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FX Series Programmable Controllers | Basic Program Instructions 2 |
2.3How to Read Ladder Logic
Ladder logic is very closely associated to basic relay logic. There are both contacts and coils that can be loaded and driven in different configurations. However, the basic principle remains the same.
A coil drives direct outputs of the PLC (ex. a Y device) or drives internal timers, counters or flags (ex. T, C, M and S devices). Each coil has associated contacts. These contacts are available in both “normally open” (NO) and “normally closed” (NC) configurations.
The term “normal(ly)” refers to the status of the contacts when the coil is not energized. Using a relay analogy, when the coil is OFF, a NO contact would have no current flow, that is, a load being supplied through a NO contact would not operate. However, a NC contact would allow current to flow, hence the connected load would be active.
Activating the coil reverses the contact status, that is, the current would flow in a NO contact and a NC contact would inhibit the flow.
Physical inputs to the PLC (X devices) have no programmable coil. These devices may only be used in a contact format (NO and NC types are available).
Example:
Because of the close relay association, ladder logic programs can be read as current flowing from the left vertical line to the right vertical line. This current must pass through a series of contact representations such as X0 and X1 in order to switch the output coil Y0 ON. Therefore, in the example shown, switching X0 ON causes the output Y0 to also switch ON. If however, the limit switch X1 is activates, the output Y0 turns OFF. This is because the connection between the left and the right vertical lines breaks so there is no current flow.
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X0 |
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I | PC Program |
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X1 | T |
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Limit switch |
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