Page 103 - Programmable Logic Controllers, Fifth Edition

Symbol
Examine If Open (XIO) Symbol Name Bit status Instruction status
Examine-o 0 FALSE
XIC
1 TRUE
Bit
number 15 14 13 12 11 10 98 76 54 32 10 XIO 0 TRUE
1 Status Figure 5-22 FALSE
1
I:1/4 Figure 5-22 Interpreting Examine-on and Examine-off
instructions.
Instruction interpreted
as false interpreted as true when there is no physical input
I:1/4
present (the bit is 0).
• If the Examine-off instruction were associated by
address with an internal relay, then the status of
15 14 13 12 11 10 98 76 54 32 10
0 the bit would be dependent on the logical status
of the internal bit with the same address as the
I:1/4 instruction.
• Like the Examine-on instruction, the status of the
Instruction interpreted instruction (true or false) determines if the instruc-
as true
I:1/4 tion will allow rung continuity through itself, like a
closed relay contact.
Figure 5-21 Examine If Open (XIO) instruction. • 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, how-
• If the instruction memory bit is a 1 (true) this instruc- ever, is determined by which instruction is used to
tion will allow rung continuity through itself, like a examine it.
closed relay contact. • Examine-on instructions always interpret a 1 status
• If the instruction memory bit is a 0 (false) this as true and a 0 status as false, while Examine-off in-
instruction will not allow rung continuity through structions interpret a 1 status as false and a 0 status
itself and will assume a normally open state just like as true, as illustrated in Figure 5-22.
an open relay contact.
The symbol for the Output Energize (OTE) instruc-
The symbol for the Examine If Open (XIO) instruc- tion is shown in Figure 5-23. The OTE instruction
tion is shown in Figure 5-21. The XIO instruction, which looks and operates like a relay coil and is associated
is also called the Examine-off instruction, looks and oper- with a memory bit. This instruction signals the PLC
ates like a normally closed relay contact. Associated with to energize (switch on) or de-energize (switch off )
each XIO instruction is a memory bit linked to the status the output. The processor makes this instruction true
of an input device or an internal logical condition in a (analogous to energizing a coil) when there is a logical
rung. This instruction asks the PLC’s processor to exam- path of true XIC and XIO instructions in the rung. The
ine if the contact is open. It does this by examining the operation of the Output Energize instruction can be
bit at the memory location specified by the address in the summarized as follows:
following manner:
• The status bit of the addressed Output Energize in-
• As with any other input the memory bit is set to 1 struction is set to 1 to energize the output and to 0 to
or 0 depending on the status of the input (physical) de-energize the output.
device or internal (logical) relay address associated • If a true logic path is established with the input
with that bit. instructions in the rung, the OTE instruction is ener-
• A 1 corresponds to a true status or on condition. gized and the output device wired to its terminal is
• A 0 corresponds to a false status or off condition. energized.
• When the Examine-off instruction is used to ex- • If a true logic path cannot be established or
amine a physical input, then the instruction will rung conditions go false, the OTE instruction is
be interpreted as false when there is a physical de-energized and the output device wired to it is
input (voltage) present (the bit is 1) and will be switched off.

84 Chapter 5 Basics of PLC Programming

pet73842_ch05_074-097.indd 84 05/11/15 4:17 PM