Logic isolator with high transient immunity

DETAILED DESCRIPTION The present invention includes a logic isolator with high transient immunity, for isolating digital logic signals, such as signals between equipment on a field side and microcontrollers on a system side in a process control system.
In one aspect, the logic isolator has an input circuit that receives a digital input signal, with edge detection circuitry that detects rising and falling edges of that input signal.
The input circuit provides an output signal indicative of those rising and falling edges to a transformer assembly which serves as an isolation barrier.
The transformer assembly replicates the signal it receives and provides it to an output circuit, while shunting common mode transients to ground.
The output circuit converts the signal from the transformer back into a digital logic signal with rising and falling edges as in the digital input signal.
The transformer assembly preferably includes a linked-coupled transformer that has a first core with a first winding, a second core with a second winding, and a grounded link wire that extends from the first core to the second core for grounding capacitively linked common mode transients.
Alternatively, a shielded transformer, such as a grounded double or single shield between two windings, could be used.
In preferred embodiments, the input circuit converts the rising and falling edges in the digital input signal to positive and negative pulses using tri-level logic, and the output circuit converts these pulses back into rising and falling edges.
The input circuit preferably also includes a pulse generator for providing pulses, referred to as refresh pulses, with a high frequency and with a pulse width that is the same as the width of the pulses created in response to detection of a rising edge or a falling edge.
The refresh pulses are logically combined with the input signal to provide an interrogating functionality that allows the isolator to determine the state of the input signal, and therefore, the isolator can recover quickly in case of a power spike or dropout, and also can quickly determine the state if an edge is missed