IEC61131-3 coding rules and guidelines series - Part1: Variable naming

Every programming language has its own coding rules and guidelines. Industrial Automation programming is no different. 
However, documentation covering Industrial automation programming coding rules and best practices is very few.

The software in the industrial environments becomes more and more important, the software projects become larger, and the costs of errors increase. 


In order to deal with the complexity of larger programs, a modern approach to software development processes should be followed supporting a structured approach. 
Also, we need to increase the efficiency in coding via re-use of pre-defined functionalities and to help better understand the program over the life cycle.

The aim of this series is to share the coding guidelines and rules that are based on IEC61131-3 standard and based on my 16 years of experience deploying automation solutions throughout different market sectors.

Part1: Naming rules for variables

1.1 Physical addresses to be avoided

Reference: 

• IEC 61131-3 6.5.5 
• IEC 61131-8 3.11.2

Programming Languages: 

• Ladder diagram (LD)
• Function block diagram (FBD)
• Structured text (ST)
• Sequential function Chart

Description

The usage of system dependent physical addresses shall be avoided. It is always preferred to use Variables.

Using physical addresses makes code less portable: every time the program should be deployed on a different manufacturer, or even a different instance on the same manufacturer it has to be edited. 

Furthermore, It makes programs less readable: Without a symbol table for reference, it will be difficult to know what each address is used for. Later system changes are also more error prone as some access to the physical addresses may be skipped or overlooked. 

Exceptions might arise for some communication protocols, where a physical addressing is needed to assign variables to a physical location to comply with the protocol mapping

1.2 Type prefixes for variables to be defined

Reference: 

• IEC 61131-3 

Programming Languages: 

• Ladder diagram (LD)
• Function block diagram (FBD)
• Structured text (ST)
• Sequential function Chart

Description

It is a good practice to define a scheme to be used to prefix a variable's name as a 
variable's name shall communicate other attributes that can improve readability and help reduce programming mistakes. 

For example:

• Knowing the size of the datatype or whether signed or unsigned can be important in the selection of instructions. 
• Knowing a variable is Global can suggest external influence, or caution about undesirable side effects when writing. 
• Writing to an "Input" signal by mistake will have no effect, and is not always shown as an error, for example if intermediate variables are mapped.

Also as programs get bigger complex, there is a tendency to group related variables (for example InfeedSpeed, InfeedAlarm, InfeedStatus). This grouping fails if groups are used inconsistently (for example Infeed_Speed, In_Feed_Alarm, ifStatus, inFault etc) so the common 'zone' names could be defined. This is especially true for large, multi-developer teams.

Here is a list of some variable's name prefix that can be used according to IEC61131-3:

Type	Prefix
BOOL	x
SINT	si
INT	i
LINT	li
DINT	di
USINT	usi
UINT	ui
UDINT	udi
ULINT	uli
REAL	r
LREAL	lr
TIME	tim
LTIME	ltim
DATE	dt
LDATE	ldt
TIME_OF_DAY / TOD	tod
LTIME_OF_DAY / TOD	ltod
DATE_AND_TIME / DT	dt
LDATE_AND_TIME / DT	ldt
STRING	str
WSTRING	wstr
CHAR	c
WCHAR	wc
BYTE	by
WORD	w
DWORD	dw
LWORD	LW
ENUM	e
ARRAY	s
STRUCT	st
FUNCTION BLOCK	fb
PROGRAM	prg
CLASS	cls
GLOBAL SCOPE	g
LOCAL SCOPE	l
TEMPORARY VARIABLE	tmp
INPUT (Read only)	i
OUTPUT (Read/Write)	o

Attention should be made when defining the prefix scheme for variable's name. Adding too many prefixes can create complexity and long variable names. 
The most commonly used is a prefix for the data type.


The next part of IEC61131-3 coding rules and guidelines series will cover the balance naming rules for Variables, Tasks, Programs, Function blocks, Functions and User defined data type.