Re: Phone Line Interfacing - FCC Part-68

On May 19, 7:16 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

I bet FGRD means equipment frame ground, which typically is connected
to the power grounding conductor, i.e. the third prong. They want
phone line surges to be shunted to earth. Of course, if you want your
equipment floating at God knows what when lightning strikes, well,
that's your choice, isn't it? Now telco will provide a protector that
is (or should be) referenced to earth, but then they designed it to
protect their equipment, not yours. And then there's the matter of
what your modem is connected to and how important it is that that
doesn't get blown, too. If you can't get at the building power ground,
I''d go with a nice groundy cold water pipe.
.

Relevant Pages

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  • Re: Phone Line Interfacing - FCC Part-68
    ...  We're using the MultiTech Socket Modem (v.34, ... In the documentation, they show several ways to interface to the phone ... We will have a choke in-line for the tip and ring for common-mode. ... some low cost POTS product that has a wall wart type powersupply. ...
    (sci.electronics.design)
  • Re: Phone Line Interfacing - FCC Part-68
    ... We're using the MultiTech Socket Modem (v.34, ... In the documentation, they show several ways to interface to the phone ... We will have a choke in-line for the tip and ring for common-mode. ... And a resettable fuse. ...
    (sci.electronics.design)
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