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Care & Maintenance Ideas for Friendlier Electronic Equipment - Part VBy: F. J. STIFTER Still More Solutions ! Modems, Fax and Phone line connected equipment Many folks think damaging spikes, transients & surges or disruptive interference can only enter sensitive high tech equipment through the AC Power Lines. That ain't necessarily so! This segment we'll take a look at modems, both phone-line and cable connected. Fax and phone line connected equipment protection is linked to everything presented here. Phone-line connected modems Phone lines rank right after AC power lines as sources for equipment damage and disruption. Phone lines extend across the countryside just as power lines, so the potential for surge and interference pickup exists. Indeed, usually phone lines are carried on power company poles and travel in parallel for many miles. I know - I hear it all the time Why worry? Modems only cost 40 - 50 bucks! Why spend $50 to $100 protecting a $40 / 50 investment? Here's why. For 1 thing, your modem is your lifeline to the internet. When it's down, your entire internet operation is down! When personnel down time, lost business opportunities and repair costs are considered - a damaged modem costs much more than the $40 / 50 hardware cost. Secondly. Your modem is a back door gateway to your system. Damaging spikes and surges entering the modem have been known to be passed through to the system, inflicting additional damage. More $$ down the drain! OK. So maybe a few surges can enter via the phone line. Sounds like that's about it. Not quite! Various interference sources continuously impinge on phone lines, sometimes causing modem slow-down; other times completely disrupting communications. An overhead lightning storm can wreck havoc with modem transmission. Phone line interference originates from lightning storms and spherics, radio, TV, police and cell phone transmitters. Electrical machinery sources and power lines in proximity to phone lines contribute to noise pick up. Note that this interference can be induced on phone lines over long wires, or locally within your building. For instance, a local air conditioner could cause severe RF radiation that finds its way into your modem, resulting in slow down and disruption. Both damaging surges and disrupting interference exit on phone lines in common mode (both lines to ground) and differential mode (line to line) orientation, and should be addressed when choosing a modem phone line protection system. What protection should be considered? First - Suppression technology Technology for modem phone line protection includes MOVs, thyrister semiconductors, suppressor diodes, and gas tubes. Any one of these can be used alone, but high end protectors generally use suppressor diodes and thyrister semiconductors or gas tubes. Modem phone line filtering must be chosen carefully to avoid affecting high speed data transfer. While MOVs are relatively inexpensive, they tend to disrupt proper phone operation. Also, as surge current increases, MOV terminal voltage increases causing added high voltage stress to connected equipment. Avoid them for modem phone lines ! Suppressor diodes have extremely fast response time, but lack ability to suppress large surge current pulses, so diodes are not often used alone. Their fast response positions them as a natural for "turn on" elements in a hybrid configuration. Gas Tubes will pass gargantuan surge currents, but have very slow response time. In addition, gas tubes have poor reliability. Upon reacting to a high voltage surge, gas tubes "trigger", pulling equipment terminal voltage down to 30 volts or less. A fair number of modem protection systems designed around gas tubes are offered in the market place. When coupled in a hybrid configuration with high speed suppressor diodes, gas tubes provide reasonable protection. Figure 1 Our very popular PDS-11 modem protection and interference control system. This hybrid thyrister semi-conductor and suppressor diode design provides common-mode and differential-mode hi speed, hi current suppression in addition to both mode RFI filtering. Each line wire is fused for expanded protection. Thyrister semi-conductors respond much faster than either MOVs or Gas Tubes, but are much slower than suppressor diodes. Capable of handling fairly large surge current, thyrister surge current capability is somewhat less than gas tubes, but considerably beyond levels required for modem-phone line protection. Like gas tubes, thyristers "trigger" upon encountering a large surge voltage, pulling equipment terminal voltage down to 30 volts or less. Thyrister semi-conductors exhibit much greater reliability than gas tubes. Hybrid modem phone line protection systems consisting of thyrister semiconductors, suppressor diodes, RFI and noise filtering present a formidable, reliable, long term defense against damage and disruption. Such a system is shown in Figure 1. Internal fusing for protection against massive hits is incorporated in the illustrated Model. Interference Filtering As previously mentioned, RFI and electrical noise interference is commonly encountered on modem phone lines. With the proliferation of cell phones, wireless and other RF carrier technology, one can only expect this situation to deteriorate. A delicate balance exists between adequate interference filtering and data transmission/reception requirements. When purchasing a protector, insist on a full refund if your system performance is compromised. Usually only high end modem protectors incorporate meaningful, non-intrusive filters. Figure 1 depicts our successful PDS-11 protection system which has successfully been used in the home, office and factory computing arena for over 20 years. These have proven so effective that radio stations purchase them for neighbors suffering interference from their transmitters ! Although very successful at removing RFI from radio and TV transmitters, the PDS-11 does not impede internet communications using 56k modems. Common Mode noise from local electrical equipment is also suppressed. Severe electrical phone line noises often found at industrial locations can be further subdued with our TURBO modem protection system. With surge protection and RFI filtering identical to the PDS-11, TURBO protectors incorporate an additional common mode electrical noise filter for added communication enhancement. Cable Modems Becoming ever more common, cable modems connect directly to our local cable provider's system providing much faster internet access. |
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