TREGers: I have a question that I hope some of you can help me with. I have heard many stories about business travelers who have destroyed their PC card modems by plugging them into digital PBXs.
According to these stories, some hotels with digital PBXs use RJ-11 jacks for the phone in the room. Apparently, some of these jacks are wired so that power for the digital phone is provided on the center two pins of the RJ-11, corresponding to tip/ring on an analog jack. The voltage and current available on these pins is sufficient to "fry" the interface circuitry of some modems. My questions are as follows: 1) How widespread is this problem? 2) Which digital PBX vendors use a connection scheme that can cause this problem? 3) What are the electrical characteristics of the voltage placed on tip/ring (open circuit voltage and short circuit current)? With regard to question #3 above, I would expect that the open circuit voltage is either 24 volts or 48 volts. These voltages are well within the range that an analog modem can handle, but perhaps the source resistance is so low that large currents end up flowing through the modem DAA. I would like to know how large these potential currents are. Any help would be greatly appreciated. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
