10 December 2015
n WITH TIM BROWN
In this column, Tim 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.
• Phone Service Blues
• CPLD vs. FPGA
• Sump Pump Alarm
n FIGURE 3.
n FIGURE 1.
Plain Old Telephone Service Blues
QMany people are going to DSL or are getting phone service from their cable, and those new systems don’t accept pulse dialing. A friend whose home is stocked with antiques —
including the phone — complains that he is stuck with the
AT&T POTS line. Our state has given permission to pull
the plug on POTS in a few more years. I’m wondering how
difficult it would be to program one of those computers
on a chip to count the pulses from the dial mechanism
(as I recall, the last/end pulse of the group is longer) and
generate the corresponding touch tone (or control a tone
chip).
There may be space to hide a circuit in those old bulky
phones but probably not a universal board, and it would
have to be powered from the phone line DC. Plan B — the
easier solution — would be to exploit the unused second
pair of wires of the phone system to send the dialer pulses
separately to a central interface, since my friend doesn’t
have a second line but does have a second phone. The
one old phone I found has a three-wire cord with two
wires paralleled in the phone, so a common wire would be
necessary to keep the old cord.
Dennis L. Green
via e-mail
AFor our “younger” readers, POTS stands for Plain Old Telephone Service (a.k.a., rotary dial, which was introduced in 1876 by Alexander Graham Bell) to distinguish it from updated
telephone systems such as touch-tone dial (started being
used over wide areas in the 1980s), fiber optic based
systems, ISDN, VoIP, cellular wireless systems, etc. The
Digital Subscriber Line (DSL) to me is the telephone
company’s gateway to the Internet, but can be used
for economical worldwide telephone service via Skype,
Vonage, Magic Jack, etc.
The POTS used 48 VDC modulated with the voice/
audio signal over a frequency range of 300 to 3,400
Hz for communicating analog signals and a 120 VAC
ringing signal. DSL uses the same lines as the POTS with
modulated channels, with 4,315.5 Hz bandwidths between
10 kHz and 100 kHz to send digitally encoded signals.
The POTS depended on
the rotary dial shown in Figure
1 (literally a round dial with
10 finger holes on the front of
the phone) which opened and closed a switch a number
of times based on the number selected to dial out (as kids
we found early on that by clicking the handset switch we
could dial equally effectively). I cannot remember when
I have seen a rotary phone in service (I may have one
somewhere in my junk collection).
The real forte of the rotary phone was reliability
because except for a strong lightning strike taking out
the hybrid transformer (isolated the residential side from
the phone company side), there wasn’t much to break
(as much as some of us tried). The transformer could be
replaced in five minutes by a competent tech in a day
n FIGURE 2.
Q & A
