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'[OT] POTS specifications???'
1998\07\09@214750 by Calvin

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Sorry to bother you again with another OT.

Does anybody know a link where I can find info on the US telephone system
specifications?

Actually, what I need to know is the voltajes that must be present on the
lines, and the signal levels.

Calvin

1998\07\09@222847 by Mitchell D. Miller

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part 0 667 bytes
       http://www.xecom.com

as a source of telephone interface devices.  Haven't used them, but they look li
ke a good deal.

-- Mitch


------------------------------
Mitch Miller
spam_OUTmitchmTakeThisOuTspamns.net
------------------------------


-----Original Message-----
From:   Calvin [SMTP:.....tgoKILLspamspam@spam@CHIH1.TELMEX.NET.MX]
Sent:   Thursday, July 09, 1998 6:46 PM
To:     PICLISTspamKILLspamMITVMA.MIT.EDU
Subject:        [OT] POTS specifications???

Sorry to bother you again with another OT.

Does anybody know a link where I can find info on the US telephone system
specifications?

Actually, what I need to know is the voltajes that must be present on the
lines, and the signal levels.

Calvin

1998\07\09@224345 by Ron Fial

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To get you started,   The open circuit voltage is 48 volts (called on hook), and
the off-hook voltage is about 6 to 15 volts, depending on the telephone used.
A typical 600 Ohm phone should yield about 12 volts when off hook.  The ring vol
tage is about 90 volts AC, the frequency can vary depending on the system (The d
ifferent frequencies used to be used to select different phones on a party line)
.  The caller ID information is sent between the first and second rings as an f
sk modulated modem signal at 1200 baud, as i recall.   The dial tone is made up
of two very accurate sine waves mixed together, so its sounds the same everywher
e and is easy to detect electronically.

You are not supposed to hook un-registered devices to the phone line (FCC regula
tes this).  Most phone interface circuits isolate the line with a 600 to 600 ohm
transformer, go off hook with a relay contact, and optically isolate the ring d
etection, so that no DC connection exists between the line and the customer equi
pment.  Be careful not to accidentally connect the phone line to a power supply
or AC.  This could be vry very expensive for you.

Regards,
  Ron Fial
===================================================================
At 07:45 PM 7/9/98 -0600, you wrote:
{Quote hidden}

1998\07\09@225427 by Calvin

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Actually, what I want (need) to do is generate this voltages to activate a
telephone without hooking it to a line. This is for testing purposes.

Calvin

{Original Message removed}

1998\07\09@225837 by Calvin

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Assuming an RJ11 connector, what would the correct polarity be for this
voltage (48V)?

What would be a safe way to drop the voltaje when off-hook? A series
resistor maybe?

Calvin

>To get you started,   The open circuit voltage is 48 volts (called on
hook), and the off-hook
>voltage is about 6 to 15 volts, depending on the telephone used.  A typical
600 Ohm phone should
>yield about 12 volts when off hook.  The ring voltage is about 90 volts AC,
the frequency can
>vary depending on the system (The different frequencies used to be used to
select different
>phones on a party line).  The caller ID information is sent between the
first and second rings as
>an fsk modulated modem signal at 1200 baud, as i recall.   The dial tone is
made up of two very
>accurate sine waves mixed together, so its sounds the same everywhere and
is easy to detect
>electronically.

>

>You are not supposed to hook un-registered devices to the phone line (FCC
regulates this).  Most
>phone interface circuits isolate the line with a 600 to 600 ohm
transformer, go off hook with a
>relay contact, and optically isolate the ring detection, so that no DC
connection exists between
>the line and the customer equipment.  Be careful not to accidentally
connect the phone line to a
{Quote hidden}

1998\07\09@231525 by David VanHorn

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>Actually, what I want (need) to do is generate this voltages to activate a
>telephone without hooking it to a line. This is for testing purposes.
>
>Calvin
>


24V through a 600 ohm resistor will do that just fine. Polarity dosen't
matter.

1998\07\09@235937 by Martin McCormick

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       Assuming you build a 48-volt power supply.

The telephone = 600 Ohms as previously stated so the problem is a
simple resistive divider in the off-hook condition.

       12 volts through 600 ohms = .02 or 20 milliamps.
v(t) = 48 Volts
v(R(s)) = 36 volts.
Since the current for the whole circuit = 20 milliamps, all that is
left is to solve for R(s)
36/.02 = 1800 Ohms.

