Delta DVP-PLC Instructions for use

PLC
PLC

DVP-PLC Application Manual
【Programming】
Table of Contents
Chapter 1 Basic Principles of PLC Ladder Diagram
Foreword: Background and Functions of PLC.......................................................... 1-1
1.1 The Working Principles of Ladder Diagram........................................................ 1-1
1.2 Differences Between Traditional Ladder Diagram and PLC Ladder Diagram........ 1-2
1.3 Edition Explanation of Ladder Diagram ............................................................. 1-3
1.4 How to Edit Ladder Diagram............................................................................. 1-8
1.5 The Conversion of PLC Command and Each Diagram Structure ......................... 1-12
1.6 Simplified Ladder Diagram ............................................................................... 1-15
1.7 Basic Program Designing Examples.................................................................. 1-17
Chapter 2 Functions of Devices in DVP-PLC
2.1 All Devices in DVP-PLC.................................................................................... 2-1
2.2 Values, Constants [K] / [H] ............................................................................... 2-6
2.3 Numbering and Functions of External Input/Output Contacts [X] / [Y].................. 2-8
2.4 Numbering and Functions of Auxiliary Relays [M] .............................................. 2-11
2.5 Numbering and Functions of Step Relays [S]..................................................... 2-11
2.6 Numbering and Functions of Timers [T]............................................................. 2-12
2.7 Numbering and Functions of Counters [C]......................................................... 2-14
2.8 Numbering and Functions of Registers [D], [E], [F]............................................ 2-28
2.8.1 Data register [D] ........................................................................................ 2-28
2.8.2 Index Register [E], [F]................................................................................ 2-29
2.8.3 Functions and Features of File Registers.................................................... 2-30
2.9 Pointer [N], Pointer [P], Interruption Pointer [I].................................................. 2-30
2.10 Special Auxiliary Relays and Special Data Registers........................................ 2-33
2.11 Functions of Special Auxiliary Relays and Special Registers............................. 2-69
2.12 Error Codes................................................................................................... 2-125
Chapter 3 Basic Instructions
3.1 Basic Instructions and Step Ladder Instructions ................................................ 3-1
3.2 Explanations on Basic Instructions ................................................................... 3-3

Chapter 4 Step Ladder Instructions
4.1 Step Ladder Instructions [STL], [RET]............................................................... 4-1
4.2 Sequential Function Chart (SFC) ...................................................................... 4-2
4.3 How does a Step Ladder Instruction Work? ....................................................... 4-3
4.4 Things to Note for Designing a Step Ladder Program......................................... 4-7
4.5 Types of Sequences......................................................................................... 4-9
4.6 IST Instruction................................................................................................. 4-17
Chapter 5 Categories & Use of Application Instructions
5.1 List of Instructions ........................................................................................... 5-1
5.2 Composition of Application Instruction .............................................................. 5-6
5.3 Handling of Numeric Values.............................................................................. 5-11
5.4 E, F Index Register Modification....................................................................... 5-14
5.5 Instruction Index.............................................................................................. 5-16
Chapter 6 Application Instructions API 00-49
●（API00～09）Loop Control .......................................................................... 6-1
●（API10～19）Transmission Comparison....................................................... 6-18
●（API20～29）Four Arithmetic Operation....................................................... 6-32
●（API30～39）Rotation & Displacement......................................................... 6-46
●（API40～49）Data Processing..................................................................... 6-57
Chapter 7 Application Instructions API 50-99
●（API50～59）High Speed Processing........................................................... 7-1
●（API60～69）Handy Instructions.................................................................. 7-39
●（API70～79）Display of External Settings.................................................... 7-59
●（API80～88）Serial I/O ............................................................................... 7-80
Chapter 8 Application Instructions API 100-149
●（API100～109）Communication................................................................... 8-1
●（API110～119）Floating Point Operation...................................................... 8-23
●（API120～129）Floating Point Operation ..................................................... 8-31
●（API130～139）Floating Point Operatio ....................................................... 8-43
●（API140～149）Others................................................................................ 8-55
Chapter 9 Application Instructions API 150-199
●（API150～154）Others................................................................................ 9-1
●（API155～159）Position Control.................................................................. 9-14

