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PLC Input Configuration, Sourcing vs. Sinking

Most Centroid PLC input/output boards allow inputs to be wired in either a "current-sourcing" or a "current-sinking" configuration.

In a current-sourcing input configuration, current flows from the PLC input terminal, through the input device (switch or sensor), to 0V/Ground. The input bank common is connected to the positive voltage (usually +24VDC).

In a current-sinking input configuration, current flows from the positive voltage source, though the input device, into the PLC input terminal. The input bank common is connected to 0V/Ground.

Note that the terminals in the diagrams above are arranged logically, to illustrate current flow. The actual terminal layout on the PLC boards is different. Typically, INP9 - INP12 would be arranged right-to-left, with their bank common terminal to the left of INP12.

"Dry-contact" devices such as electromechanical limit switches, pushbuttons, and relay contacts can be connected equally well to current-sourcing or current-sinking PLC inputs, since they are not sensitive to the direction of current flow.

NPN proximity sensors and similar devices can only be connected to current-sourcing PLC inputs.

PNP proximity sensors and similar devices can only be connected to current-sinking PLC inputs.

Centroid factory wiring diagrams generally show most PLC inputs in a sourcing configuration. A notable exception is for tool-changer sensors on typical umbrella and swing-arm ATC machines. Since many of these machines come with PNP proximity sensors, one or two banks of inputs must be wired in a current-sinking configuration to receive those sensors.

Most Centroid PLC input/output boards group the inputs into banks of four. Each bank of four inputs can be configured independently for either sourcing or sinking operation, and also for different voltages.

The choice of sourcing or sinking is determined simply by the bank common connection: +V for sourcing inputs, 0V/Ground for sinking inputs.

The choice of input voltage is determined by the SIP resistors installed on the I/O board. See Input SIPs on Centroid PLC Boards.

A notable exception is the Acorn DIY stepper control board. Its eight on-board PLC inputs all share the same common, and can only operate in a 24V current-sourcing configuration.

Earlier-generation boards, such as the DC3IO, SERVO3IO, RTK2 and PLC 15/15, used only 5V current-sourcing inputs.

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Last updated 06-Apr-2023 MBL