TELECOM Digest OnLine - Sorted: Re: Question About "Network Interface" Phone Jack

Re: Question About "Network Interface" Phone Jack

William Warren (
Wed, 27 Jul 2005 14:28:38 -0400

Tony P. wrote:

> In article <>,
> says:

>> After living in the same place in New York City for years, I recently
>> moved to another place (also in New York City). The room I moved to
>> has an existing phone jack but it looks different from the old
>> square-ish ones I'm accustomed to. (The old ones consisted only of 4
>> terminals inside the case). This new one is rather rectangular and
>> has a label on the outside that says:

>> Network Interface
>> *Caution
>> Disconnect plug from this jack during installation and repair
>> of wiring.
>> *Testing
>> Plug working phone directly into this jack. If phone operates,
>> fault is in wiring. If phone does not operate, call repair
>> service.

>> When I opened the case, I noticed that the red and green wires (the
>> only ones that will be actually used by the telephone itself) are also
>> connected to a little circuit board whose most conspicuous component
>> is a yellow cylinder-shaped object (about 3/4" long and about 3/8"
>> diameter) with the following markings ...

>> 250V
>> TI
>> 0.47 MFD
>> +/- 10%

>> * What is the purpose of this circuit board?
>> * Is it really necessary? (How come the old-fashioned jacks
>> didn't have this?)
>> * What if I were to disconnect it?

> The capacitor is probably part of an RF shield on the jack.[snip]

The capacitor is used to "terminate" the line with a value that
approximates one standard ringer, so that if someone calls in a
trouble report, the tester can check if (s)he "sees" the capacitor
with the NI jack disconnected. Depending on local practice, the
Central Office might be programmed to report a fault on any line that
it rings which doesn't show a ringer attached, so you should leave it

It doesn't hurt anything.


(Filter noise from my address for direct replies)

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