The B837–016 24 Vac/Vdc (True High) input module senses off and on input signals from its field circuitry and converts them to logic levels used by a Modicon PLC. The module’s 16 inputs are separated into two groups of eight channels, each group being totally isolated from the other.

Although both groups use common return wires, none has a definite relationship to system ground unless established in the user’s field circuitry. Since both groups nominally employ independent power return sources, both ac and dc powered field circuits may input to the module at the same time.

When the user’s ac/dc powered field circuit goes on - as the result of a limit switch for example - it presents the field power voltage at the modules appropriate input channel. When the input voltage meets or exceeds the module’s guaranteed on threshold, the resulting voltage turns the channel monitor lamp on, current flows through the bridge rectifier and subsequently the opto-isolator (OPTO-ISOL) circuit.

Given a nominal 24 V field power supply and 1000Ω maximum input source impedance, the module’s channel monitor lamps will indicate on and off when voltages are 20.4 Vac/19.2 Vdc for the high level on; and 6 Vac/10 Vdc for the low level off respectively. The optical energy goes to the communications board (COMM BOARD) where the Ourbus output register is set to represent the field circuit’s on state. As long as the field input status remains true, the module will communicate this status each time it is polled by the controller.