Differences Between The Conventional And Plc Ladder Diagram - FATEK FBS Series Manual

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.
↓
↓
↓
↓
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
the Motor is ON (running) at stage
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
The following diagram illustrates the scanning process of a PLC Ladder Diagram.
Conventional Ladder Diagram
X5 X6
Y3
X5 switch
(NO)
Released
Pressed
Released
Released
Released
and is OFF (stopped) at stage
Y3
X6 switch
Motor（Relay）status
(NC)
Released
Released
Released
Pressed
Released
and stage
. This sequential control with the feedback of the
-PLC takes approximately 0.33 ms for IK steps of contact.
E
1 - 3
PLC Ladder Diagram
X5 X6
Y3
OFF
ON
ON
OFF
OFF
, X5 and X6 switches are both released, but
Y3