●（API160～169）Real Time Calendar ............................................................ 9-39
●（API170～171）Gray Code Conversion........................................................ 9-49
●（API172～175）Floating Point Operation ..................................................... 9-51
●（API180～190）Matrix................................................................................. 9-59
●（API191～199）Positioning Instruction ........................................................ 9-76
Chapter 10 Application Instructions API 215-246
●（API202～203）Others................................................................................ 10-1
●（API215～223）Contact Type Logic Operation Instruction............................. 10-7
●（API224～246）Contact Type Compare Instruction....................................... 10-10

1 Basic Principles of PLC Ladder Diagram
DVP-PLC Application Manual 1-1
Foreword: Background and Functions of PLC
PLC (Programmable Logic Controller) is an electronic device, previously called “sequence controller”. In 1978,
NEMA (National Electrical Manufacture Association) in the United States officially named it as “programmable logic
controller”. PLC reads the status of the external input devices, e.g. keypad, sensor, switch and pulses, and execute by
the microprocessor logic, sequential, timing, counting and arithmetic operations according the status of the input
signals as well as the pre-written program stored in the PLC. The generated output signals are sent to output devices
as the switch of a relay, electromagnetic valve, motor drive, control of a machine or operation of a procedure for the
purpose of machine automation or processing procedure. The peripheral devices (e.g. personal computer/handheld
programming panel) can easily edit or modify the program and monitor the device and conduct on-site program
maintenance and adjustment. The widely used language in designing a PLC program is the ladder diagram.
With the development of the electronic technology and wider applications of PLC in the industry, for example in
position control and the network function of PLC, the input/output signals of PLC include DI (digital input), AI (analog
input), PI (pulse input), NI (numeric input), DO (digital output), AO (analog output), and PO (pulse output). Therefore,
PLC will still stand important in the industrial automation field in the future.
1.1 The Working Principles of Ladder Diagram
The ladder diagram was a diagram language for automation developed in the WWII period, which is the oldest
and most widely adopted language in automation. In the initial stage, there were only A (normally open) contact, B
(normally closed) contact, output coil, timer and counter…the sort of basic devices on the ladder diagram (see the
power panel that is still used today). After the invention of PLC, the devices displayable on the ladder diagram are
added with differential contact, latched coil and the application commands which were not in a traditional power panel,
for example the addition, subtraction, multiplication and division operations.
The working principles of the traditional ladder diagram and PLC ladder diagram are basically the same. The
only difference is that the symbols on the traditional ladder diagram are more similar to its original form, and PLC
ladder diagram adopts the symbols that are easy to recognize and shown on computer or data sheets. In terms of the
logic of the ladder diagram, there are combination logic and sequential logic.
1. Combination Logic
Examples of traditional ladder diagram and PLC ladder diagram for combination logic:
Traditional Ladder Diagram PLC Ladder Diagram
X4
X0
X2
X3
X1
Y0
Y2
Y1
X0
Y0
X1
Y1
Y2
X2
X3
X4
Row 1: Using a normally open (NO) switch X0 (“A” switch or “A" contact). When X0 is not pressed, the contact
will be open loop (Off), so Y0 will be Off. When X0 is pressed, the contact will be On, so Y0 will be On.

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Row 2: Using a normally closed (NC) switch X1 (“B” switch or “B” contact). When X1 is not pressed, the contact
will be On, so Y1 will be On. When X1 is pressed, the contact will be open loop (Off), so Y1 will be Off.
Row 3: The combination logic of more than one input devices. Output Y2 will be On when X2 is not pressed or
X3 and X4 are pressed.
2. Sequential Logic
Sequential logic is a circuit with "draw back” structure, i.e. the output result of the circuit will be drawn back as an
input criterion. Therefore, under the same input criteria, different previous status or action sequence will follow by
different output results.
Examples of traditional ladder diagram and PLC ladder diagram for sequential logic:
Traditional Ladder Diagram PLC Ladder Diagram
Y3
X5
Y3
X6
Y3
X5
Y3
X6
When the circuit is first connected to the power, though X6 is On, X5 is Off, so Y3 will be Off. After X5 is pressed,
Y3 will be On. Once Y3 is On, even X5 is released (Off), Y3 can still keep its action because of the draw back (i.e.
the self-retained circuit). The actions are illustrated in the table below.
Devicestatus
Action sequence X5 X6 Y3
1 No action No action Off
2 Action No action On
3 No action No action On
4 No action Action Off
5 No action No action Off
From the table above, we can see that in different sequence, the same input status can result in different output
results. For example, switch X5 and X6 of action sequence 1 and 3 do not act, but Y3 is Off in sequence 1 and
On in sequence 3. Y3 output status will then be drawn back as input (the so-called “draw back”), making the
circuit being able to perform sequential control, which is the main feature of the ladder diagram circuit. Here we
only explain contact A, contact B and the output coil. Other devices are applicable to the same method. See
Chapter 3 “Basic instructions” for more details.
1.2 Differences Between Traditional Ladder Diagram and PLC Ladder Diagram
Though the principles of traditional ladder diagram and PLC ladder diagram are the same, in fact, PLC adopts
microcomputer to simulate the motions of the traditional ladder diagram, i.e. scan-check status of all the input devices
and output coil and calculate to generate the same output results as those from the traditional ladder diagram based
on the logics of the ladder diagram. Due to that there is only one microcomputer, we can only check the program of
the ladder diagram one by one and calculate the output results according to the program and the I/O status before the
cyclic process of sending the results to the output interface Ære-reading of the input status Æcalculation Æoutput.
The time spent in the cyclic process is called the “scan time” and the time can be longer with the expansion of the
program. The scan time can cause delay from the input detection to output response of the PLC. The longer the delay,

