actually, no. all that is opto-isolated on most modems is the ring detect, and activating the relay to connect the rest of the modem. the data and any other "sound" signals are transformer coupled, which doesn't provide nearly the isolation that an opto-isolator does, particularly with cheap transformers (and consumer items, like modems tend to be made with the cheapest available parts that work at all which is part of why i hate servicing consumer electronics). in any case, a large surge will arc over small gaps between the phone line side and computer side of a modem, dirt can allow enough leakage current to be a problem (particularly in humid areas), and there's always the risk of bugs (the crawling kind) getting in there. as surges tend to be high frequency in nature, it's very easy for them to arc or couple from one side of the modem electronics to the other and cause problems. unless you use a surge suppresor with a fuse on the phone line, many surges will hopelessly overload the the surge suppression and make those parts fail. no surge arrestor completely blocks the transient signals, so there's always some surge that gets through, particularly if the surge arrestor isn't grounded or well grounded in which case many surges will be dissipated in the modem rather than the surge suppressor (if there's a large inductance in the ground line because of how the house is wired, or the whole house ground is poor then it isn't much help). also, after repeated surges surge arrestors often fail, usually shorted so the phone won't work and it's easy to find, but it is possible for them to fail open and essentially just stop working without interfering with normal phone function so there's no warning.
you also have to consider the source of "surges" in phone lines, often it is from a lightning hit to exposed phone wiring (even other people's phone lines as they readily arc over) or strikes near a phone line and induces a surge, or worse (because it continues) is when a phone line is accidentally shorted to a power line, usually the 13.8kv that runs on most power lines at a very high current, there is essentially unlimited energy available (when a utility pole falls down or there is a construction accident, consider how often phone lines are cut by back hoes). the telco in older days put very good fuses on phone lines where they entered the house, fuses designed to stop even a high voltage fault from getting into the house, but they don't do that anymore, probably to save cost and increase signal quality (the fuses probably caused problems with high speed modems). i always, always unplug the phone line from the computer when storms come, even if they aren't close enough that i completely unplug the computer. also consider that if phone lines run parallel to network or other cabling that they can induce surges in that as well, and that any network cable can pick up surges during a thunderstorm (as can the power lines of course), possibly (likely) at the same time the phone line does, so a modem may be fried and there may be another route for a surge to enter the computer as well which may be incorrectly assumed to be from the modem when it may not be or may be in addition to surge power on the phone line. also, another problem with surges is that sometimes they don't fry the electronics directly, but may make an arc that leaves a carbon trail on the board, allowing large leakage currents after the surge leaves, either from the phone line (which is definitely incompatible with serial port signals) or from the line power or part of the power supply that are more than the computer can tolerate. they can also temporarily create an arc that once started lets line power or other power sources couple where they shouldn't, though that's usually a brief insult at least, assuming no path is carbonized into what were insulators. > From: Bruce Johnson <[EMAIL PROTECTED]> > Actually, most modem phone signals are opto-isolated from the main > computer circuitry. Doesn't help, as the phone-line side of the modem > still fries, but it's just replacing the modem that's necessary. -- Philip Stortz -- To be nobody but yourself when the whole world is trying its best night and day to make you everybody else is to fight the hardest battle any human being will ever fight. -- E.E. Cummings -- PCI-PowerMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | -- Sonnet & PowerLogix Upgrades - start at $169 | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> PCI-PowerMacs list info: <http://lowendmac.com/lists/pci-powermacs.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive:<http://www.mail-archive.com/pci-powermacs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
