Marc Unangst <mju@mudos.ann-arbor.mi.us> (06/05/91)
I recently had one modem in the middle of a 15-line hunt group go out. Not wanting to move the rest of the modems up a line each, I did some investigation with a VOM and a telephone, and discovered that plugging a 270 ohm resistor into the phone line (across tip and ring) should have almost the exact same effect as an off-hook telephone does -- thus busying out the line. Now, my question is, is there anything wrong with doing something like this? Will a 1/2W resistor be enough? Is this an "accepted" way of busying out a phone line? Marc Unangst mju@mudos.ann-arbor.mi.us !hela!mudos!mju
tnixon@uunet.uu.net> (06/06/91)
In article <telecom11.425.15@eecs.nwu.edu>, Marc Unangst <mju@mudos. ann-arbor.mi.us> writes: > I recently had one modem in the middle of a 15-line hunt group go out. > Not wanting to move the rest of the modems up a line each, I did some > investigation with a VOM and a telephone, and discovered that plugging > a 270 ohm resistor into the phone line (across tip and ring) should > have almost the exact same effect as an off-hook telephone does -- > thus busying out the line. Now, my question is, is there anything > wrong with doing something like this? Will a 1/2W resistor be enough? > Is this an "accepted" way of busying out a phone line? The FCC has said that it will no longer allow Part 68 registration of modems that busy out the line in this manner (i.e., by just going "off-hook"). Centrex is supposed to have a way to tell the CO that a particular line in a hunt group is unavailable. Bellcore is working on a DTMF signal that can be sent on the line to tell the switch that the circuit is busied-out (like the "Do Not Disturb" function on many PBXes). And FCC already specifies an RJ4MB data jack, in which, if you close the MB/MBC pair, notifies the switch that the circuit is busied-out. Nobody likes the RJ4MB solution, though, because it takes up an extra pair of wires for every circuit. The DTMF solution is what everybody is looking for. In the meantime, about the only way you can do it is by going "off-hook". Not being a hardware engineer, I can't comment on whether the resistor value you've chosen is appropriate for this function. I assume some of the telco experts here will comment for you. Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net
"Barton F. Bruce" <Barton.Bruce@camb.com> (06/06/91)
In article <telecom11.425.15@eecs.nwu.edu>, mju@mudos.ann-arbor.mi.us (Marc Unangst) writes: > I recently had one modem in the middle of a 15-line hunt group go out. > Not wanting to move the rest of the modems up a line each, I did some > investigation with a VOM and a telephone, and discovered that plugging > a 270 ohm resistor into the phone line (across tip and ring) should Back in pre-electronic days, your line was apt to be fed by a balanced pair of 200 ohm relay windings fed 48 volts DC. That is 400 ohms total, and a short rignt in the CO would pull 120 ma. Many 1A2 KTU cards used a 135 ohm 5 watt resistor for hold. If that resistor were to be put right across the line in the CO, it would only get 1.1 watts. Any farther out on the loop, obviously gets less. Sure there were dial long line units that used 96 volt battery, but those were on LONG LINES. At the other end, a TT pad spec expects 23 ma min to work. Sure many run with less. If you were on a VERY long loop, you might want something lower that you picked to really hold the line busy reliably. For years our NYC office busied ALL lines except the first at night. The answering machine was on the first line, and 135 ohm resistors were slammed across all others by a 12 pole wire spring relay. If they went up on permanent signal,and 'dropped a card' each night, so what. Every morning everything would go back to normal. If the test board ever looked at it, they soon knew what to expect and ignored it. I bet in NYC they never noticed. Today I never bother with a resistor. Take a four inch piece of modular cord, crunch a plug on one end, strip the outer jacket about an inch on the other, and stuff the two center wires into a 'chicklet' (eight connector, or whatever) and squeeze it in a presser to connect T + R together. This will never fail to busy a loop start line. There is no reason to try to be 'nice' with a resistor. To busy a ground start line, you could just ground ring, but for several reasons it is best to ground BOTH tip and ring. The simple answer is just short Tip to Ring and be done with it.
Mike Bell <mb@sparrms.ists.ca> (06/06/91)
In article <telecom11.425.15@eecs.nwu.edu>, Marc Unangst <mju@mudos. ann-arbor.mi.us> writes: > I recently had one modem in the middle of a 15-line hunt group go out. > Not wanting to move the rest of the modems up a line each, I did some Dumb question: couldn't you just move the modem at the end of the hunt group into the broken modem's position?