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DVP-PLC Application Manual 1-3
the bigger the error is to the control. The control may even be out of control. In this case, you have to choose a PLC
with faster scan speed. Therefore, the scan speed is an important specification requirement in a PLC. Owing to the
advancement in microcomputer and ASIC (IC for special purpose), there has been great improvement in the scan
speed of PLC nowadays. See the figure below for the scan of the PLC ladder diagram program.
The output result is calculated
based on the ladder diagram.
(The result has not yet sent to the
external output point, but the
internal device will perform an
immediate output.)
Y0
X0 X1
Y0
Start
M100 X3
Y1
X10
:
:
X100 M505
Y126
End
Send the result to the output point
Read input status from outside
Executing in cycles
Besides the difference in the scan time, PLC ladder and traditional ladder diagram also differ in “reverse current”.
For example, in the traditional ladder diagram illustrated below, when X0, X1, X4 and X6 are On and others are Off,
Y0 output on the circuit will be On as the dotted line goes. However, the PLC ladder diagram program is scanned from
up to down and left to right. Under the same input circumstances, the PLC ladder diagram editing tool WPLSoft will be
able to detect the errors occurring in the ladder diagram.
Reverse current of traditional ladder diagram
X6
X0 X1 X2
X3 X4 X5
ab
Y0
Reverse current of PLC ladder diagram
X6
X0
Y0
X1 X2 Y0
X3 X4 X5
ab
Error detected in the third row
1.3 How to Edit Ladder Diagram
Ladder diagram is a diagram language frequently applied in automation. The ladder diagram is composed of the
symbols of electric control circuit. The completion of the ladder diagram by the ladder diagram editor is the completion
of the PLC program design. The control flow illustrated by diagram makes the flow more straightforward and
acceptable for the technicians of who are familiar with the electric control circuit. Many basic symbols and actions in

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the ladder diagram come from the frequently-seen electromechanical devices, e.g. buttons, switches, relay, timer and
counter, etc. in the traditional power panel for automation control.
Internal devices in the PLC: The types and quantity of the devices in the PLC vary in different brand names.
Though the internal devices in the PLC adopts the names, e.g. transistor, coil, contact and so on, in the traditional
electric control circuit, these physical devices do not actually exist inside the PLC. There are only the corresponding
basic units (1 bit) inside the memory of the PLC. When the bit is “1”, the coil will be On, and when the bit is “0”, the coil
will be Off. The normally open contact (NO or contact A) directly reads the value of the corresponding bit. The
normally close contact (NC or contact B) reads the opposite state of the value of the corresponding bit. Many relays
will occupy many bits. 8 bits equal a “byte”. 2 bytes construct a “word” and 2 words combined is “double word”. Byte,
word or double words are used when many relays are processed (e.g. addition/subtraction, displacement) at the
same time. The other two devices, timer and counter, in the PLC have coil, timer value and counter value and they
have to process some values in byte, word or double word.
All kinds of internal devices in the value storage area in the PLC occupy their fixed amount of storage units.
When you use these devices, you are actually read the contents stored in the form of bit, byte or word.
Introductions on the basic internal devices in the PLC (See Ch 2. Functions of Devices in DVP-PLC for more details.)
Device Functions
Input relay
The input relay is an internal memory (storage) unit in the PLC corresponding to a external
input point and is used for connecting to the external input switches and receiving external
input signals. The input relay will be driven by the external input signals which make it “0” or
“1". Program designing cannot modify the status of the relay, i.e. it cannot re-write the basic
unit of a relay, nor can it force On/Off of the relay by HPP/WPLSoft. SA/SX/SC/EH/EH2/SV
series MPU can simulate input relay X and force On/Off of the relay. But the status of the
external input points will be updated and disabled, i.e. the external input signals will not be read
into their corresponding memories inside PLC, but only the input points on the MPU. The input
points on the extension modules will still operate normally. There are no limitations on the times
of using contact A and contact B of the input relay. The input relays without corresponding input
signals can only be left unused and cannot be used for other purposes.
&Device indication: X0, X1,…X7, X10, X11,… are indicated as X and numbered in octal
form. The No. of input points are marked on MPU and extension modules.
Output relay
The output relay is an internal memory (storage) unit in the PLC corresponding to a external
output point and is used for connecting to the external load. The output relay will be driven by
the contact of an input relay, contacts of other internal devices and the contacts on itself. A
normally open contact of the output relay is connected to the external load. Same as the input
contacts, there are no limitations on the times of using other contacts of the output relay. The
output relay without corresponding output signals can only be left unused and can be used as
input relay if necessary.
&Device indication: Y0, Y1,…Y7, Y10, Y11,…are indicated as Y and numbered in octal
form. The No. of output points are marked on MPU and extension modules.

