Bryan L. Programmable controllers. Theory and implementation

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start and kill of the Batch_Pump program are both pulse actions. The status

action (step 10) is a nonstored action used to send a message, perhaps to a

display, to inform the operator of whether the batch is running or not running.

IEC 1131-3

Software System

(“Soft PLC”)

I/O Devices

Figure 10-79. A software PLC interfaced with I/O devices.

In addition to the implementation of the IEC 1131-3 in PLCs, many

manufacturers of software systems provide the IEC 1131-3 standard in

different hardware platforms and operating systems, such as Windows and

Unix. These software systems emulate the operation of a programmable

controller (i.e., they are software PLCs or “soft PLCs”) in the hardware

platform being used (e.g., a PC). They support either a third-party I/O

system or one or more of a PLC manufacturer’s I/O through the use of

built-in drivers that communicate with an I/O rack (see Figure 10-79).

10-5 IEC 1131-3 SOFTWARE SYSTEMS

The Paradym-31 software system from Wizdom Controls, Inc. provides an

IEC 1131-3 graphical programming environment in a Windows-based soft-

ware platform. This system allows the user to employ LD, FBD, or a custom-

built function block language to program the actions in the SFC application.

The user must program custom function blocks in C code. In fact, the

Paradym-31 system compiles the entire IEC 1131 program in an ANSI C

code and then downloads it to a hardware platform or to a third-party

controller and its system.

Another software system, which offers a full implementation of all five IEC

1131-3 languages, is ISaGRAF from TranSys, Inc. and CJ International. This

system provides a thorough set of instructions for all languages and several

SFC-type actions. ISaGRAF also allows the user to test or simulate a PLC

program in a personal computer, making it easier to debug an entire

application or parts of it without actual hardware and I/O connections.

ISaGRAF can run in a variety of operating systems, including OS-9, VRTX,

VXWorks, ControlWare, DOS, and Windows NT. This software package can

also transfer a control program to a programmable controller using a PortPack

tool driver. Table 10-5 lists the ISaGRAF set of instructions for each of the

IEC 1131-3 languages.

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Boolean • NOT: Boolean negation

• AND(&): logical AND

• OR: logical OR

• XOR: logical exclusive-OR

• REDGE: rising-edge detection

• FEDGE: falling-edge detection

Arithmetic • Addition: +

• Subtraction: –

• Multiplication: ×

• Division: /

Comparison • Less than: <

• Greater than: >

• Less than or equal to: <=

• Greater than or equal to: >=

• Equal to: =

• Not equal to: <>

Basic statements • Assignment (:+)

• RETURN statement

• IF-THEN-ELSE structure

• CASE statement

• WHILE iteration statement

• REPEAT iteration statement

• FOR iteration statement

• EXIT statement

Extensions • TSTART–TSTOP: timer control

• SYSTEM function

• OPERATE function

Controls the • GSTART: starts an SFC program

execution of SFC • GKILL: kills an SFC program

child programs • GFREEZE: freezes an SFC program

• GRST: restarts a frozen program

• GSTATUS: gets current status of an SFC program

Acesses the status • GSnnn.x: Boolean value that represents the

of an SFC step activity of the step

• GSnnn.t: time elapsed since last activation of the

step (

nnn

is the reference number of the SFC step)

—(SET)—

—(RST)—

—(RET)—

—(JMP)—

STRUCTURED

TEXT OPERATORS

AND S

TATEMENTS

LADDER

DIAGRAM SYMBOLS

Symbol

Description

Normally open contact (examine-ON)

Normally closed contact (examine-OFF)

Output coil

NOT, or inverted, output coil

Set output coil

Reset output coil

Return—used as the conditional end of a program

or to return from a ladder diagram subprogram

Jump to a label “LAB”

INSTRUCTION

LIST

OPERATORS

LD N Variable, constant Loads operand

ST N Variable Stores current result

S Bool variable Sets to TRUE

R Bool variable Resets to FALSE

AND (&) N( Bool variable Boolean AND

OR N( Bool variable Boolean OR

XOR N( Bool variable Exclusive-OR

ADD ( Variable, constant Addition

SUB ( Variable, constant Subtraction

MUL ( Variable, constant Multiplication

DIV ( Variable, constant Division

GT ( Variable, constant Test:>

GE ( Variable, constant Test:>=

EQ ( Variable, constant Test:=

LE ( Variable, constant Test:<=

LT ( Variable, constant Test:<

JMP C N Label Jump to a label

RET Returns from subprogram

) Delayed operation/execution

Note:

is used for negation (NOT) of the operand.

