18 May 2017
n WITH KRISTEN A. McINTYRE
QI am trying to repair some old 300 baud modems from the late ‘70s/early ‘80s, and I need to connect a phone line to verify that hey are working properly. The problem is that
everything is now digital. Do you know what the off-the-hook and ring voltages were for ‘Ma Bell’ for that time?
I have a function generator and a good AC/DC power
supply. Can you show/tell me a quick way to set up a test
circuit to simulate these two voltages?
ABelieve it not, I still have a Plain Old Telephone Service (or POTS) pair coming into my home. It’s for emergencies when other things might be down. I do understand what you’re saying
about the local phone companies phasing out good
old loop-start POTS service. It’s becoming less and less
economical for the telephone companies to maintain those
wires for the price that they charge. In addition, they can
make a lot more by running DSL on the pair, or instead
removing them completely to be replaced by fiber optic
cables that can carry high speed data.
The type of signaling on POTS lines is generally called
loop start. In a loop start system, there is a moderate open
circuit voltage on the pair coming in from the telephone
company’s central office. In the US, nominally that’s 48
VDC. The two signals are called tip and ring, for historical
reasons (think about a 1/4” phone plug like an operator
used to use). Tip is the positive terminal and ring is the
negative terminal. The loop is powered by a current source,
with the nominal voltage being the upper limit of the
current source’s voltage.
Figure 1 shows an example of just the basic loop
current source with a nominal current of 30 mA. That
might or might not get something going. Ring voltage is
usually a sine wave of 20 Hz at 90V. Be careful, it bites!
With the basic electrical characteristics established, we
have to start thinking about what other signals a modem
might be looking for on the line. For example, most
modems look for dial tone. Again in the US, dial tone is
established by mixing a 350 Hz signal with a 440 Hz signal.
This forms a 90 Hz beat tone. The level is not entirely clear
to me, but a little experimentation would probably help
find the right amplitude. Ringback tone might or might not
be needed, as long as the other side picks up. US ringback
tone is 440 Hz mixed with 480 Hz, and that is modulated
at two seconds on and four seconds off.
To build a device to simulate all of these conditions
would be a bit of work. It could be done with some
additional analog circuitry and probably an Arduino or
Raspberry Pi to handle timing and sequencing. Designing
such a device is beyond the scope of this column, but it’s
certainly doable. There are telephone line simulators out
there like the Teltone TLS- 4, but they’re quite expensive.
An easier way to go might be to make up a small
analog PBX using Asterisk ( www.asterisk.org), which I’ve
mentioned here before. Unfortunately, analog line cards
for Asterisk are quite expensive. Perhaps the cheapest
alternative to building something would be the Cisco
SPA2102 VoIP phone adapter with router. This device has
two POTS ports on it with full signaling support, I would
guess. It expects a SIP server, however.
It might be possible to get two numbers cheaply on
one of the many SIP servers on the Internet and have those
call each other. I’m not sure if you could build a SIP server
yourself to just utilize two fake phone numbers. It sounds
like it might be possible, but without looking into the
In this column, Kristen answers questions about
all aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest to
the hobbyist. Feel free to participate with your
questions, comments, or suggestions. Send all
questions and comments to: Q&A@nutsvolts.com.
• Modem Testing
• Identifying Components
• Drawing Schematics
Q & A
n FIGURE 1. Loop start current source example.