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Current To Voltage and Multiplexing Circuits for Direct Wire Monitoring

IP.com Disclosure Number: IPCOM000081578D
Original Publication Date: 1974-Jul-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Hellwarth, GA: AUTHOR [+2]

Abstract

DC amplifier circuits provide the capability of monitoring short circuits open circuits or intermediate levels as these signals are presented by a remote location. The amplifiers are adaptable for use in multiplexing of a large number of inputs to a computer interface.

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Current To Voltage and Multiplexing Circuits for Direct Wire Monitoring

DC amplifier circuits provide the capability of monitoring short circuits open circuits or intermediate levels as these signals are presented by a remote location. The amplifiers are adaptable for use in multiplexing of a large number of inputs to a computer interface.

Direct-wire systems used in applications such as security monitor stations are energized by a voltage source which is frequently 130 volts DC, to provide a current supply through a single-wire line to the user with an earth-ground return. Current in the line is monitored to determine the state of the security of the user. Typical current conditions which may occur depending upon events monitored by the user, include open circuit, short circuit (45 milliamps in many applications) and intermediate levels such as 20 and 30 milliamps. The open and short conditions indicate security breaches and require fast responses, while the intermediate conditions correspond to day/night customer conditions and can be scanned more slowly. When implemented in the IBM System/7 using the 5012 I/O module, the circuitry shown is capable of monitoring 1,536 direct wires for each 5012 module.

A current-sensing circuit is inserted in the line to convert the current to voltage, which can be sensed by an isolated digital input point at the computer interface. The circuits of Figs. 1 and 2 provide these functions. Current-to- voltage conversions are effected with negligible disturbance to the line. An output voltage of approximately zero for both security breach conditions is produced. An output voltage greater than 2 volts for normal operating conditions such as day/night security monitoring is provided.

Since the circuits must operate at a high common-mode voltage such as 130 VDC, the circuits are powered from the line and eliminate the need of isolated power supplies. The circuits use only inexpensive transistors and resistors. An output is available which may be measured by the analog input of a monitoring computer, to determine the exact line current and to check for line integrity. The two security breach conditions and the intermediate normal conditions map into opposite voltage states so that the monitoring computer may be interrupted, such as through an interrupt-on-change or process interrupt apparatus, whenever a security problem occurs.

Figs. 1 and 2 operate as described below using typical component values as shown. It is assumed the transistors turn on when the base-emitter voltages exceed 0.6 volt.

Fig. 1: For currents below 10 ma, transistors 1 and 2 are off and the voltage Vout is zero. For currents between 10 ma...