Hmm... sounds interesting.
Now at least I understand how it works. Thanks alot!

"Van Gardner" wrote in message
...
: "Vangelis Pantazis" wrote in message
...
: Hello everybody !
: Can somebody tell me the technical difference between analog phone lines
: (PSTN) versus digital ones (ISDN or aDSL) ?
: Is there any way to distinguise them with an electronic device ?
: I've heard of these digital phone line tester but I'm not sure if this
is
: the one I'm looking for to build.
:
: Any ideas ? Many thanks, in advance
:
: V.P.
:
: I have such a device made by IBM called a Line Current Checker P/N
: 16F1621. They came out with the device about 15 years ago when laptop
: computers and digital phone system were meeting in motel rooms. If
: you plugged one of these early laptops into a digital line it would
: blow up the modem. The analog line is current limited to 100ma and
: the digital is not. The device is 5 1/2 inch long and 1/2 inch square
: with a clip to wear in your shirt pocket. It has a RJ11 phone plug on
: one end to plug into the line you want to test. It has three LEDs
: that are powered by the phone line. The first light (green)is labeled
: OK and indicates that current is limited to 100ma or less. The second
: light (Red)indicates current not limited, over 100ma. The third light
: (green) labeled REV indicates that the tip and ring wires are
: reversed. This device does not indicate performance of the line but
: lets you know what type line it is before you plug your laptop in.
:
: Van Gardner