Data communication interfaces are commonly used to transmit digital data between computers or other types of programmable or control systems. These data communication interfaces typically include a plurality of pins, which are each associated with a data line, and function to transmit a particular signal through the data line or provide a ground connection. A number of different standards exist for standardizing the interfaces and associated data lines across applications, so that a number of different types of equipment can communicate with each other by way of the interfaces. Some of the more common interface standards are the RS-standards, such as, for example, the RS-232 and RS-485 data interface standards. In order to transmit data over the data lines, one or more transmitters and receivers, or "transceivers" are provided in the interface. These transceivers control the transmission of data through the associated data lines in accordance with the selected standard.
The transceivers used in RS-standard interface applications are typically designed to operate within specified voltage and current ranges, and these ranges are typically specified in a manufacturer's catalog or data sheet for the interface. Operation within the voltage range specified for the interface is recommended in order to minimize errors in the transmitted data, as well as protect the interface components. If a voltage which exceeds the recommended level is applied to the interface, or any of the data lines associated with the interface, data transmission may be interrupted, or, in more severe cases, components in the interface may be destroyed.
An example of an application in which an RS-standard interface may be used to transmit signals is a magnetostrictive linear displacement transducer. In a magnetostrictive linear displacement transducer, an excitation pulse transmitted through a conductive wire in a waveguide, interacts with a magnetic field from a magnet adjacent the waveguide, to produce a torsion wave within the waveguide. The torsion wave is detected and converted into an electrical signal by a mode converter, such as a coil, and the time interval between the transmission of the excitation pulse and the detection of the converted signal is used to determine the position of an object which is connected to the magnet. In the transducer, a data interface may be used for receiving and transmitting the interrogation and reply pulse from the signal electronics associated with the waveguide pulse. In the transducer application, as in other situations in which a data interface is utilized, the application of an excessive voltage for more than a brief period of time to any of the data lines associated with the interface can cause adverse effects. In particular, the application of an excessive voltage can cause a reverse current to flow through the data interface which can cause permanent damage of the interface.
A number of circuit arrangements and components have been provided in the past for eliminating the problems associated with an overvoltage condition on one or more data lines associated with an interface. These arrangements have included shunting the data lines of the interface through a zener or transzorb diode to either ground or a positive power supply, to prevent the power in the lines from exceeding the rating of the diode or power supply. This shunting of the data lines to ground through a diode can provide protection against short overvoltage signals, such as power spikes or transients associated with an electrostatic discharge. However, because the ability of either a zener or transzorb diode to absorb power in an overvoltage situation is limited, these arrangements have not been able to provide protection in the event of an overvoltage situation which occurs for more than a brief period of time, such as would result from the inadvertent connection of an excessive power supply to a data line. Therefore, if a data line is mistakenly connected, or otherwise comes into contact with a power supply which exceeds the maximum operating voltage for the interface, for more than a brief period of time, the excess voltage may produce a reverse current through the interface transceiver to ground. This reverse current may cause irreparable damage to the transceiver.
Accordingly, to overcome the above and other problems, it is desirable to have an overvoltage protection apparatus which disables the transmission of power through a data interface upon application of a voltage either to the interface, or any data line associated with the interface, that exceeds the manufacturer's recommended voltage level. Further, it is desirable to have such an overvoltage protection apparatus which continues to protect the interface for as long as the overvoltage condition exists.