Symbol Bit number Examine If Open (XIO) Examine-o 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 I:1/4 1 Instruction interpreted as false 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 I:1/4 0 Instruction interpreted as true Figure 5-21 Examine If Open (XIO) instruction. I:1/4 I:1/4 84 Chapter 5 Basics of PLC Programming Symbol Name XIC Bit status Instruction status 0 FALSE TRUE TRUE FALSE 1 0 1 XIO interpreted as true when there is no physical input present (the bit is 0). • If the Examine-off instruction were associated by address with an internal relay, then the status of the bit would be dependent on the logical status of the internal bit with the same address as the instruction. • Like the Examine-on instruction, the status of the instruction (true or false) determines if the instruction will allow rung continuity through itself, like a closed relay contact. • The memory bit always follows the status (true 5 1 or false 5 0) of the input address or internal address assigned to it. The interpretation of that bit, however, is determined by which instruction is used to examine it. • Examine-on instructions always interpret a 1 status as true and a 0 status as false, while Examine-off instructions interpret a 1 status as false and a 0 status as true, as illustrated in Figure 5-22. The symbol for the Output Energize (OTE) instruction is shown in Figure 5-23. The OTE instruction looks and operates like a relay coil and is associated with a memory bit. This instruction signals the PLC to energize (switch on) or de-energize (switch off ) the output. The processor makes this instruction true (analogous to energizing a coil) when there is a logical path of true XIC and XIO instructions in the rung. The operation of the Output Energize instruction can be summarized as follows: • The status bit of the addressed Output Energize instruction is set to 1 to energize the output and to 0 to de-energize the output. • If a true logic path is established with the input instructions in the rung, the OTE instruction is energized and the output device wired to its terminal is energized. • If a true logic path cannot be established or rung conditions go false, the OTE instruction is de-energized and the output device wired to it is switched off. Status • If the instruction memory bit is a 1 (true) this instruction will allow rung continuity through itself, like a closed relay contact. • If the instruction memory bit is a 0 (false) this instruction will not allow rung continuity through itself and will assume a normally open state just like an open relay contact. The symbol for the Examine If Open (XIO) instruction is shown in Figure 5-21. The XIO instruction, which is also called the Examine-off instruction, looks and operates like a normally closed relay contact. Associated with each XIO instruction is a memory bit linked to the status of an input device or an internal logical condition in a rung. This instruction asks the PLC’s processor to examine if the contact is open. It does this by examining the bit at the memory location specified by the address in the following manner: • As with any other input the memory bit is set to 1 or 0 depending on the status of the input (physical) device or internal (logical) relay address associated with that bit. • A 1 corresponds to a true status or on condition. • A 0 corresponds to a false status or off condition. • When the Examine-off instruction is used to examine a physical input, then the instruction will be interpreted as false when there is a physical input (voltage) present (the bit is 1) and will be Figure 5-22 Figure 5-22 Interpreting Examine-on and Examine-off instructions.
Petruzella_ProgrammableLogicControllers__5e
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