RS-232 line drivers and receivers are made for voltages ranging from +15 volts to -15 volts. Voltages exceeding the typical 23 volt device limit can be induced and damage the RS-232 drivers and receivers. Excessive voltages can be caused by power surges, power dropouts, ground loop currents and lightning. Lightning does not have to strike a building directly to cause damage to data comm equipment. The expanding magnetic field from a nearby lightning strike or even a cloud-to-cloud discharge can easily induce voltage in data lines. Even without lightning storms, ground loop currents can occur when cables are run between buildings, when there is improper electrical grounding, and when a multiplexer and the terminal are connected to different power line transformers or when earth ground points are separated by hundreds of feet.
The consequence of exceeding these voltages is electrical failure of the RS-232 drivers and receivers. Failure modes include invisible electrical damage, chips with the tops blown off, and traces burned.
1. Keep cable distances to a minimum. A large building may be served by more than one power line transformer. A multiplexer powered from one power line transformer, connected by a long RS-232 cable to a terminal connected to a different power line transformer, may have a substantial voltage differential between the earth grounds. These ground currents will attempt to equalize across the data cable, often with the multiplexer or terminal devices acting as a "fuse". If long distances are needed, use transformer or optically isolated short haul modems.
2. Do not run RS-232 cables between buildings. Cables between buildings can invite direct or induced lightning damage, and ground loop currents. Use transformer or optically isolated short-haul modems instead.
3. Use properly grounded electrical outlets. If there is a faulty ground, the RS-232 link may provide an excellent but destructive path to ground. Grounding can be checked with a ground fault testing tool readily available at hardware stores for about $5.00.
4. Do not use frame ground (protective ground) for a data return ground. This increases the probability of ground loop damage to RS-232 components. Use the signal ground pin for data return ground.
5. Consider placing all equipment on an uninterruptable power supply (UPS). These systems supply conditioned, battery-backed power, isolating valuable equipment from the local power grid. Many of these units are now priced such that all the data comm equipment in a location may be protected for less than the repair cost of a single piece of the equipment.
6. Install Surge suppressors. Electrical surges can come through the RS-232 cables to the multiplexer and terminal device, through the phone line to the modem or through the power source to any attached equipment. Surge suppressors are available to protect RS-232 links from potentially damaging voltage spikes. It may be necessary to install surge suppressers on all of the RS-232 links. Surges suppressers should protect all of the connections used in an RS-232 link. Pins 1 through 8 and 20 are typically used at a terminal or printer. If the surge suppressor purchased comes with a "pigtail" or wire braid, it MUST be connected to a good earth ground in order for the suppressor to function.
Surge suppressors are resonably priced, readily available from industrial suppliers, and we recommend their use on all longer RS-232 lines.
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