The L (latch) and U (unlatch) instructions can be used to lock outputs on. When initially energized, the OSR (one-shot relay) instruction will turn on for one scan, but then be off for all scans after, until it is turned off. This type of output is not available on all PLC types. When it is energized, the output will turn off. The circle with a diagonal line through it is a normally on output. The first is a normal output when energized it will turn on and energize an output. Six types of outputs are shown in Figure 7.7(c).
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Some will be externally connected, as may the internal memory locations in the PLC.
![ladder logic program examples light sequence ladder logic program examples light sequence](https://automationcommunity.com/wp-content/uploads/2020/07/3-way-traffic-light-control-ladder-logic.jpg)
In ladder logic, there are multiple types of outputs, but these are not consistently available on all PLCs. Immediate inputs will take current values, but not those from the previous input scan. This allows ladder logic to examine input values more often than once every cycle. The IIT (Immediate Input Terminal) function allows inputs to be read after the input scan, while the ladder logic is being scanned. Normally closed: power flows when the input X is not open. Normally open: an active input X will close the contact and allow power to flow. The first two are normally open and closed inputs, discussed previously.
![ladder logic program examples light sequence ladder logic program examples light sequence](https://ladderlogicworld.com/wp-content/uploads/2018/10/WhatIsLadderLogic-LadderLogicDiagram.png)
Figure 7.7(b) shows the three types of inputs. PLC inputs are easily represented in ladder logic. Relay ladder logic representations: (a) a seal-in circuit (b) ladder logic inputs (c) ladder logic outputs.