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DVP-PLC Application Manual 1-5
Internal relay
The internal relay does not have connection with the external. It is an auxiliary relay inside the
PLC with the functions same as those of the auxiliary (middle) relay in the electric control
circuit. Every internal relay corresponds to a basic internal storage unit and can be driven by
the contacts of the input relay, contacts of the output relay and the contacts of other internal
devices. There are no limitations on the times of using the contacts of the internal relay and
there will be no output from the internal relay, but from the output point.
&Device indication: M0, M1,…, M4095 are indicated as M and numbered in decimal form.
Step
DVP series PLC offers a step-type control program input method. STL instruction controls the
transfer of step S, which makes it easy for the writing of the control program. If you do not use
any step program in the control program, step S can be used as a internal relay M as well as an
alarm point.
&Device indication: S0, S1,…S1023 are indicated as S and numbered in decimal form.
Timer
The timer is used for timing and has coil, contact and register in it. When the coil is On and the
estimated time is reached, its contact will be enabled (contact A closed, contact B open). Every
timer has its fixed timing period (unit: 1ms/10ms/100ms). Once the coil is Off, the contact iwlwl
be disabled (contact A open, contact B closed) and the present value on the timer will become
“0”.
&Device indication: T0, T1,…,T255 are indicated as T and numbered in decimal form.
Different No. refers to different timing period.
Counter
The counter is used for counting. Before using the counter, you have to give the counter a set
value (i.e. the number of pulses for counting). There are coil, contact and registers in the
counter. When the coil goes from Off to On, the counter will regard it as an input of 1 pulse and
the present value on the counter will plus “1”. We offer 16-bit and 32-bit high-speed counters
for our users.
&Device indication: C0, C1,…,C255 are indicated as C and numbered in decimal form.
Data register
Data processing and value operations always occur when the PLC conducts all kinds of
sequential control, timing and counting. The data register is used for storing the values or all
kinds of parameters. Every register is able to store a word (16-bit binary value). Double words
will occupy 2 adjacent data registers.
&Device indication: D0, D1,…,D9,999 are indicated as D and numbered in decimal form.

