Method and apparatus for analogue output current control

A method of driving an analogue output current in an industrial process control comprising one or more input modules, one or more processing modules and one or more analogue output modules, the method comprising:

• generating a first driven current from an output drive circuit comprising a first transistor, the first driven current having an analogue output in a continuous range of 4-20 mA in an analogue current loop driving a field termination assembly;
• using a processor of a first controller in communication with the output drive circuit to determine a value for the first driven current in the continuous range of 4-20 mA according to a commanded current from digital communication data to control the first driven current to output the first driven current according to the digital communication data for communication with, the remote device;
• and
• using a processor of a second controller in communication with the output drive circuit, the second controller receiving the commanded current and a measure of the first driven current, to determine a value for an expected first driven current in the continuous range of 4-20 mA according to the commanded current, and
• activating a cutoff switch receiving the first driven current, the cutoff switch comprising a second transistor, the cutoff switch being configured to block the first driven current from an output, when a difference between the first driven current and the expected first driven current is greater than a predetermined threshold,
• wherein the processor of the second controller determines the expected first driven current independently from the processor of the first controller which determines the first driven current, and
• further comprising superimposing a HART communications signal in the analogue current loop.

An apparatus for industrial process control comprising one or more input modules, one or more processing modules and one or more analogue output modules, a first analogue output module of said one or more analogue output modules comprising:

• an output drive circuit comprising a first transistor, the output drive circuit being configured to generate a first driven current having an analogue output in a continuous range of 4-20 mA in an analogue current loop driving a field termination assembly;
• a cutoff switch comprising a second transistor, the cutoff switch being configured to receive the first driven current and being configured to block the first driven current from an output when activated;
• a first controller in communication with the output drive circuit, the first controller having a processor configured to determine a value for the first driven current in the continuous range of 4-20 mA according to a commanded current to control the first driven current;
• and
• a second controller in communication with the output drive circuit, the second controller receiving the commanded current and the first driven current, the second controller having a processor configured to determine a value for an expected first driven current in the continuous range of 4-20 mA according to the commanded current,
• wherein the processor of the second controller determines a difference between the first driven current and the expected first driven current to activate the cutoff switch when the difference between the first driven current and the expected first current is greater than a predetermined threshold,
• wherein the processor of the second controller determines the expected first driven current independently from the processor of the first controller which determines the first driven current, and
• wherein the first analogue output module superimposes a Highway Addressable Remote Transducer (HART) communications signal in the analogue current loop.