A method for providing galvanic isolation between an input or trunk interface (30) and multiple outputs or spur interfaces (38) for connecting field devices (14) to a trunk (20) of a two-wire (44a, 44b) process control network (10) includes connecting multiple sets of multiple spur interfaces to respective isolating elements (34). Each isolating element (34) connects a respective set of outputs or spur interfaces (38) to the trunk interface (30) and galvanically isolates (40, 42) the respective set of spur interfaces (38) from the trunk interface (30). Field devices (14) attached to different sets of spur interfaces are also galvanically isolated from one another.

An isolated signal transmission apparatus isolatingly transmits, from a control-side apparatus to a control target apparatus, an electric signal obtained by superimposing a communication signal on a direct-current signal. The apparatus includes a lower limiter circuit configured to output, to the control target apparatus, a direct-current signal, whose value is a predetermined lower limit value, if a value of the direct-current signal from the control-side apparatus is less than the predetermined lower limit value.

A method for providing galvanic isolation between an input or trunk interface (30) and multiple outputs or spur interfaces (38) for connecting field devices (14) to a trunk (20) of a two-wire (44a, 44b) process control network (10) includes connecting multiple sets of multiple spur interfaces to respective isolating elements (34). Each isolating element (34) connects a respective set of outputs or spur interfaces (38) to the trunk interface (30) and galvanically isolates (40, 42) the respective set of spur interfaces (38) from the trunk interface (30). Field devices (14) attached to different sets of spur interfaces are also galvanically isolated from one another.