1 -3
Conventional Ladder Diagram
PLC Ladder Diagram
X5
Y3
X6
Y3
X5
Y3
X6
Y3
When we first connect this circuit to the power source, X6 switch is ON but X5 switch is OFF, therefore the relay Y3 is OFF.
The relay output contacts 1and 2 are OFF because they belong to A contact (ON when relay is ON). Motor does not run. If
we press down the switch X5, the relay turns ON as well as contacts 1and 2 are ON and the Motor starts. Once the relay
turns ON, if we release the X5 switch (turns OFF), relay can retain its state with the feedback support from contact 1 and it
is called Latch Circuit. The following table shows the switching process of the example we have discussed above.
X5 switch
(NO)
X6 switch
(NC)
Motor
(
Relay
)
status
Released Released
OFF
Pressed Released
ON
Released Released
ON
Released Pressed
OFF
c
↓
d
↓
e
↓
f
↓
g
Released Released
OFF
From the above table we can see that under different stages of sequence, the results can be different even the input
statuses are the same. For example, let’s take a look at stage
c
and stage
e
, X5 and X6 switches are both released, but
the Motor is ON (running) at stage
e
and is OFF (stopped) at stage
c
. This sequential control with the feedback of the
output to the input is a unique characteristic of Ladder Diagram circuit. Sometimes we call the Ladder Diagram a "Sequential
Control Circuit" and the PLC a "Sequencer”. In this section, we only use the A/B contacts and output coils as the example.
For more details on sequential instructions please refer to chapter 5 - "Introduction to Sequential Instructions."
1.2
Differences Between Conventional and PLC Ladder Diagram
Although the basic operation principle for both conventional and PLC Ladder Diagram are the same, but in reality, PLC uses
the CPU to emulate the conventional Ladder Diagram operations; that is, PLC uses scanning method to monitor the
statuses of input elements and output coils, then uses the Ladder Diagram program to emulate the results which are the
same as the results produced by the conventional Ladder Diagram logic operations. There is only one CPU, so the PLC has
to sequentially examine and execute the program from its first step to the last step, then returns to the first step again and
repeats the operation (cyclic execution). The duration of a single cycle of this operation is called the scan time. The scan
time varies with the program size. If the scan time is too long, then input and output delay will occur. Longer delay time may
cause big problems in controlling fast response systems. At this time, PLCs with short scan time are required. Therefore,
scan time is an important specification for PLCs. Due to the advance in microcomputer and ASIC technologies nowadays
the scan speed has been enhanced a great deal. A typical FB
E
-PLC takes approximately 0.33 ms for IK steps of contact.
The following diagram illustrates the scanning process of a PLC Ladder Diagram.
Содержание FBs-CBE
Страница 78: ...MEMO ...
Страница 111: ...4 6 X0 X1 Node A Y0 Y1 Node B differential down Incerse differential up t Scan time Inverse t t ...
Страница 141: ...Basic Function Instruction 6 16 FUN 2 SKP SKIP START FUN 2 SKP Y2 Y1 Y0 T201 10S X0 X1 X2 0 10 0 ...
Страница 305: ...MEMO ...
Страница 348: ...MEMO ...