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page 234

-I/O error -- problem with one of the I/O cards

-etc.

19.3 MEMORY TYPES

• Basic memory types include,

ROM -

RAM -

EPROM -

EEROM -

FLASH RAM -

19.4 SOFTWARE BASED PLCS

• Software based PLCs run on normal personal computers, but they use software that emulates dedicated PLCs.

19.5 PROGRAMMING STANDARDS

19.5.1 IEC 1131

The IEC 1131 standard is an effort towards standardizing PLC oriented control. This is more necessary now that personal computers and software are starting to open the PLC market.

These standards were not designed to force a rigid style, so the different PLC vendors will still have programming environments that vary, but the conceptual elements will be consistent.

The most notable differences between implementation of PLCs will be addresses of outputs, inputs, internal memory, etc.

There are a few components to the standard,

page 235

IEC 1131-3 Data types and programming

IEC 1131-5 Communications

IEC 1131-7 Fuzzy control

• IEC 1131-3 defines the basic programming languages,

IL (Instruction List) - This is effectively mnemonic programming ST (Structured Text) - A BASIC like programming language

LD (Ladder Diagram ) - Relay logic diagram based programming

FBD (Function Block Diagram) - A graphical dataflow programming method SFC (Sequential Function Charts) - A graphical method for structuring programs

Many of the functions described in this note set support the standard.

The standard also defines data types,

Name

Type

Bits

Range

BOOL

boolean

1

0 to 1

SINT

short integer

8

-128 to 127

INT

integer

16

-32768 to 32767

DINT

double integer

32

-2.1e-9 to 2.1e9

LINT

long integer

64

-9.2e19 to 9.2e19

USINT

unsigned short integer

8

0 to 255

UINT

unsigned integer

16

0 to 65536

UDINT

unsigned double integer

32

0 to 4.3e9

ULINT

unsigned long integer

64

0 to 1.8e20

REAL

real numbers

32

 

LREAL

long reals

64

 

TIME

duration

not fixed

not fixed

DATE

date

not fixed

not fixed

TIME_OF_DAY, TOD

time

not fixed

not fixed

DATE_AND_TIME, DT

date and time

not fixed

not fixed

STRING

string

variable

variable

BYTE

8 bits

8

NA

WORD

16 bits

16

NA

DWORD

32 bits

32

NA

LWORD

64 bits

64

NA

19.5.2 The Future of Open Architecture Controllers

• a PLC is not simply a computer. It contains a computer, with special interfacing abilities. But the

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