What is the difference between positive and negative logic in digital I/O circuits?
“When a circuit requires logic 1 to operate, engineers may refer to this condition as positive logic. Thus, the more positive voltage causes the action to take place. On the other hand, if a circuit requires a logic 0 to cause action, this type circuit is referred to as negative logic. There is nothing negative or positive about these various circuits. The notation simply provides a shorthand that tells engineers and users whether a logic 1 or a logic 0 causes an action.”1
Sealevel digital I/O adapters that use the SeaIO driver support configuration for positive or negative logic. The relay outputs are configured for positive logic by default, meaning that the relays are in the open or reset position and logic 1 must be written to close or set the relays. If the relays are configured for negative logic (through Windows Device Manager), then logic 0 would be written to close or set the relays.
Optically isolated inputs are also configured for positive logic by default, meaning that the inputs report logic 1 when a voltage is applied across the contacts. If the inputs are configured for negative logic (through Windows Device Manager), then logic 0 would be reported when a voltage is applied across the contacts.
The SeaI/O data acquisition modules use the SeaMAX driver and support positive logic.
1Citation: O’Hanlan, Tom and Jon Titus, “Chapter 1 Logic Principles,” in The Digital I/O Handbook
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