Introduction to Industrial Automation

282

 

◾

  Introduction to Industrial Automation

Ta

bl

e 7

.1

 

Pro

gr

am

m

in

g I

ns

tr

uc

ti

on

s i

n T

hr

ee B

as

ic L

an

gu

ag

es

Languag

e

Oper

and

Inst

ruc

tion Oper

at

ion

Boole

FBD

LAD

A

A

I, Q

, M, T

, C

Look f

or 1 

and

 per

form AND with previous RL

O

Boolean L

ogic 

Instructions

AN

A

I, Q

, M, T

, C

Look f

or 0 

and

 per

form AND with previous RL

O

O

O

I, Q

, M, T

, C

Look f

or 1 

and

 per

form OR with previous RL

O

ON

O

I, Q

, M, T

, C

Look f

or 0 

and

 per

form OR with previous RL

O

A(

AND logic of a comple

x (brack

eted) e

xpression

O(

OR logic of a comple

x (brack

eted) e

xpression

)

Termination of brack

eted e

xpression

=

Q,

 M

Assign v

alue 1 to the operand if RL

O = 1

Assign v

alue 0 to the operand if RL

O = 0

Activ

ation 

Instructions

S

S

S

Q

, M, C

Set, i.e., assign v

alue 1 to the operand, if RL

O = 1

Do nothing if RL

O = 0 

 

(Latch/Unlatch)

1

SX

T

T

T

Activ

ate timer T

 of kind X at positiv

e g

oing edg

e of RL

O

.

R

R

R

Q

, M, T

, C

Reset, i.e., assign v

alue 0 to the operand, if RL

O = 1

Do nothing if RL

O = 0

L

IB, IW

, QB, QW

, 

MB, MW

, 

Constant

Load the content of the operand or the v

alue of 

constant 

in 

the 

accumulator

, independentl

y 

of 

RL

O 

            

            

            

       

(Get/Put)

1

Complemental 

Instructions

(

C

ont

inued

)

---

Basic Programming Principles of PLCs 

◾

 

283

Ta

bl

e 7

.1 (

C

on

ti

nu

ed

) 

Pro

gr

am

m

in

g i

ns

tr

uc

ti

on

s i

n t

hr

ee b

as

ic la

ng

ua

ge

s

Languag

e

Oper

and

Inst

ruc

tion Oper

at

ion

Boole

FBD

LAD

T

IB, IW

, QB, QW

, 

MB, MW

Transf

er the content of the accumulator to the operand 

independentl

y of RL

O

Complemental 

instructions

CU

C

xx

CU

C

xx

CU

C

Increase the content of the counter C

xx

 b

y 1 on positiv

e 

going edg

e of RL

O

CD

C

xx

CD

C

xx

CD

C

Decrease the content of the counter C

xx

 b

y 1 on 

positiv

e g

oing edg

e of RL

O

≥, ≤, ≠

CMP ≥1

IN

1

IN

2

CMP ≥1

IN

1

IN

2

Accumulator

Per

form comparison betw

een the contents of tw

o 

accumulators

JU

Label

JM

P

(JMP)

Label

Label

Jump to another program instruction (instead of the 

ne

xt instruction) unconditionall

y

JC

Label

JM

P

(JMP)

Label

Label

Jump to another program instruction (instead of the 

ne

xt instruction) if RL

O = 1

CALL

DB

YY

FB

XX

DB

YY

FB

XX

FB, FC

Call a F

unction Block or a F

unction

Notes:

(1) Alternativ

e terms instead of Set/Reset and L

oad/T

ransf

er respectiv

el

y

IB

 

=

 

Input By

te

 

CU

 

=

 

Counter Up

 

FB

 

=

 

Function Block

IW

 

=

 

Input W

ord

 

CD

 

=

 

Counter Down

 

FC

 

=

 

Function

QB

 

=

 

Output By

te

 

JMP

 

=

 

Jump

 

DB

 

=

 

Data Block (Data F

ile)

QW

 =

 

Output W

ord

 

JU

 

=

 

Jump Uncondionall

y 

RL

O

 =

 

Result of L

ogic Operation

MB

 

=

 

Memor

y Bit By

te

 

JC

 

=

 

Junp Conditionall

y

MW

 =

 

Memor

y Bit W

ord

 

CMP

 

=

 

