December 2015 11
when we didn’t throw
away anything that could
be repaired. Under POTS,
the phone company
owned the phone
hardware and internal
phone lines — not the
The touch tone
or Dual Tone Multiple
I found the Rotatone SMD Version 2 shown in Figure
4 available from Old Phone Works (see Q&A SIDELINES)
which converts the rotary dial signal pulses to DTMF. For
the cost of parts (about the same as the Rotatone SMD
V2) and the hassle of developing a one-off design, a proven
device is the way to go. In my case, I even considered
wireless phones for my home because one wire to the
base unit would operate all of the phones I will ever need
(we have seven), and would allow me to place the cordless
handsets anywhere in the house without the hassle of
rewiring each time (sorry Alf, my days of crawling under
houses are over).
As far as residential telephone wiring, take a look
at Figure 5 to see the four-wire standard used in many
areas with the modular jacks, and plugs RJ 11 for two-wire
systems and RJ 14 for four-wire systems. Our area only uses
the two-wire system, so I could use the black-yellow pair
for a second phone line or in case the red-green pair fail
(not super successful since what took out the R-G probably
got the B-Y too).
A side note is that I have found phone lines in our
area indicate reversed wiring on the phone line testers. I
would really avoid an external device like the plague, and
a central unit could be a real nightmare. The Rotatone
device appears to do what you need and should be able to
fit inside the old POTS phones which were manufactured
before the days of “engineering Olympics” of trying to stuff
more and more into smaller and smaller spaces.
I hope this information helps your friend maintain his
nostalgic phone within the modern phone network.
CPLD vs. FPGA
QI noticed in the July NV magazine the subject on FPGAs written on by Ryan Clarke. I have been using a CPLD in different applications, but have never used a FPGA. Can someone explain the
differences between an FPGA and CPLD?
Also, there is a wide range of parts with different
speeds and operating voltages. Is there a table that
compares the different parts? This subject could make an
QUESTIONS and ANSWERS
Post comments on this article at
n FIGURE 4.
n FIGURE 5.
Plain Old Telephone Service Blues
Old Phone Works
Breaker Tripped/Sump Pump Alarm
Power-out Power Failure Alarm
and Safety Light LED
Failed Circuit Alarm