k2ph@pacbell.com (Bob Schreibmaier) (06/06/91)
mju@mudos.ann-arbor.mi.us (Marc Unangst): > I recently had one modem in the middle of a 15-line hunt group go out. > Not wanting to move the rest of the modems up a line each, I did some > investigation with a VOM and a telephone, and discovered that plugging > a 270 ohm resistor into the phone line (across tip and ring) should > have almost the exact same effect as an off-hook telephone does -- > thus busying out the line. Now, my question is, is there anything > wrong with doing something like this? Will a 1/2W resistor be enough? > Is this an "accepted" way of busying out a phone line? On most lines the 1/2 watt resistor would be plenty. However, if you happened to be REAL close to a central office you could see as much as 24 volts across the resistor and, therefore, might have to dissipate as much as two watts. As long as you have that VOM handy, measure the voltage across the 270 ohm resistor. Then, square the voltage reading and divide by 270. That is the amount of power you are asking the resistor to dissipate. Should be 1/4 watt or less if you want to use a 1/2 watt resistor (safety factor of two). By the way, I have used this trick myself and it seems to work. Doesn't seem to be any problem with the telco "siren" signal that they put on off-hook lines that are not calling anyone. Bob Schreibmaier K2PH | UUCP: ...!att!dxis!k2ph a.k.a. "The QRPer" | Internet: k2ph@dxis.att.com Middletown, New Jersey | ICBM: 40o21'N, 74o8'W
Julian Macassey <julian%bongo.UUCP@nosc.mil> (06/06/91)
In article <telecom11.425.15@eecs.nwu.edu> mju@mudos.ann-arbor.mi.us (Marc Unangst) writes: X-Telecom-Digest: Volume 11, Issue 425, Message 15 of 15 > I recently had one modem in the middle of a 15-line hunt group go out. > Not wanting to move the rest of the modems up a line each, I did some > investigation with a VOM and a telephone, and discovered that plugging > a 270 ohm resistor into the phone line (across tip and ring) should > have almost the exact same effect as an off-hook telephone does -- > thus busying out the line. Now, my question is, is there anything > wrong with doing something like this? Will a 1/2W resistor be enough? > Is this an "accepted" way of busying out a phone line? A regular telephone measured with an Ohm Meter looks pretty much like a 300 Ohm resistor. As I recall the "DC Resistance" Used for testing phone devices is considered to be 200 Ohms. But a real phone (AT&T, ITT, Comdial et al), has "Loop Compensation", this is a usually a silicon carbide varistor. The varistor resistance drops as the voltage is raised. This works as an "Automatic Gain Control (AGC)", so if you are near the CO, the audio won't clean the wax out of your ears. When the phone is on a short loop (Short loop means close to the CO and the loop of wire between your phone and the CO is short), the resistance is quite low. This means there is no absolute DC value for a phone. But read on, and you will see that it doesn't really matter. Part of the whole equation is, how far are you from the CO. Here is why that matters. You can consider that the average distance from a CO is 15 Kilo-Feet (This is another dumb US measurement, the rest of the world uses Kilometers). The wire is a pair, that is a total of 30,000 feet of wire. The wire is usually 24 AWG (0.5mm). The resitance of this wire is 26.17 Ohms per thousand feet. So you will have about 785.1 Ohms worth of resistance between you and the CO. Add to this, about 400 Ohms for the CO. So if you shorted your wires, the total current that can flow assuming the CO voltage is 48V DC is 40 Ma. So for the hell of it, clear the short and add a phone. This adds 200 Ohms more to the loop resistance for a grand total of 1385 Ohms. Your loop current is now 34.6 Ma. My point is, it doesn't really matter whether you put a paper clip across the line or a 300 Ohm resistor. But yes, a half Watt resistor is fine, as is the paper clip. The paper clip has better transient handling capabilities though. Note that after all this resistance drop, what was 48 Volts at the CO is now between 3 and 9 volts depending on your loop length. I have used 1,000 Ohm resistors in the past to busy out lines. Note that many COs will go off hook and feed dialtone supplying only 8 Ma of current. Not many phones will work with 8 Ma. The US minimum current is 20 Ma. The lowest current spec is Sweden with 12 Ma. Sweden is a big country with not many people. Like the Western US, they have some very long loops. I recall when this question last came up, Brian Kantor mentioned that he used an LED and resistor combo which lit up and reminded him that he had a bad line or modem at that location. This is an excellent idea. It is also cheap and simple to implement. Note that the above discussion has been strictly about the DC characteristics of a phone line. They do not address the complex impedances of miles of wet cable, aerial wire, bad splices and crummy quad cable. Julian Macassey, julian@bongo.info.com N6ARE@K6VE.#SOCAL.CA.USA.NA 742 1/2 North Hayworth Avenue Hollywood CA 90046-7142 voice (213) 653-4495
Al L Varney <varney@ihlpf.att.com> (06/07/91)
