Sinking and Sourcing Inputs and Outputs, Wiring Diagrams

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rjagodowski
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Joined: Fri Sep 04, 2015 6:59 pm

Sinking and Sourcing Inputs and Outputs, Wiring Diagrams

Post by rjagodowski »

Here are some links I've used to explain sinking and sourcing as well as 2, 3 & 4 wire sensors. There was a discussion at the end of last week's lab but most had left before, so I'm including this for those interested.

In summary: For inputs and outputs, the device connected to the PLC I/O must be of a type opposite of the PLC I/O connection. Also remember with sourcing or sinking PLC inputs, IF a current is flowing into or out of the input, that would be reported as a TRUE condition with a XIC instruction. IF there is no current flowing into or out of the PLC input, then that would be reported as FALSE with an XIC instruction. TLDR: TRUE if current flows, FALSE if no current flows.

Example 1: PLC has sourcing inputs, therefore a sensor connected to the input MUST BE a sinking device to accept the PLC current. (In other words, the PLC supplies a voltage to allow current to flow OUT of the PLC input.)

Example 2: PLC has sinking inputs, therefore a sensor connected to the input MUST BE a sourcing device to provide the current IN to the PLC input. (In other words, the PLC looks like a ground to allow current to flow IN the PLC input.)

Example 3: PLC has sourcing outputs, therefore an actuator/load connected to the output MUST BE a sinking device to accept the current out of the PLC output. (In other words, the PLC output looks like a voltage source supplying current to the load.)

Example 4
: PLC has sinking outputs, therefore an actuator/load connected to the output MUST BE a sourcing device to provide the current into the PLC output. (In other words, the PLC output looks like a ground to accept current from the load.)

Here's a good summary presenting the type of transistor (NPN or PNP) used in the sensor and how it determines if the sensor is sourcing or sinking. TLDR: PNP is sourcing, NPN is sinking. Sensor Connections from Allied Electronics & Automation. Here's a pdf of the above link:

PLC Sinking and Sourcing Output Modules

PLC Sinking & Sourcing Input Modules

Two Wire Sensors
Three Wire Sensors
Four Wire Sensors


Lastly, the topic of Wiring Diagrams and Symbols came up in today's discussion. Here's a document from Allen-Bradley that helps to explain differences between various wiring diagram schemes.
Global_Reference_Guide_for_Reading_diagrams.pdf
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