Just moved into a new house, plugged in my computer and modem, and
suddenly I was unable to receive inbound calls. Outbound calls work
just fine. The computer is powered off. The same computer & modem (56K
internal, need to check make & model) worked fine at my last house.

For inbound calls, the caller either hears 1-2 rings and then a fast
busy signal, or modem connection tones.

Unplugging the modem from the phone jack remedies the problem. The
cables are not reversed or anything like that.

Any ideas?

"Mark G" <im_a_glover@yahoo.com> wrote in message
news:2c1905a5.0308050442.aca71e3@posting.google.co m...
I had a similar problem a couple of years ago with a corroded jack. It
appears telephone switching equipment gets confused if on-hook and off-hook
line values are not correct.

Sounds like the modem is not completely releasing the line or one of the
lines are shorted to ground.

If you have a multimeter connect a telephone test cord to the modem and
measure the resistance between tip/ring (red/green conductors) with the
modem "hung up." It should be millions of ohms. Then measure the resistance
from tip to computer case and ring to computer case. It should also be
millions of ohms.

My guess you will find one of these values is in the hundred or thousand ohm
range. If the modem looks good by itself plug the modem back in and repeat
the measurement at the Telco NID. Isolate inside wiring by unplugging the
test jack, or plugging a test phone into the test jack (depends on what type
of NID you have). The idea is to isolate inside wiring from the outside
phone line. The measurement should be the same as the modem by itself. IF
you have multiple phone lines repeat for each line and test to make sure
none of the conductors of one line are shorted to another.

/Tom

If wires are shorted so that ringer does not ring, then
phone cannot be used for conversation. If wire is open so
that ringer does not ring, then again, phone cannot be used
for conversation. If phone line can make an outgoing call
without noise, then phone line is not the problem.

Connect modem. Ask someone outside to call in. Disconnect
modem. Ask someone outside to call in again. Use same line
to make outgoing call. Best if phone makes outgoing call
connection through second jack on modem. Results of that test
should isolate (or confuse) problem completely.

Too many think eyes will see electrons. Without basic test
equipment, no one can identify a failed part.

Then the NID. Its function - a demarcation between your
wires and theirs. Open your side, unplug house from their
wires, connect your modem direct to the line - and test
without anything else in your building to confuse your
thoughts. This procedure so that others can stop posting what
should be irrelevant possibilities. Is it the modem or not?
Do other phones make outgoing calls when modem is and is not
connection? Those are the simple questions.

Mark G wrote:

On 06 Aug 2003 16:20:58 -0800, Floyd Davidson <floyd@barrow.com> put
finger to keyboard and composed:

Actually, that's very old technology and hasn't been used in "modern"
telephone sets in over a decade.

The varistors one finds in modern telecomms equipment are metal oxide
varistors (MOV), typically made of zinc oxide, and have an entirely
different V-I characteristic than the voltage dependent resistors of
those early telephones. In fact MOVs are essentially open circuit
until the applied potential exceeds their breakover voltage, usually
around 250V, at which point they look like a dead short. This means
they have absolutely no impact in normal usage, and exist merely to
shunt surges. Older varistors (possibly the silicon carbide type) have
a smoother characteristic which allows their resistance to fall
gradually with rising loop current.

See the table of V-I characteristics in Table 2 of this document:
http://www.littelfuse.com/PDFs/AppNotes/an9768.pdf

See also this URL which discusses the ways in which older and newer
equipment compensates for loop length:
http://engr.smu.edu/~levine/ee8302/telset98.ppt

Older equipment used varistors whose V-I characteristic could be
approximated by the relationship, i= K•v^2.

The aforementioned document has this to say about the older gear:

================================================== ===================
Three varistors used in a type 500 tel set:

One parallel with earphone to bypass high peak voltage audio (from
power crossing or manual switchboard clicks)

Two in parallel with microphone and matching network, to bypass more
mike audio on short loops (where loop current Io is large) when high
mike audio level is not required at the CO.
================================================== ===================

.... and says this about modern equipment:

================================================== ===================
Integrated Circuit Telephone Sets

Today most inexpensive one-piece telephone sets use an integrated
subscriber line circuit (SLC), which performs the 2-to-4 wire
functions of the telephone by means of unidirectional transistor
amplifiers.

A variable gain amplifier controlled by loop current is used to
compensate the microphone signal level for different loop lengths (no
varistors needed)

The earphone signal level is automatically controlled to prevent
overly loud audio (no earphone varistor used)
================================================== ===================

There are no significantly sized inductors in a modem ringer circuit,
if any at all. A typical modem ringer consists of a capacitor,
resistor, anti-tinkle zeners, and an optocoupler for isolation.

Some time ago we had a discussion about ringer circuits which prompted
me to look at a lot of IC datasheets, telecommunications texts,
telephone and modem circuit diagrams, and actual telephones. My
research told me that the most common ringer configuration in a modern
telephone looks something like this:

tip o---- C -- R -- IC -----o ring
|
speaker

The IC contains a bridge rectifier and logic to discriminate whether
the AC signal is a ring signal, or something else.

For a modem, the typical circuit is something like this:

tip o---- C -- R -- zeners -- optocoupler -----o ring
|
DSP

The modem could have a silicon DAA which would mean that faults are
not necessarily confined to the ringer circuit. However, I agree with
you that if the modem is generating tones, then the problem may not be
a hardware one.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.