       If you build the 48-volt power supply and connect a 1800-Ohm
resistor in series, you will make a circuit that looks just like the
telephone line as far as the phone is concerned.  If you don't mind a
little mains ripple, your 48-volt supply doesn't have to be much more
than approximately a 30-volt secondary connected to a bridge and filter
capacitor.  The output voltage of the un-loaded bridge with the filter
capacitor across it will be the square-root of two times the RMS
voltage of the transformer's secondary.

       A 35-volt transformer should give you about 50 volts to play
around with.  Use at least a 100uf filter to cut down on the ripple.

       The actual voltage is not terribly important as long as it is
within five or 10 volts of 48 volts.  Our Northern Telecom DMS100 that
serves Stillwater sends us about 51 volts.  The old #5 cross bar
switch we had before that sent right at 48 volts.  Other systems may
vary a bit.  Your telephone circuit should be very forgiving of line
variations because they are out there and they can be nasty.

Martin McCormick

1998\07\10@084755 by Peter L. Peres

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On Thu, 9 Jul 1998, Calvin wrote:

> Sorry to bother you again with another OT.
>
> Does anybody know a link where I can find info on the US telephone system
> specifications?
>
> Actually, what I need to know is the voltajes that must be present on the
> lines, and the signal levels.
>
> Calvin

You are looking for a booklet published by AT&T or Bell for general
instruction of technicians or engineers in the mysteries of telephony.
There should be one available at a local technical college (If they made
it to here, in Israel, they have to be in Mexico somewhere <G>).

Peter

1998\07\10@084807 by Peter L. Peres

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On Thu, 9 Jul 1998, Martin McCormick wrote:

> telephone line as far as the phone is concerned.  If you don't mind a
> little mains ripple, your 48-volt supply doesn't have to be much more
> than approximately a 30-volt secondary connected to a bridge and filter
> capacitor.  The output voltage of the un-loaded bridge with the filter

Beg pardon, if there is anything electronic to be hung to the line (such
as a modem), then it had better be filtered VERY well. The switches go as
far as using batteries and expensive power supplies just to remove any
unwanted AC from the line, and the DC in the phone lines is among the
cleanset seen from any kind of utility.

Bad DC ruins any kind of data connection, especially high speed modems,
even though there should be no energy in the 50 and 100 Hz band. There is
something about additional phase shifting and whatnot that forces the
modem to slow down when this happens.

All 'good quality' phones and speaker phones sound absolutely lousy with
hum over the sometimes faint audio.

Peter

1998\07\10@120607 by Martin McCormick

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       You are absolutely 100% correct.  I was talking about a crude
circuit that can provide the look and feel so to speak of a telephone
line.  Some systems, especially old PBX's are not much better than the
type of circuitry I described.  They have all kinds of nice hum on
them.  I was describing a quick and very dirty way to power telephones
and not the ideal test setup.  I don't even address the delivery of
ringing voltage which is usually superemposed on the 48-volt DC
component.

Martin

"Peter L. Peres" writes:
{Quote hidden}

1998\07\10@140343 by Peter L. Peres

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On Fri, 10 Jul 1998, Martin McCormick wrote:

>         You are absolutely 100% correct.  I was talking about a crude
> circuit that can provide the look and feel so to speak of a telephone

The 'un-ideal' quick and dirty testing method that I use (and not only for
testing), when the line simulator is not available consists of two 12V
lead gel batteries in series, a 0.4 A slow blow fuse (actually an ICP-N10
chip protector if I remember well), a 470 uFd capacitor in parallel with
the batteries after the fuse and a 24V/0.05A bulb in series with the line,
and shunted by a 100 uF capacitor.  A current limiting resistor in series
with all this prevents damage in pulse dial contacts, it's an 80 ohm (or
100 ohm), 15 Watt resistor.

I use 2 batteries because I can connect two phones in series to the
circuit (or two modems and quite a bit of twisted pair between them). One
should be enough but twice over holds better. BTW for short duration
testing two 9 Volt batteries in series instead of the lead gel ones also
do the trick. The BULB is the essential part of the circuit. Beware.

For a semi-permanent setup (modem to modem in 2 buildings about 50 meters
apart, buried single twisted pair, and a phone in parallel with each modem
at each end) I have used a 24 V 0.3 A (ex charger) wall wart supply in
this circuit. There is no ringer system - this one would be a nice target
for signalling ring with PICs ;).