(

is used to indicate that the operation is to be delayed.

is used to indicate a conditional operation.

Type

Operators

Table 10-5. ISaGRAF instruction set.

Operator

Modifier Operand

Description

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Table 10-5 continued.

FUNCTION

BLOCK DIAGRAM

OPERATORS

AND FUNCTIONS

Standard Operator

Data manipulation • Assignment

• Analog negation

Boolean operations • Boolean AND

• Boolean OR

• Boolean exclusive-OR

Arithmetic operations • Addition

• Subtraction

• Multiplication

• Division

Logic operations • Analog bit-to-bit AND mask

• OR mask

• Exclusive-OR mask

Comparison tests • Less than

• Less than or equal to

• Greater than

• Greater than or equal to

• Equal to

• Not equal to

Data conversion • Convert to Boolean

• Convert to integer analog

• Convert to real analog

• Convert to timer

• Convert to message

Other message concatenations • System access

• Operate I/O channel

Standard Function Bloc

Boolean • SR: set dominant bistable

• RS: reset dominant bistable

• R_Trig: rising-edge detection

• F_Trig: falling-edge detection

• SEMA: semaphore

Counting • CTU: up counter

• CTD: down counter

• CTUD: up/down counter

Timers • TON: ON-delay timing

• TOF: OFF-delay timing

• TP: pulse timing

Integer analogs • CMP: full comparison function block

• StackInt: stack of integer analogs

Real analogs • AVERAGE: running average for

samples

• HYSTER: Boolean hysteresis on difference of reals

• LIM_ALRM: high/low limit alarm with hysteresis

• INTEGRAL: integration over time

• DERIVATE: differentiation according to time

• PID: proportional-integral-derivative control

Signal generation • BLINK: blinking Boolean signal

• SIG_GEN: signal generator

Standar

d Functions

Math

• Absolute value

• Exponent

• Power calculation

• Logarithm

• Square root

• Truncate decimal part

Trigonometric

• Arc cosine

• Arc sine

• Arc tangent

• Cosine

• Sine

• Tangent

• Rotate left

• Rotate right

• Shift left

• Shift right

Data manipulation

• Minimum

• Maximum

• Limit

• Modulo

• Multiplexer (4 or 8 entries)

• Binary selector

• Odd parity

• Random value

String management

• Get string length

• Insert string

• Delete substring

• Find substring

• Replace substring

• Extract left, middle, or part

• Get time of day

Data conversion

• Character→ASCII code

• ASCII code→character

Array manipulation

• Create array of integer values

• Read/write array element

Functions

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PLC LANGUAGES SIMILAR TO THE IEC 1131-3

PLC manufacturers may adapt their programmable controller languages to

embrace some of the qualities of the IEC 1131-3 standard. These qualities

usually reflect the ease of programming found when using sequential function

charts to encapsulate parts of a ladder program into an action. This added

software versatility enhances a programmable controller system tremen-

dously by speeding up program development, minimizing interlocking

sequences, and reducing system troubleshooting time.

For instance, PLC Direct, a PLC manufacturer, offers programmable

controllers with both standard ladder programming language instructions

(RLL—relay ladder logic) and RLL Plus, which is their software language

that incorporates some of the features of sequential function charts. In fact,

the RLL Plus language closely follows the activation of a horizontal flow-

chart. As an example, let’s examine a machine press application. The

sequence chart in Figure 10-80 shows the sequential steps for implementing

the pressing and stamping routine, which can be programmed using either

standard ladder diagrams (see Figure 10-81a) or RLL Plus (see Figure 10-

81b). The highlighted sections of the program in Figure 10-81a indicate

the interlocking requirements for the operation shown in the flowchart.

While both the ladder diagram and the RLL Plus programs implement the

same control and use the same inputs and outputs, the RLL Plus program is

much easier to understand and troubleshoot. For example, if the press system

stops at SG S0003 (stage step 0003) and the coil output does not jump to

SG S0004 (stage step 0004), then the fault must have occurred in either the

Press Down output (Y1) or the Lower Limit input (X4). By investigating

just this area of the PLC program, rather than the whole ladder diagram, the

troubleshooting technician can find the fault more quickly.