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File register
The file register is used for storing the data or all kinds of parameters when the data registers
required for processing the data and value operations are insufficient. Every file register is able
to store a 16-bit word. Double words will occupy 2 adjacent file registers. In SA/SX/SC series
MPU, there are 1,600 file registers. In EH/EH2/SV series MPU, there are 10,000 file registers.
There is not an actual device No. for a file register. The reading and writing of file registers
should be executed by instructions API 148 MEMR, API 149 MEMW, or through the peripheral
device HPP02 and WPLSoft.
&Device indication: K0 ~ K9,999, numbered in decimal form.
Index register
E and F index registers are 16-bit data registers as other data registers. They can be read and
written and can be used in word devices, bit devices or as a constant for index indication.
&Device indication: E0 ~ E7, F0 ~ F7 are indicated as E and F and numbered in decimal
form.
The structure of a ladder diagram:
Structure Explanation Instruction Devices Used
Normally open, contact A LD X, Y, M, S, T, C
Normally closed, contact B LDI X, Y, M, S, T, C
Normally open in series
connection AND X, Y, M, S, T, C
Normally closed in series
connection ANI X, Y, M, S, T, C
Normally open in parallel
connection OR X, Y, M, S, T, C
Normally closed in parallel
connection ORI X, Y, M, S, T, C
Rising-edge trigger switch LDP X, Y, M, S, T, C
Falling-edge trigger switch LDF X, Y, M, S, T, C
Rising-edge trigger in series
connection ANDP X, Y, M, S, T, C
Falling-edge trigger in series
connection ANDF X, Y, M, S, T, C
Rising-edge trigger in parallel
connection ORP X, Y, M, S, T, C
Falling-edge trigger in parallel
connection ORF X, Y, M, S, T, C
Block in series connection ANB -
Block in parallel connection ORB -

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DVP-PLC Application Manual 1-7
Structure Explanation Instruction Devices Used
Multiple output
MPS
MRD
MPP
-
Coil driven output instruction OUT Y, M, S
SS
Step ladder STL S
Basic instruction
Application instruction
Application
instructions
See Ch.3 for basic instructions
(RST/SET and CNT/TMR) and Ch.5 ~
10 for application instructions
Inverse logic INV -
Block:
A block is a series or parallel operation composed of more than 2 devices. There are series block and parallel block.
Series block
Parallel block
Separation line and combination line:
The vertical line is used for separating the devices. For the devices on the left, the vertical line is a combination line,
indicating that there are at least 2 rows of circuits on the left connected with the vertical line. For the devices on the
right, the vertical line is a separation line, indicating that there are at least 2 rows of circuits interconnected on the right
side of the vertical line).
12
Combination line for block 1
Separation line for block 2
Combination line for block 2
Network:
A complete block network is composed of devices and all kinds of blocks. The blocks or devices connectable by a
vertical line or continuous line belong to the same network.

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An independent network
Network 1
Network 2
An incomplete network
1.4 How to Edit a PLC Ladder Diagram
The editing of the program should start from the left power line and ends at the right power line, a row after
another. The drawing of the right power line will be omitted if edited from WPLSoft. A row can have maximum 11
contacts on it. If 11 is not enough, you can continuously connect more devices and the continuous number will be
generated automatically. The same input points can be used repeatedly. See the figure below:
X0 X1 X2 X3 X4 X5
Y0
X11 X12 X13
X6 X7 X10 C0 C1
00000
00000
Continuous number
The operation of the ladder diagram program is scanning from top left to bottom right. The coil and the operation
frame of the application instruction belong to the output side in the program and are placed in the right if the ladder
diagram. Take the figure below for example, we will step by step explain the process of a ladder diagram. The
numbers in the black circles indicate the order.
X0 X1 Y1 X4
M0
X3 M1
T0 M3
Y1
TMR T0 K10
The order of the instructions:
1 LD X0
2 OR M0
3 AND X1
4 LD X3
AND M1

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DVP-PLC Application Manual 1-9
ORB
5 LD Y1
AND X4
6 LD T0
AND M3
ORB
7 ANB
8 OUT Y1
TMR T0 K10
Explanations on the basic structures in the ladder diagram:
1. LD (LDI) instruction: Given in the start of a block.
AND block OR block
LD instruction LD instruction
The structure of LDP and LDF instructions are the same as that of LD instruction, and the two only differ in their
actions. LDP and LDF instructions only act at the rising edge or falling edge when the contact is On, as shown in the
figure below.
X0
OFF ON OFF
Time
Falling edge
X0
OFF ON OFF
Time
Rising edge
2. AND (ANI) instruction: A single device connects to another single device or a block in series
AND instructio
n
AND instruction
The structure of ANDP and ANDF instructions are the same. ANDP and ANDF instructions only act at the rising
edge or falling edge.
3. OR (ORI) instruction: A single device connects to another single device or a block
OR instruction OR instruction OR instruction
The structure of ORP and ORF instructions are the same. ORP and ORF instructions only act at the rising edge
or falling edge.

