Basic rules of ladder-logic

The basic rules of ladder-logic can be stated to be:
• The vertical lines indicate the power supply for the control system (12 V DC to 240 V AC). The ‘power flow’ is visualized to move from left to right.

• Read the ladder diagram from left to right and top to bottom (as in the normal Western convention of reading a book).

• Electrical devices are normally indicated in their normal de-energized condition. This can sometimes be confusing and special care needs to be taken to ensure consistency.

• The contacts associated with coils, timers, counters and other instructions have the same numbering convention as their control device.

• Devices that indicate a start operation for a particular item are normally wired in parallel (so that any of them can start or switch the particular item on).

• Devices that indicate a stop operation for a particular item are normally wired in series (so that any of them can stop or switch the particular items off).

• Latching operations are used, where a momentary start input signal latches the start signal into the on condition, so that when the start input goes into the OFF condition, the start signal remains energized ON. The latching operation is also referred to as holding or maintaining a sealing contact. See the previous two diagrams for examples of latching.

• Interactive logic: Ladder-logic rungs that appear later in the program often interact with the earlier ladder-logic rungs. This useful feed back mechanism 40 Practical SCADA for Industry can be used to provide feed back on successful completion of a sequence of operations (or protect the overall system due to failure of some aspect).