The RLL Plus programming language, like sequential function charts, ex-

ecutes each stage’s ladder diagram actions when that stage is active. When the

control program starts, the initial stage (ISG) is activated. Jump instructions,

driven by the ladder diagram contacts that form the transition logic, pass the

token from stage to stage. The last rung in the active stage performs the

transition logic. The RLL Plus software also supports divergences and

convergences, along with the use of timers and counters in the implementa-

tion of transitions. Subroutine implementations are also available through

the use of call instructions in the stage programming. Figure 10-82 presents

the stage (SFC step) instructions typically used with the RLL Plus program-

ming language.

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Figure 10-80. Pressing and stamping routine.

Step 0 Step 1

Step 6

Step 2 Step 3 Step 4 Step 5

(0) The machine is inactive

(1) The operator presses the Start PB to start the machine.

(2) The machine checks for a part. If the part is present, the process

continues. If it is not, the conveyor moves until a part is present.

(3) A clamp locks the part in place.

(4) The press stamps the part.

(5) The clamp is unlocked and the finished piece is moved out of the press.

(6) The process stops if the machine is in one-cycle mode or continues

if it is in automatic mode.

Start PB

Part Present

Part Locked

Part Unlocked

Lower Limit

Upper Limit

Conveyor Indexed

One-Cycle Switch

Clamp

Press

Conveyor

Operation

Inputs Outputs

Note: For this PLC an X denotes an input and a Y denotes an output.

Press Arm

Part

Detection

Sensor

Part

Clamp

Conveyor

Movement

Machine Press

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Figure 10-81. Pressing/stamping routine programmed in (a) ladder diagrams and (b) RLL

Plus. SG denotes a stage step and ISG denotes an initial stage step.

Start

Run 1 Cycle Stop*

Start

C3 X11

Run

Out

Release

Clamp

Part

Unlocked

Clamp

Out

Press

Lower

Limit

Index

Conveyor

Press

Complete

X4 X6

Press

Complete

Out

Release

Clamp

Out

Lower

Limit

Part

Locked

Press

Complete

Press

X4 C1

Press

Out

Run

Part

Present

C0 C3

MLS

Index

Conveyor

Part

Unlocked

Press

Complete

Run

X3 X6

Conveyor

Out

1 Cycle

Index

Conveyor

X7 X5

1 Cycle

Out

Upper

Limit

Press

Complete

C1 X5

Out

Executes all rungs left to right, top to bottom

Only executes logic in stages that are active

ISG

S0000

JMP

Wait for start

Part Present

S0001

JMP

Part Present

JMP

Check for a part

S0002

Clamp

SET

Part Locked

JMP

Lock the clamp

S0004

Clamp

RST

Part Unlocked

Top Limit

JMP

Unlock the clamp

S0003

Press

Down

Lower Limit

JMP

Press the part

S0005

Move

Conveyor

Conveyor Moved

JMP

Index the conveyor

S0006

Automatic

One Cycle

JMP

One cycle or automatic?

(a) (b)

*wired N.C.

(b)

(a)

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Referring to Figure 10-81b, note that the program uses set and reset output

instructions (SG2 and SG4, respectively) to turn ON and OFF the clamp

(output Y0). Just like in an SFC, this is required because standard outputs in

a stage are turned OFF once the control token has been passed to another

stage. In this case, set and reset parameters were used because the clamp

output solenoid needed to be ON from stage 2 through stage 4. Figure 10-

83a shows the equivalent sequential function chart diagram of the program

shown in Figure 10-81b. Figure 10-83b illustrates the flowchart of the

process, which closely resembles the operation of the SFC program.

Figure 10-82. RLL Plus stage instructions.

Initial Stage (ISG

)

The initial stage instruction is used to signal the starting point of the user

application program. The ISG instruction will be active on power up and

PROGRAM to RUN transitions.

(

aaa

= Stage memory location)

Stage (SG)

Stage instructions are used to create structured programs. They are program

segments that can be activated or deactivated with control logic.

(

aaa

= Stage memory location)

Jump (JMP)

The JMP coil deactivates the active stage and activates a specified stage when

there is power flow to the coil.

(

aaa

= Stage memory location)

Not Jump (NJMP)

The NJMP coil deactivates the active stage and activates a specified stage

when there is no power flow to the coil.