For now, the far phone 'clicks' with its ringer (piezo)  as one depresses
the other phone's on hook switch repeatedly ;). This I have obtained by
doctoring the ringer circuit in both phones and adding fuses in case
someone will plug them into the POTS system at some later date.

The circuit has the advantage that you can measure the leakage current in
your {circuit,phone,modem,answering machine} vs. ground without any
misleading influences, even if far away (buried cable etc).

The voltage is high enough to make any 'line in use' gizmos think that the
phones are on hook when they are on hook.

The ASCII schematic is:

           100 ohm 10 W   Lamp, 24V/0.05A (1 Watt)
To phone(s) O--/\/\/\-------+-(X)-+----o~~o----------O + Power (18-24V dc)
                           |    +|    Fuse 0.4A
                           +-| |-+
                        100 uFd  | +
                        25 V    ---  470 uFd 25 V
                                ---
                                 |
To phone(s) O---------------------+------------------O - Power gnd.

This circuit is not appropriate for testing phones connected in parallel.
I have used a series hookup for the situation described above (mode 2
modem...). For parallel, remove the 100 uFarad capacitor and select a bulb
that will give a reasonable voltage on the line with all the phones off
hook (probably 24V/0.15A (3W) for 4 phones).

The role of the 100 uFd is to test newly designed the dialler and off-hook
circuit under 'worst case' conditions. it should withstand the current
surge from charging the 100 uFd and lighting the bulb. (works out to
24V/100ohm = 0.24 Amp peak through the pulse dialer or line hold circuit).

To test ringers, I use a 110/6 V (about 10 Watt spec) transformer wired as
step-up transformer. The ring voltage comes from a signal generator's
speaker output, through a 4.7 ohm 5 Watt resistor, into the secondary of
the transformer. The primary goes to the phone. (NOT to the phone line).
The generator is set to the desired ring frequency (20 Hz), and keyed
manually. Its output amplitude is calibrated with an AC voltmeter at the
transformer secondary BEFORE connecting the phone to a spec of 90 Vac.

It is *essential* not to pick up the ringing phone being tested with this
makeshift ringer test circuit. Also, if testing answering machines and
modems set to auto-answer, be aware that this can fry them thoroughly and
immediately (although it should not).

Use this circuit and any information supplied in this message at your own
risk, as I supply this information without warranty, know that you cannot
sue me for damages. NEVER connect any of these circuits to a live phone
line.

Neither of these things are related to PICs, can replace a line simulator,
be used to qualify equipment for some standard, or are safe to use by
technicians who are unaware of the damage they can do to phone equipment
with them.

Peter

PS: The shouting is for the benefit of newbies and people who will find
this message in the news archives.

1998\07\10@212150 by Harold M Hallikainen

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       Have a look at part 68 of the FCC Rules at
http://hallikainen.com/FccRules .

Harold



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1998\07\14@155734 by Sean Breheny

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On Fri, 10 Jul 1998, Peter L. Peres wrote:

> circuit (or two modems and quite a bit of twisted pair between them). One
> should be enough but twice over holds better. BTW for short duration
> testing two 9 Volt batteries in series instead of the lead gel ones also
> do the trick. The BULB is the essential part of the circuit. Beware.
>

Please excuse my ignorance, but what is the bulb for? Just to get an
indication that the phone is drawing current?

Thanks,

Sean

1998\07\14@160739 by Max Toole

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The bulb acts as a current regulator.

Max

1998\07\14@164932 by Mike Keitz

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On Tue, 14 Jul 1998 15:56:53 -0400 Sean Breheny <.....shb7KILLspamspam.....CORNELL.EDU>
writes:
>On Fri, 10 Jul 1998, Peter L. Peres wrote:

>
>Please excuse my ignorance, but what is the bulb for? Just to get an
>indication that the phone is drawing current?

The bulb replaces a resistor to limit the current.  Using a bulb is
better because it is not linear.  Over a rather wide range (from when it
starts to glow up to full rated voltage), a bulb will approximate a
constant current source.  In the olden days, special "ballast bulbs" were
made for this purpose.  I think they used a different metal for the
filament, maybe iron.


>
>Thanks,
>
>Sean
>

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1998\07\15@002920 by paulb

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Mike Keitz wrote:

> In the olden days, special "ballast bulbs" were made for this purpose.
> I think they used a different metal for the filament, maybe iron.

 Iron in hydrogen gas.  Called "Barreter" bulbs if I spelt that right!

--
 Cheers,
       Paul B.

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