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4. ANB instruction: A block connects to a device or another block in series
ANB instruction
5. ORB instruction: A block connects to a device or another block in parallel
ORB instruction
If the ANB and ORB operations are with several blocks, the operation should be performed from up to down or
left to right, combining into a block or network.
6. MPS, MRD, MPP instructions: Bifurcation point of multiple outputs, for generating many and diverse outputs.
MPS instruction is the start of the bifurcation point. The bifurcation point is the intersection of the horizontal line
and vertical line. We will have to determine whether to give a contact memory instruction by the contact status of the
same vertical line. Basically, every contact can be given a memory instruction, but considering the convenience of
operating the PLC and the limitation on its capacity, some parts in the ladder diagram will be omitted during the
conversion. We can determine the type of contact memory instruction by the structure of the ladder diagram. MPS is
recognized as “┬” and the instruction can be given continuously for 8 times.
MRD instruction is used for reading the memory of the bifurcation point. Due to that the same vertical line is of
the same logic status, in order to continue analyzing other ladder diagrams, we have to read the status of the original
contact again. MRD is recognized as “├”.
MPP instruction is used for reading the start status of the top bifurcation point and popping it out from the stack.
Since MPP is the last item on the vertical line, the vertical line ends at this point.
MPP is recognized as “└”. Using the method
given above for the analysis cannot be wrong.
However, sometimes the compiling program will ignore
the same output status, as shown in the figure.
MPS
MRD
MPP
MPP
MPS

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7. STL instruction: Used for designing the syntax of the sequential function chart (SFC).
STL instruction allows the program designer a clearer and readable picture of the sequence of the program as
when they draw a sequence chart. From the figure below, we can see clearly the sequence to be planned. When the
step S moves to the next step, the original S will be “Off". Such a sequence can then be converted into a PLC ladder
diagram and called “step ladder diagram”.
M1002
S0
SET S0
S0
SET S21
SET S22
S
S21
S
RET
S22
S
M1002
8. RET instruction: Placed after the completed step ladder diagram.
RET also has be placed after STL instruction. See the example below.
RET
S20
S
RET
S20
S
X1
X1
See step ladder instructions [STL], [RET] in Ch. 4 for the structure of the ladder diagram.

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1-12
1.5 The Conversion of PLC Command and Each Diagram Structure
Ladder Diagram
X0 X2 X1
X1
M1
C0
Y0
SET S0
M2 Y0
M0
X10
Y10
SET S10
S0
S
X11
Y11
SET S11
S10
S
SET S12
SET S13
X12
Y12
SET S20
S11
S
X13
S0
RET
S20
S
S12
S
S13
S
X0
CNT C0 K10
X1
M0
C0
X1
M2
RST C0
M1
M2
END
LD X0
OR X1
LD X2
OR M0
ORI M1
ANB
LD M2
AND Y0
ORB
AN I X1
OUT Y0
AND C0
SET S0
STL S0
LD X10
OUT Y10
SET S10
STL S10
LD X11
OUT Y11
SET S11
SET S12
SET S13
STL S11
LD X12
OUT Y12
SET S20
STL S20
STL S12
STL S13
LD X13
OUT S0
RET
LD X0
CNT C0 K10
LD C0
MPS
AND X1
OUT M0
MRD
AN I X1
OUT M1
MPP
AN I M2
OUT M2
END
OR
block
ANI
Multiple
outputs
RST C0
OR
block
Series
connection blcok
AND
block
Parallel
connection block
The output will continue
following the status of
Step ladder Start
Status working item and
step point transfer
Withdraw S10 status
Withdraw X11 status
Status working item and
step point transfer
Withdraw S11 status
Withdraw X12 status
Status working item and
step point transfer
Bifurcation
convergence
End of step ladder
Status working item
and step point transfer
Return
Read C0
Multiple
outputs
End of program
Status S0 and X10 operation
Fuzzy Syntax
The correct ladder diagram analysis and combination should be conducted from up to down and left to right.
However, without adopting this principle, some instructions can make the same ladder diagram.

