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

For some reasonable ideas go to a discount electronics store and find
some low cost POTS product that has a wall wart type powersupply. Buy
it and then rip that open and it will likely answer almost all your
questions.

- mkaras
.

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