In article <telecom11.427.9@eecs.nwu.edu> hayes!tnixon@uunet.uu.net (Toby Nixon) writes: > In article <telecom11.425.15@eecs.nwu.edu>, Marc Unangst <mju@mudos. > ann-arbor.mi.us> writes: >> I recently had one modem in the middle of a 15-line hunt group go out. >> Not wanting to move the rest of the modems up a line each, I did some >> investigation with a VOM and a telephone, and discovered that plugging >> a 270 ohm resistor into the phone line (across tip and ring) should Any DC value less than 330 ohms should do it, if it's legal. >> Is this an "accepted" way of busying out a phone line? > The FCC has said that it will no longer allow Part 68 registration of > modems that busy out the line in this manner (i.e., by just going > "off-hook"). On the other hand, would a modem that had a switch that caused the modem to refuse to drop new calls still conform to Part 68? The first incoming call would "hang" the line, but you wouldn't have made it "busy" by just going "off-hook". How picky is the FCC on this section? > Centrex is supposed to have a way to tell the CO that a ^^^^^^^ I believe this can be ANY line, for a price. Another option is to use a Hunt Group option that doesn't always start from the same line, assuming that there aren't many "bad" modems. > Bellcore is working > on a DTMF signal that can be sent on the line to tell the switch that > the circuit is busied-out (like the "Do Not Disturb" function on many > PBXes). And what will this "feature" cost on a per-line basis??? Al Varney, AT&T Network Systems, Lisle, IL varney@ihlpf.att.com
Proctor & Associates <0003991080@mcimail.com> (06/08/91)
In issue 425, Marc Unangst writes: > I recently had one modem in the middle of a 15-line hunt group go out. > Not wanting to move the rest of the modems up a line each, I did some > investigation with a VOM and a telephone, and discovered that plugging > a 270 ohm resistor into the phone line (across tip and ring) should > have almost the exact same effect as an off-hook telephone does -- > thus busying out the line. Now, my question is, is there anything > wrong with doing something like this? Will a 1/2W resistor be enough? A half watt resistor is not enough. The worst case would be if you were next door to an old C.O. running fixed battery feed of 52 vdc and A-relay resistance of about 400 ohms. This would give you about 78 ma of loop current and the resistor would have to dissipate around 1.6 watts. So that the resistor wont heat up too bad, it is best to spec the resistor wattage around twice that, so figure a three watt resistor. As long as you can draw at least 20 ma of loop current, the C.O. should detect an off hook condition. A safe bet is to use a 400 ohm resistor, which will give you a lower current draw. But the higher the resistance, the more power must be dissipated by the resistor for a GIVEN amount of current. If the C.O. battery was 48 volts and total loop resistance was about 700 ohms, the power load on the 400 ohm resistor (this all is emitted as heat) would be about 3/4 watt. Paul Cook Proctor & Associates Redmond, WA 206-881-7000 3991080@mcimail.com
Brian Kantor <brian@ucsd.edu> (06/10/91)
I use a 270 ohm 1/2w resistor in series with a red LED, and either crimp it into a modular plug, or solder it to one of those nifty AM-P widgets that can be stuffed over the pins on the 66 blocks. That way, I've got a red light glowing to remind me I've busied out the line. About a year ago, someone asked this same question and prompted a bunch of replies here on the net. Problem is, the archives are far to large to rummage through looking for an answer to a specific question. Ah well, maybe someday we'll have a really nice automated indexing scheme that can be queried by mail. Brian
tnixon@uunet.uu.net> (06/14/91)
In article <telecom11.434.3@eecs.nwu.edu>, varney@ihlpf.att.com (Al L Varney) writes: > On the other hand, would a modem that had a switch that caused the > modem to refuse to drop new calls still conform to Part 68? The first > incoming call would "hang" the line, but you wouldn't have made it > "busy" by just going "off-hook". How picky is the FCC on this section? The FCC, so far, isn't being very picky about it. For example, most AT command set modems include the "ATH1" command, which can very easily be used as a "make busy" ccommand. But the FCC certifies modems that have it, because there are other legitimate uses for the command. I don't think they'll certify any new modems that use Pin 25 of an EIA-232-E interface for "make busy". >> Bellcore is working >> on a DTMF signal that can be sent on the line to tell the switch that >> the circuit is busied-out (like the "Do Not Disturb" function on many >> PBXes). > And what will this "feature" cost on a per-line basis??? I assume that will be tariffed separately by each LEC in each state. There are certainly INDIVIDUALS who would PAY for a Do Not Disturb feature, that either gave a busy signal or invoked their forward-on-busy number, so they wouldn't have to deal with the warbling tone if they take the phone off hook to engage in some activity without being disturbed. I'm surprised it hasn't already been offered. Toby Nixon, Principal Engineer | Voice +1-404-840-9200 Telex 151243420 Hayes Microcomputer Products Inc. | Fax +1-404-447-0178 CIS 70271,404 P.O. Box 105203 | UUCP uunet!hayes!tnixon AT&T !tnixon Atlanta, Georgia 30348 USA | Internet hayes!tnixon@uunet.uu.net