Overvoltage protection for RS232

RS-232C serial port lines are quite prone to be damaged by overvoltages. The damaged to computer serial ports have become more and more expensive to replace because of higher intergaration: usually you have to buy new motherboard if one serial port in it gets broken. So it is wise to use some extra protection in situations where overvoltages surges are quite propabe and repairing gets expensive. For examples of this type of situations are long serial port lines side the building and situation where the computer is connected to ungrounded outlet or the electricity distribution wiring is is not well organized.
Overvoltage protection for RS232 - schematic

Overvoltage protection for RS232 - img1

Protecting RS-232 serial port against overvoltages usually quite easy. The standard says that the signal can be voltage between +25 and -25 volts, so anything outside this range is not allowed. Typical RS-232C receiver chips can handle well voltages up to +/-30V. So an overvoltage protection which cuts out everyhing above 25V and acts fast is the solution. Suitable components for this are varistors (VDR), zener diodes and other fast semiconductor based overvoltage protection devices. Another question is what needs to be protected. The answer is that every communication line wire which is connected between computers must be protected. In simplest case there are only transmit data, receive data and ground lines used. In this case only protection componets are needed: one between transmit data and ground and another between receive data and ground. If more lines are used, then all of then must be protected with a protection component between signal wire and signal ground. This another circuit is based on zener diodes and the basic idea is quite similar to some commercial circuits also. The zener diode is faster to react to the surges than those VDRs so they provide better protection. Zener diodes also have lower capacitance than VDRs, so they suite better for high speed data lines. Unfortuantely the surge capacity of those zener diodes is not very high, so that they can't handle the large spikes by themselves. That's why I...

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