Practical SCADA for Industry David Bailey - FER-a

Figure 2.20
Ladder-logic start operation (& logic diagram)
SCADA systems, hardware and firmware 39
• 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). See
Figure 2.21 for an example of this.
Figure 2.21
Ladder-logic stop operation (& logic diagram)
• 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