(

aaa

= Stage memory location)

Converge Stages (CV)

Converge stages is a group of stages that, when all stages are active, will

activate another stage specified by the associated converge jump(s) (CVJMP).

One scan after the CVJMP is executed, the converge stages will be deactivated.

(

aaa

= Stage memory location)

Converge Jump (CVJMP)

The CVJMP coil deactivates the active CV stages and activates a specified

stage when there is power flow to the coil.

(

aaa

= Stage memory location)

Block Call/Block/Block End (BCALL w/BLK and BEND)

The BCALL coil activates a block of stages when there is power flow to the coil.

BLK is the label that marks the beginning of a block of stages. BEND is the

label used to mark the end of a block of stages.

(

aaa

= C memory location)

ISG

aaa

BLK

aaa

JMP

aaa

NJMP

aaa

CVJMP

aaa

BCALL

BEND

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Figure 10-83. (a) An SFC program for the press/stamp control program and (b) its

corresponding flowchart.

(a)

(b)

Part

Present

Part

Locked

Part_Present

Part_Locked

Lower_Limit

Part_Unlocked

Initial Stage IS00

Wait_for_Start

Stage S01

Check_for_Part

NOT Part_Present

Start

Stage 02

Lock_the_Clamp

Conveyor_Moved

Stage 03

Press_the_Part

Stage S05

Index_the_Conveyor

Stage S06

Check_Mode

One_Cycle_or_Auto

Stage 04

Unlock_the_Clamp

One_Cycle NOT One_Cycle

Begin

Start

SG 01

Check for Part

SG 02

Lock the Clamp

SG 03

Press the Part

Lower

Limit

SG 04

Unlock the Clamp

Part

Unlocked

SG 05

Index (move) Conveyor

Conveyor

Moved?

One

Cycle or

Auto

SG 06

Check Mode

Auto

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EXAMPLE 10-8

Referencing Figures 10-81b and 10-83a, implement an additional

stage that monitors a normally closed stop push button and resets the

completed pressing operation. This stage should be monitored at all

times and, upon activation (i.e., after resetting all outputs), should

return to the initial stage.

OLUTION

The monitoring stage of the Stop PB must be activated as soon as the

Start PB is pressed, which is when the program starts executing

control. Figure 10-84 illustrates the Stop PB monitoring implementa-

tion. Note that stage S500 is ON (set) as soon as Start is pressed in the

initial stage. As the PLC scans the control program during execution,

Start

Only executes logic in stages that are active

ISG

S0000

S500

SET

JMP

Wait for start

Part Present

S0001

JMP

Part Present

JMP

Check for a part

S0002

Clamp

Set

Part Locked

JMP

Lock the clamp

S0500

Y0–Y2

RST

Stop

X10

S0–S6

RST

Monitor for stop

JMP

When Start is pressed, stage

500 is set and program

execution continues in stages.

If the N.C. stop PB is pressed,

outputs Y0–Y2 and stages

S0–S6 are reset. Program

control goes to initial stage.

Figure 10-84. Implementation of a stop-monitoring block.

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it also scans the Stop signal, which if pressed, resets all outputs (Y0

to Y2) and stages (S0 to S6) and then jumps to the initial stage (S0).

Figure 10-85 shows the equivalent SFC level 1 implementation chart.

Note that in the SFC, stage (step) 499 has been included so that a

parallel AND divergence can be implemented and the Stop PB can be

scanned (the actions in steps 4 and 5 do not execute any instructions).

As the NOT Stop transition occurs (NOT Stop because of the normally

closed wiring), the token passes to stage 500 for a one scan reset of

all outputs, then, the token goes back to the initial stage.

Figure 10-85. SFC level 1 implementation of Figure 10-84.

Part_Present

Part_Locked

NOT Stop

Always_True

Lower_Limit

Part_Unlocked

Initial Stage IS00

Wait_For_Part

Stage 501

Check_for_Part

499

500

Stage 500

Reset_Outputs

NOT Part_Present

Start

Stage 502

Lock_the_Clamp

Conveyor_Moved

Stage 503

Press_the_Part

Stage 05

Index_the_Conveyor

Stage 06

Check_Mode

One_Cycle_or_Auto

Stage 499

Wait/Monitor

Stop_Signal

Stage 504

Unlock_the_Clamp

One_Cycle NOT One_Cycle