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Example Program 1
See the ladder diagram below. There are 2 ways to indicate the ladder by instruction programs with the same result.
Ideal way Less ideal way
LD X0 LD X0
OR X1 OR X1
LD X2 LD X2
OR X3 OR X3
ANB LD X4
LD X4 OR X5
OR X5 ANB
X0 X2 X4
X5X3X1
ANB ANB
The two instruction programs will be converted into the same ladder diagram. The difference between the ideal
one and less ideal one is the operation done by the MPU. For the ideal way, the combination is done block by block
whereas the less idea way combines all the blocks combine with one another in the last step. Though the length of
the program codes of the two ways are equal, the combination done in the last step (by ANB instruction, but ANB
cannot be used continuously for more than 8 times) will have to store up the previous calculation results in advance.
In our case, there are only two blocks combined and the MPU allows such kind of combination. However, once the
number of blocks exceed the range that the MPU allows, problems will occur. Therefore, the best way is to execute
the block combination instruction after a block is made, which will also make the logic sequence planned by the
programmer more in order.
Example Program 2
See the ladder diagram below. There are 2 ways to indicate the ladder by instruction programs with the same result.
Ideal way Less ideal way
LD X0 LD X0
OR X1 LD X1
OR X2 LD X2
OR X3 LD X3
ORB
ORB
X0
X1
X2
X3
ORB
In this example, the program codes and the operation memory in the MPU increase in the less ideal way.
Therefore, it is better that you edit the program following the defined sequence.
Incorrect Ladder Diagram
PLC processes the diagram program from up to down and left to right. Though we can use all kinds of ladder
symbols to combine into various ladder diagrams, when we draw a ladder diagram, we will have to start the diagram
from the left power line and end it at the right power line (In WPLSoft ladder diagram editing area, the right power line
is omitted), from left to right horizontally, one row after another from up to down. See bellows for the frequently seen
incorrect diagrams:

1Basic Principles of PLC Ladder Diagram
DVP-PLC Application Manual
1-14
OR operation upward is not allowed.
Rever se flow
“Reverse flow” exists in the signal circuit from the
beginning of input to output.
The up-right corner should output first.
Combining or editing should be done from the
up-left to the bottom-right. The dotted-lined area
should be moved up.
Parallel operation with empty device is not allowed.
Empty device cannot do operations with other
devices.
No device in the middle block.
Devices and blocks in series should be horizontally
aligned.
Label P0 should be in the first row of a complete
network.
Blocks connected in series should be aligned with
the upmost horizontal line.

1 Basic Principles of PLC Ladder Diagram
DVP-PLC Application Manual 1-15
1.6 Simplified Ladder Diagram
When a series block is connected to a parallel block in series, place the block in the front to omit ANB instruction.
Ladder diagram complied into instruction
LD X0
LD X1
OR X2
X0 X1
X2
ØANB
Ladder diagram complied into instruction
LD X1
OR X2
X0X1
X2
AND X0
When a single device is connected to a block in parallel, place the block on top to omit ORB instruction.
Ladder diagram complied into instruction
LD T0
LD X1
AND X2
T0
X1 X2
ØORB
Ladder diagram complied into instruction
LD X1
AND X2
T0
X1 X2
OR T0
In diagram (a), the block on top is shorter than the block in the bottom, we can switch the position of the two
blocks to achieve the same logic. Due to that diagram (a) is illegal, there is a “reverse flow” in it.
Ladder diagram complied into instruction
LD X0
OR X1
AND X2
LD X3
AND X4
X0
X1 X2
X3 X4
(a)
ØORB
Ladder diagram complied into instruction
LD X3
AND X4
LD X1
OR X0
AND X2
X0
X1 X2
X3 X4
(b)
ORB

1Basic Principles of PLC Ladder Diagram
DVP-PLC Application Manual
1-16
MPS and MPP instruction can be omitted when the multiple outputs in the same horizontal line do not need to
operate with other input devices.
Ladder diagram complied into instruction
MPS
AND X0
OUT Y1
MPP
X0
Y1
Y0
Ø
OUT Y0
Ladder diagram complied into instruction
OUT Y0
AND X0
Y0
Y1
X0
OUT Y1
Correct the circuit of reverse flow
In the following two examples, the diagram in the left hand side is the ladder diagram we desire. However, the illegal
“reverse flow” in it is incorrect according to our definition on the ladder diagram. We modify the diagram into the
diagram in the right hand side.
Example 1
X0
X3
X6
X1
X4
X7
X2
X5
X10 LOOP1
reverse flo
w
Ö
X0 X1 X2
X3 X4 X5
X10
X6 X7 X5
X10 LOOP1
Example 2
X0
X3
X6
X1
X4
X7
X2
X5
X10 LO OP 1
re ver se fl ow
X0
X3
X6
X1
X4
X7
X2
X5
X10
LOOP2
Reverse fl ow
Ö
LOOP1
X0 X1 X2
X3 X4 X5
X6
X3 X7 X10
X6
X0 X1 X7 X10
LOOP 2
X4