Compare

---

284

 

◾

  Introduction to Industrial Automation

language. In the column of variables, the following notations are depicted with the corresponding 

explanations:

I 

= Input

Q  = Output

*

M 

=  Auxiliary memory bit (logic coil)

T  = Timer

C  = Counter

FB  =  POU function block

FC  =  POU function

IB 

=  Input byte

IW  =  Input word

†

QB  =  Output byte

QW  =  Output word

†

MB  =  Auxiliary memory byte

MW  =  Auxiliary memory word

†

Label  =  Alphanumeric label

‡

Each PLC contains at least two accumulators with widths of two digital words each, starting 

from the least significant byte on the right to the most significant byte on the left, as shown in Figure 

7.8. The two words and the four bytes are marked with the initials L or H, representing “low” or 

“high” significance. The contents of the register 1 are modified by using the load instruction (abbre-

viation L). In the register, a byte, a word, or a double word can be loaded after the initial shift of 

the old content of register 1 to register 2, and the reset of register 1. Obviously, loading a numeric 

constant to the register creates an equivalent digital content of the constant in binary form, unless 

otherwise specified by the loading instruction (e.g., BCD code). The transfer instruction (abbrevia-

tion T) always transfers the contents of register 1 to where it is specified with the instruction.

In the column named “instruction operation”, the action performed by the CPU is summa-

rized when executing each instruction respectively. But before proceeding with the actions of the 

various programming instructions, it is necessary first to introduce and explain the term “result of 

instruction operation”.

7.4.1   The Result of an Instruction Execution

The term “result of a logic operation” (RLO) is characterized by the logical result created in the 

CPU after an instruction execution. Every time that an instruction is executed, a new RLO is cre-

ated that depends on the type of instruction and on the previous RLO, while the last one executed 

is erased afterwards. The concept of the RLO will be further explained with the help of the classic 

automation circuit presented in Figure 7.9. The activation of the relay C is dependent on the status 

of the three switching contacts S

1

, S

2

, and S

3

. Initially, it is assumed that the relay is not energized, 

and the status of the circuit branch needs to be examined. In this case, it is assumed further that 

the status of the three switching contacts (their status is not known in advance) when testing the 

*

 

The normal letter for the output notation is “O”, but since OR instruction in Boolean language uses the same 

letter, “Q” has been adopted in order to avoid any confusion.

† 

There are additional variables for double word (DW) with similar notation and handling that are omitted.

‡ 

An alphanumeric word called “label” is written in front of an instruction to define the point of program transi-

tion after a jump instruction execution.

---

Basic Programming Principles of PLCs 

◾

 

285

circuit is exactly as indicated in Figure 7.9. It is initially examined with a simple voltmeter if there 

is any electrical voltage at the power supply. If voltage exists, the next step is to examine if there is 

voltage just after the contact S

1

. Since S

1

 is a closed contact, the voltmeter will show us the trend. 

Continuing the examination of this circuit branch, the next point to be checked is the one imme-

diately below the S

2

 contact, as shown in Figure 7.9. Since the S

2

 contact is open, the voltmeter will 

not show a voltage, and therefore it explains why the relay C is not energized. However, in the wired 

branch, the presence or absence of an electrical voltage, as well as the flow of the electric current, 

occurs naturally by itself and automatically as a consequence of the existence of a potential differ-

ence. In programmable logic, where the switching contacts are replaced by logical instructions, the 

role of the electrical voltage is carried out by the RLO. In the classic circuit, the relay C is activated 

if there is voltage at point A, i.e., in its ends as depicted in Figure 7.9. In programmable logic, the 

RLO is what determines—as a voltage—the result of the activating instructions. In this case, the 

output C (and hence the C relay connected to it) is activated only when RLO equals 1.

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

HH Byte

HL Byte

LH Byte

LL Byte

H Word

L Word

ZB 20

ZB 20

ZB 21

ZB 20

ZB 21

ZB 22

ZB 23

Accumulator 1

Load ΖΒ 20  

Load ΖW 20   

Load ΖDW 20   

= ΖW 20

= ΖDW 20

Where Ζ = I or Q or Μ, that is a Byte or Word of Inputs or Outputs or Memory Bits

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

ZB 20

ZB 20

ZB 21

ZB 20

ZB 21

ZB 22

ZB 23