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Ex-OR versus Ex-NOR: A lucky logic equivalence

Some years ago, back in 2005 to be specific, the project was a power supply for a low Earth orbiting satellite.

Part of our digital circuitry involved some logic gating using Exclusive-OR (Ex-OR) devices of type CD4070BMS. All was well until we were advised that the 4070 parts were not obtainable as radiation hardened products. Whether that would still be the case today I have no idea, but back then, it was so.

We were also told, however, that the part CD4077BMS exclusive-NOR (Ex-NOR) devices were available with radiation hardening. The two devices compared as follows:

Figure 1 The original Exclusive-OR device (left) versus the Exclusive-NOR replacement (right).

Someone must have been smiling on us that day because we discovered that our logic functions were not affected by having originally chosen exclusive-OR rather than exclusive-NOR devices. Our outputs F and G as seen in Figure 2 were the same versus the A and B inputs for using either the ‘4070 or the ‘4077 parts.

Figure 2 As it turned out, our logic usage resulted in virtually the same outputs from both the original Ex-OR devices and the Ex-NOR replacements.

Although the intermediate C signal was inverted from its original states in the Ex-NOR implementation, that didn’t matter because each signal path comprised two logic inversions that cancelled each other out.  

It wasn’t planned that way. We just got lucky.

is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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