Abstract: A control system of the type used in process control and comprising a controller which provides a control signal on a signal line, and a DC output circuit for converting the control signal on the signal line into a DC current output signal varying in a range of, e.g., 4 to 20 milliamperes, which is supplied to a current loop including one or more load devices. The control system is characterized by open circuit detecting means for detecting an open circuit condition in the load current loop by detecting the fall of the DC current output signal below a predetermined minimum value, and shorting means in the DC output circuit for shorting the signal line in response to detection of an open circuit condition. Level detecting means are provided for detecting the shorting of the signal line, and thus an indication of an open-circuited load condition may be supplied back to the controller without requiring added signal carrying lines.

Abstract: A process control system of the type arranged to accept varied forms of analog sensing signals from field sensing devices, to digitally process data represented thereby, and to supply varied forms of analog control signals to field control devices. The system is characterized by signal converter means which convert data between the varied analog forms generated by the field devices and a standard form of intermediate signal, e.g., a pulse-width signal, which carries data as a function of duration. The intermediate signals facilitate signal transmission, editing, and isolation, and permit the process control system to have a simple, noise-free, easily adaptable structure. Digital interface means then convert data between the intermediate signals and digital signals in a form suitable for use by digital data processing means.

Abstract: A process control system of the type arranged to accept sensing signals from field sensing devices, to digitally process data represented thereby, and to supply control signals to field control devices. The system is characterized by a centralized maintenance backup system for supplying backup control signals to selected field control devices while their normal control signals are interrupted during servicing of various system components, e.g., signal conversion units. A maintenance backup device, centrally located at a control station, includes means for providing an adjustable backup control signal, a selector unit having jack terminals connected through prewired leads to a plurality of field control devices, and jack plug means for connecting the backup control signal means to a selected jack terminal and the field control device corresponding thereto. Servicing of circuit boards used, e.g.

Abstract: A variable resistance circuit comprising a field effect transistor, a plurality of amplifiers and a feedback circuit for one of the amplifiers with the equivalent resistance value derived from a circuit comprising the field effect transistor electrodes and a reference resistor being controlled by a digital signal or a pulse-width signal supplied to the feedback circuit.

Abstract: A signal transmission system of the type used in process control systems for conveying a signal between a field telemetering instrument and a central controller, or between a central controller and a process control device. The signal transmission system is arranged with a transmitter accepting an input signal, a transmission line for carrying a transmission signal, and a receiver receiving the transmission signal and developing an output signal.In accordance with the invention, converter means in the transmitter develop a pulse width signal with a duration corresponding in a predetermined manner to the value of the input signal, and means in the transmitter provide the transmission signal (e.g., a current signal) with two states (e.g., opposite polarities), one of which has a duration corresponding to the duration of the pulse width signal.