Open topic with navigation

Constant Expressions

Constant expressions can be used in all languages for assigning a variable with a value. All constant expressions have a well-defined data type according to their semantics. If you program an operation between variables and constant expressions having inconsistent data types, it leads to syntactic errors when the program is compiled.

Below is the list of prefixes according to possible data types:

Type Prefix Description
BOOL   There are only two possible Boolean constant expressions. They are reserved keywords TRUE and FALSE.
SINT SINT# Small integer constant expressions are valid integer values (between -128 and 127). All integer expressions having no prefix are considered as DINT integers
USINT/BYTE USINT# Unsigned small integer constant expressions are valid integer values (between 0 and 255). All integer expressions having no prefix are considered as DINT integers.
INT INT# 16-bit integer constant expressions are valid integer values (between -32768 and 32767). All integer expressions having no prefix are considered as DINT integers.
UINT/WORD UINT# Unsigned 16-bit integer constant expressions are valid integer values (between 0 and +65535). All integer expressions having no prefix are considered as DINT integers.
DINT  

32-bit integer constant expressions must be valid numbers between -2147483648 to +2147483647. DINT is the default size for integers: such constant expressions do not require a prefix.

You can use 2#,8# or 16# prefixes to specify an integer in binary, octal or hexadecimal basis respectively.
UDINT/DWORD UDINT# Unsigned 32-bit integer constant expressions are valid integer values (between 0 and 4294967295). All integer expressions having no prefix are considered as DINT integers.
LINT LINT# Long integer (64-bit) constant expressions are valid integer values. All integer expressions having no prefix are considered as DINT integers.
ULINT/LWORD ULINT# Unsigned 64-bit integer constant expressions are valid integer values. All integer expressions having no prefix are considered as DINT integers.
REAL  

Real constant expressions must be valid numbers, and must include a dot ("."). If you need to enter a real expression having an integer value, add ".0" at the end of the number. You can use "F" or "E" separators for specifying the exponent in case of a scientific representation. REAL is the default precision for floating points: such expressions do not require a prefix.

REAL is restrictive, but because it is the default, it is recommended to explicitly declare your real constants with the LREAL# prefix.

REAL constants are limited to 6-7 digits of accuracy. Any digits after these significant digits will be lost, leading to a loss of precision.
LREAL LREAL#

Real constant expressions must be valid numbers, must include a dot ("."). If you need to enter a real expression having an integer value, add ".0" at the end of the number. You can use "F" or "E" separators for specifying the exponent in case of a scientific representation.

LREAL constants are limited to 14-15 digits of accuracy. Any digits after these significant digits will be lost, leading to a loss of precision.
TIME T# or TIME# Time-constant expressions represent durations that must be less than 24 hours.
They are expressed as a number of hours followed by "h", a number of minutes followed by "m", a number of seconds followed by "s", and a number of milliseconds followed by "ms".
The order of units (hour, minutes, seconds, milliseconds) must be respected.
You cannot insert blank characters in the time expression.
There must be at least one valid unit letter in the expression.
STRING  

String expressions must be written between single quote marks.
The length of the string cannot exceed 255 characters.
You can use the following sequences to represent a special or not-printable character within a string:

$$ a "$" character
$' a single quote
$T a tab stop (ASCII code 9)
$R a carriage return character (ASCII code 13)
$L a line feed character (ASCII code 10)
$N carriage return plus line feed characters (ASCII codes 13 and 10)
$P a page break character (ASCII code 12)

$xx

any character (xx is the ASCII code expressed on two hexadecimal digits

Table 5-1: List of Prefixes for Constant expressions

Examples

Below are some examples of valid constant expressions:

TRUE TRUE Boolean expression
FALSE FALSE Boolean expression
SINT#127 small integer
INT#2000 16 bit integer
123456 DINT (32 bit) integer
16#abcd DINT integer in hexadecimal basis
8#34712 DINT integer in octal basis
2#1000100 DINT integer in binary basis
LINT#1 long (64 bit) integer having the value "1"
0.0 0 expressed as a REAL number
1.002E3 1002 expressed as a REAL number in scientist format
LREAL#1E-200 Double precision real number
T#23h59m59s999ms maximum TIME value
TIME#0s null TIME value
T#1h123ms TIME value with some units missing
'hello' character string
'name$Tage' character string with two words separated by a tab
'I$'m here' character string with a quote inside (I'm here)
'x$00y' character string with two characters separated by a null character (ASCII code 0)

Below are some examples of typical errors in constant expressions

BooVar := 1; 0 and 1 cannot be used for Booleans
1a2b basis prefix ("16#") omitted
1E-200 "LREAL#" prefix omitted for a double precision float
T#12 Time unit missing
'I'm here' quote within a string with "$" mark omitted
hello quotes omitted around a character string

Additionally, there are pre-defined constants. See Use the Defines List for information about Internal and user-defined Defines.

 

Stay Connected with Kollmorgen

Copyright © 2015 Kollmorgen™

 

Open topic with navigation