Re: Phone Line Interfacing - FCC Part-68
- From: rickman <gnuarm@xxxxxxxxx>
- Date: Thu, 22 May 2008 20:19:20 -0700 (PDT)
On May 19, 10:16 pm, mpm <mpmill...@xxxxxxx> wrote:
I'm on a data-collection project, and the terminal will download via
POTS.
My question is: We're using the MultiTech Socket Modem (v.34, because
it's low cost).
In the documentation, they show several ways to interface to the phone
line.
We will have a choke in-line for the tip and ring for common-mode.
And a resettable fuse. So far, so good.
But I have a question:
MultiTech shows a paralleled 220pf 5kV cap & a sidactor (transorb),
with one set each on both the tip and ring. The other side goes to
"FGND". Any idea what this means, as it's not referenced anywhere
else in the document??
I am assuming Earth ground, or at least some ground other than the
power supply ground driving the rest of the circuity. (There is also
an analog ground for the modem speaker - which we're not
implementing...)
Our box will be just that: A plastic box with a membrane keypad, an
LCD, a 9VDC 2-conductor wall-wart power supply, and of course, an
RJ-11 for the phone line. There will not be an earth-grounded
conductor.
Should we bother with the (Y2-rated safety) caps and sidactor
(transorb) protection, or just go with the common mode choke and
inline fuse? What is "safe", if anything, to connect to power
supply ground? Will the choke & fuse only arrangement pass FCC -68
requirements?
Thanks. (Seems like ages since I did any work with embedded dial-up
modems!)
-mpm
If your equipment does not have an electrical connection to anything
else, then you don't need the fancy earth ground and transorbs. The
transorbs aren't really safety related, IIRC. There are two things
you need to protect phone line equipment from, lightning caused surges
(not necessarily a direct strike) and AC mains crosses. The lightning
stuff is more of an equipment issue. There is not much of anything
that you can do which will protect in the event of a direct strike to
the phone line. It will roll in on the wire and fry every thing
connected, protection or no. But a close by strike will create a
voltage/current surge in the line which can be protected against
depending on the strength.
A power mains cross with the phone line is a different matter. This
can very easily kill anyone touching the equipment if it is not
isolated. This is not just the 110 or 220 power coming into the
house, but can be one of the higher voltages common on the poles. I
want to say the requirements are for 5kV of isolation. That can only
be done with transformers, opto-isolators or choppers/capacitors.
Maybe the requirement is only for 2kV. I seem to recall that on one
design I did about 8 years ago, the chip I was using chopped the
signal into a square wave at high frequency which was passed through 2
kV caps. On the protected side the signal was recovered. BTW, it was
a CP Clare chip and it has ***NO*** power supply rejection. If you
had 10 mV of noise on the power rail, you had 10 mV of noise in your
signal. And believe it or not, you can hear 10 mV of noise on a phone
line.
.
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