Oscillating output improves system security

-February 06, 2003

Many electronic-control systems have digital outputs that use transistors. One method of improving the security in these outputs is to use an oscillating signal to represent a logic-high state instead of a fixed voltage level (Figure 1). This type of signal, a dynamic variable, can drive the circuit shown in Figure 2. This circuit connects to the output transistor of the electronic-control system. The dynamic-variable signal connects to the output transistor, whose collector connects to a pulse transformer. If the system cannot produce pulses because of a fault, the relay deactivates. If a fault exists in the drive circuitry, the output system stays off. This solution guarantees the security function when one fault occurs. However, it does not guarantee the detection of all faults. However, you can use a DPDT relay and connect one of the contacts to one of the electronic-control inputs; thus, you can confirm the relay's activation or deactivation in the control program.

The circuit in Figure 2 performs efficiently if the system can generate a frequency greater than 1 kHz to excite the pulse transformer. Many electronic systems that use an output transistor can generate a dynamic variable. Many systems, such as PLCs (programmable-logic controllers), have long cycle times and, thus, cannot generate signals of adequate frequencies. In these cases, you can obtain an appropriate signal using the low-frequency dynamic variable from the PLC. To accomplish that task, you must use an external oscillator and a pulse detector implemented with a monostable multivibrator. The oscillator produces a square-wave signal of a frequency that suits commercial pulse transformers. This signal drives the MOSFET when the pulse detector receives pulses from the PLC. Because the pulse detector can fail, you must duplicate the external circuit for redundancy. In this way, the final result is the same as that of Figure 2, except that the safe output comprises two relays.

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