The Modem Protector
Loop Current is the amount of electrical
energy flowing through the telephone and line. Many telephone companies are using special
pair gain equipment (called SLICs - Subscriber Line Interfaces) to bring phone lines to
neighborhoods. These SLICs are made to allow the
phone company to serve subscribers as far away as possible without further conditioning
the lines. They put out tons of loop current - enough to destroy many modern
telecommunications devices. Even new Central Offices themselves are being made to put out
as much power (loop current) as possible to serve customers far away from the CO. If you
happen to be near the SLIC or CO, watch out!
The specs for maximum loop current were developed in the
70's before the advent of all of the new electronic telecommunications products. Even
though the phone companies know the high loop current is damaging subscriber equipment,
they're using the old specs to their benefit to serve subscribers far away from their
COs, without having to add more equipment to their lines. The more current, the farther
you'll be able to use the phone line.
Nobody seems to be interested in changing the old spec, and
modern equipment manufacturers seem to ignore the spec and pretend the problem's not
there. The Modem Protector has been designed to fix all problems associated
with high loop current. It's a quick and easy fix to many of the problems encountered with
modern telecommunications equipment caused by high loop current.
Is this something new? No. We've sold lots of Loop
Current Attenuators to phone companies who use them to prevent strange problems
with telephone equipment and modems. The Loop Current Attenuator has a
series of dip switches, which a technician flips while watching a loop current meter. When
he finds a combination of dip switches that brings the loop current between 23 and 27ma
(the ideal setting), he just leaves the device there on the line. If the loop current
later changes on the line, the technician needs to go out and manually set the dip
switches again. Our Modem Protector handles all of this automatically.
It keeps the loop current at a constant 25ma, and also includes surge and lightning
protection, and a polarity indicator (some modem manufacturers claim that you can get
better speed by using the correct polarity).
PCMCIA (PC Card) and built-in laptop modems seem to take a real beating from high
loop current problems. It not only messes up fax and data transmissions, but it blows up
modems! Take a look at this chart, and you'll see why:
This graph shows the amount of POWER that a
modem must dissipate just to keep the line off-hook.
That power is ALL converted directly into heat
that a modem must somehow dissipate. It's virtually impossible for a PCMCIA card
or internal laptop modem to
dissipate 2 watts under any circumstances. The Modem Protector automatically
extracts the heat that can burn up your modem.
It turns out that according to sources at an FCC testing lab
almost all new faxes and modems and some new telephones use a device called an OTA
(operational transconductance amplifier), a special type of op amp with a small capacitor
to simulate the inductor that would normally absorb the loop current in a telephone. Why?
Because they're smaller and cheaper than using real inductors. They look like any 16 pin
IC chip, and have the same power handling limitations as IC chips. The best OTA can handle
0.57 watts - about in the middle of the above chart. Any closer to the phone company CO or
the PBX, and the OTA will fail. This limit translates to roughly 40 to 45ma of loop
current. The Modem Protector automatically reduces the loop current on its
output jacks to 25ma, no matter how high the loop current. If the loop current is at or
below 25ma, there is no effect on the loop current. In any case, surge
protection is provided within The Modem Protector. For
lighting protection our
Modular MLP-270M Lightning Protector will give you the maximum
lighting protection possible as long as you provide it a good ground.
Notice also that the effect of distance is not linear. As
telephone lines get shorter the power increases substantially. This explains why many
problems occur with modems plugged into PBX extensions, which often have short runs.
IBM was marketing a device called the Modem Saver for a
while. It was a little pen shaped device that you plug into a jack with indicators for
the polarity of the line and if the loop current is too high. Unfortunately, the warning
doesn't glow until about 90ma. At 85 ma, it's unlikely you'll be able to transmit data
reliably at any speed, and you may smoke your modem, but the warning light won't be on. I
have no idea why they designed this product like they did? As you can see from the above
chart, The Modem Protector doesn't need to warn you of high loop current
because it just takes care of the problem for you.
The Modem Protector now adds a Modem
Filter to reduce common mode noise
on the line that may interfere with modem transmissions. Common mode noise comes down both
sides of the line (the tip and ring). Because it's equal on both sides of the line, you
don't hear it with a regular telephone, but the modem will hear it because
it is referenced to ground, and not perfectly
balanced, and the data can be
slowed or you'll have connection problems. Because the Modem Filter™ section
of the Modem Protector™ is an inductor, it also helps to stop
spikes on the phone line from getting to the modem or phone equipment
attached to it.
Click HERE to go to the Modem Protector with
Filter product page
Click HERE to see the nitty gritty
about Loop Current in
our Loop Current Tech Bulletin