An I/O module circuit for use in an industrial control system comprising: a housing; terminals supported by the housing for receipt of electrical power to be controlled by the I/O module circuit; a zero crossing detector connected to the terminals for detecting a zero crossing of an AC waveform connected to the terminals and for generating a zero crossing signal corresponding to a zero crossing of the AC waveform; a module controller for receiving the zero crossing signal from the zero crossing detector and generating a switch state signal indicating a desired control of electrical power at the terminals of the I/O module circuit coincident with the zero crossing of the AC waveform; and an output circuit contained in the housing providing: at least two field effect transistors (76,78) having a source and drain in series between the terminals to control current flow through the terminals according to a first electrical signal received by a gate of each of the at least two field effect transistors; an input receiving the switch state signal indicating the desired control of electrical power at the terminals of the I/O module circuit; a power isolator being a photovoltaic optical isolator having a photovoltaic stack, the power isolator receiving, at a first controller-side, the switch state signal and providing at a second field-side output power and the first electrical signal communicating information of the switch state signal; wherein the output power is derived from input electrical power received at the first controller-side of the power isolator but isolated and floating with respect to that input power; and wherein the first electrical signal is isolated and floating with respect to the switch state signal. a housing; terminals supported by the housing for receipt of electrical power to be controlled by the I/O module circuit; a zero crossing detector connected to the terminals for detecting a zero crossing of an AC waveform connected to the terminals and for generating a zero crossing signal corresponding to a zero crossing of the AC waveform; a module controller for receiving the zero crossing signal from the zero crossing detector and generating a switch state signal indicating a desired control of electrical power at the terminals of the I/O module circuit coincident with the zero crossing of the AC waveform; and an output circuit contained in the housing providing: at least two field effect transistors (76,78) having a source and drain in series between the terminals to control current flow through the terminals according to a first electrical signal received by a gate of each of the at least two field effect transistors; an input receiving the switch state signal indicating the desired control of electrical power at the terminals of the I/O module circuit; a power isolator being a photovoltaic optical isolator having a photovoltaic stack, the power isolator receiving, at a first controller-side, the switch state signal and providing at a second field-side output power and the first electrical signal communicating information of the switch state signal; wherein the output power is derived from input electrical power received at the first controller-side of the power isolator but isolated and floating with respect to that input power; and wherein the first electrical signal is isolated and floating with respect to the switch state signal. at least two field effect transistors (76,78) having a source and drain in series between the terminals to control current flow through the terminals according to a first electrical signal received by a gate of each of the at least two field effect transistors; an input receiving the switch state signal indicating the desired control of electrical power at the terminals of the I/O module circuit; a power isolator being a photovoltaic optical isolator having a photovoltaic stack, the power isolator receiving, at a first controller-side, the switch state signal and providing at a second field-side output power and the first electrical signal communicating information of the switch state signal; wherein the output power is derived from input electrical power received at the first controller-side of the power isolator but isolated and floating with respect to that input power; and wherein the first electrical signal is isolated and floating with respect to the switch state signal.