Negative-true logic is the standard logic for industrial automation I/O Module control. 

Industry Standard I/O is configured using individual “negative-true” I/O signal control lines for each I/O position.  Standard I/O Modules are designed for “open-collector” sinking logic. Standard I/O Mounting racks employ a pull-up resistor on each I/O signal line (usually in the range of 3.3k to 4.7k).  In the inac-tive or “off” state, the control signal line is pulled high to the +Logic power. Likewise, in the active or “on” state, the I/O signal line is asserted to Logic 0 (low) to logic common.

For informationon Digital I/O Interfacing, please see the adjacent link for Brentek’s Negative-True Logic Application Note.