here but because people complained that my * * signature file was too long I deleted it. Now only this short * * message, explaing what happened to my signature remains. *
larry@kitty.UUCP (Larry Lippman) (06/03/91)
In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes: > Telephone line circuit switch hook detectors are designed to operate >at currents ABOVE 12mA. They are also designed NOT TO operate at >currents LESS than 7mA. > >No problem will result if you draw less than the 7mA from the loop. There could well be a problem if rotary dial pulsing is used. Such current leakage of 7 mA or less can well result in dial pulse distortion, especially if pulsing an "A-relay" on a longer loop in an electromechanical central office. Loop leakage effects are much more evident due to residual magnetism in a relay, as opposed to rotary dial pulsing with electronic loop current detection. This is not, of course, a problem when DTMF signaling is used. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry
larry@kitty.UUCP (Larry Lippman) (06/03/91)
In article <107170@sgi.sgi.com> rpw3@sgi.com (Rob Warnock) writes: >| Telephone line circuit switch hook detectors are designed to operate >| at currents ABOVE 12mA. They are also designed NOT TO operate at >| currents LESS than 7mA. >| >| No problem will result if you draw less than the 7mA from the loop. > >No *immediate* operational problem... However, in the nightly automatic test >scans [forgot the Telco acronym], your line will be flagged as "leaky", and >may eventually spit out a trouble ticket. When they figure out the "leak" is >inside your house, they'll send you the "repair bill" for a "false service >call due to customer premises equipment". In general, automatic line insulation test apparatus (ALIT) is concerned with leaks to ground or presence of foreign EMF from another pair. Such conditions are indicative of cable faults. The primary purpose of ALIT is as an early warning of outside plant cable trouble - and not as an indicator of subscriber instrument trouble. While it is possible to configure more intelligent ALIT apparatus to perform a tip-to-ring resistance test, this is generally not done due to the possibilty of false alarms. Such false alarms are most commonly caused by old, leaky series ringer capacitors. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry
larry@kitty.UUCP (Larry Lippman) (06/03/91)
In article <1991May31.135017.13702@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes: >>No *immediate* operational problem... However, in the nightly automatic test >>scans [forgot the Telco acronym], your line will be flagged as "leaky", and >>may eventually spit out a trouble ticket. > >Hey! Thats neat! Do you know what current the test flags as a leak? Convention in the telephone industry is to measure leakage to ground in terms of resistance in ohms rather than current. The alarm threshhold for automatic line insulation test apparatus is generally set between 50,000 ohms and 150,000 ohms - depending upon the type of central office and the existing condition of its outside plant. This corresponds to a current leakage of 0.3 to 1.0 mA. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry
grayt@Software.Mitel.COM (Tom Gray) (06/03/91)
In article <1991May31.135017.13702@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes: :rpw3@rigden.wpd.sgi.com (Rob Warnock) writes: : :>In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes: :>+--------------- :>| Telephone line circuit switch hook detectors are designed to operate :>| at currents ABOVE 12mA. They are also designed NOT TO operate at :>| currents LESS than 7mA. :>| :>| No problem will result if you draw less than the 7mA from the loop. :>+--------------- : :>No *immediate* operational problem... However, in the nightly automatic test :>scans [forgot the Telco acronym], your line will be flagged as "leaky", and :>may eventually spit out a trouble ticket. When they figure out the "leak" is :>inside your house, they'll send you the "repair bill" for a "false service :>call due to customer premises equipment". : The periodic tests on the subscriber loop will not normally look for faults between tip and ring. Also to render a little bit of reality back into this discussion, there is automatic equipment designed to operate at the end of telephone loops. In the days before microprocessors and integrated op amps (when dinosaurs ruled the world and I travelled daily past the brontosaurus store), these detectors consisted of high resistance relays. If your tip to ring resistance is greater than 10koms nobody is going to get excited (ie 4mA). If your tip to ring resistance is greater than 30kohms, nobody will ever know.