Page 109 - Programmable Logic Controllers, Fifth Edition

pushbuttons are represented by the Examine If Closed
5
4
1 2 3 Discrete inputs 6 7 B3:1/3 same results. You will note that both the NO and the NC
Rung 1 symbol. This is because the normal state of an input (NO
Internal or NC) does not matter to the controller. What does matter
relay is that if contacts need to close to energize the output, then
coil
Discrete inputs the Examine If Closed instruction is used. Since both PB1
B3:1/3 8 9 10 11 12 and PB2 must be closed to energize the pilot light, the
Rung 2 Examine If Closed instruction is used for both.
Internal Discrete A simple program using the Examine If Open (XIO)
relay output instruction is shown in Figure 5-44. Both the hardwired
contact
circuit and user program are shown. In the hardwired cir-
Figure 5-42 Programmed internal relay control. cuit, when the pushbutton is open relay coil CR is de-
energized and its NO contact closes to switch the pilot
5.8 Programming Examine If Closed light on. When the pushbutton is closed, relay coil CR
and Examine If Open Instructions is energized and its NC contact opens to switch the pilot
light off. The pushbutton is represented in the user pro-
A simple program using the Examine If Closed (XIC) gram by an Examine If Open instruction. This is because
instruction is shown in Figure 5-43. This figure shows a
hardwired circuit and a user program that provides the Hardwired circuit User program providing
PB1 the same results
User program providing
Hardwired circuit the same results PB_1
CR PL
PB1
PB2 PB_1 PB_2 PL CR
PL PL

Figure 5-43 Simple program that uses the Examine If Figure 5-44 Simple program that uses the Examine If
Closed (XIC) instruction. Open (XIO) instruction.

The status of the instruction is
If the data table bit XIC XIO OTE
is EXAMINE IF CLOSED EXAMINE IF OPEN OUTPUT ENERGIZE

Logic 0 False True False

Logic 1 True False False

Input instructions Output instruction
XIC XIO OTE

Instruction outcome Input bit status
Time XIC XIO OTE XIC XIO OTE
t (initial) False True False 0 0 0

1
t 2 True True Goes true 1 0 1
t 3 True False Goes false 1 1 0
t 4 False False Remains false 0 1 0

Figure 5-45 Simple program using both the XIC and XIO instructions.

90 Chapter 5 Basics of PLC Programming

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