Conversion circuit for process control system

Conversion apparatus adapted to couple to a field transmitter through a primary process control loop, comprising:

• a digital receiver adapted to couple to the primary process control loop having a first process variable signal input, a second process variable signal input and a digital output related to the first and second process variable inputs;
• a controller having a digital signal input from the digital receiver and having a secondary loop control output as a function of first and second process variable signals;
• and
• a secondary loop controller adapted to couple to a secondary process control loop, the loop controller including a current source having an analog current output coupled to the secondary process control loop responsive to the secondary loop control output.

Conversion circuitry for coupling to a field transmitter over a primary process control loop, comprising:

• means for receiving a first digital process variable signal and a second digital process variable signal from the primary process control loop and providing a digital output related to the first and second digital process variables;
• means for receiving the digital output and responsively providing a secondary loop control output as a function of the first and second digital process variables;
• and
• means for controlling current flowing through the secondary loop in response to the secondary loop control output whereby an analog current in the secondary loop is a function of the first digital process variable signal and the second digital process variable provided by the primary process control loop.

A method of controlling current in a secondary process control loop, comprising:

• receiving a first digital process variable signal and a second digital process variable signal from a primary process control loop;
• providing a digital output related to the first and second digital process variables;
• providing a secondary loop control output responsive to the digital output;
• and
• controlling analog current in the secondary control loop in response to the secondary loop control output as a function of the first digital process variable signal and the second digital process variable.