Supervision Device for Normally-Open Contacts
Original Publication Date: 1985-Apr-01
Included in the Prior Art Database: 2005-Feb-18
This article describes a supervision device for line supervision of normally-open contact sense points. In the past, supervision of normally-open lines required the use of end-of-line resistors, and overall loop resistance became critical. Additionally, these circuits functioned on a static current value and were easily defeated. Utilization of circuitry of the present disclosure allows variable values of time to be utilized and can be made virtually impossible to defeat. While the disclosure depicts one circuit, digital input cards will contain a plurality of these line- supervision circuits. In the drawings, Fig. 1 is a schematic view showing the digital input card containing the present improved supervision circuit connected to a normally-open sensor. Fig. 2 is a circuit diagram of the supervision circuit.