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The IEC 61131-3 ST (Structured Text) is a high-level programming language designed for industrial automation systems, developed by the International Electrotechnical Commission (IEC). Introduced in 1993, it stands as part of the IEC 61131 standard aimed at unifying PLC programming languages under one international standard. This text-based language follows syntax similar to Pascal and is widely used for creating complex algorithms due to its flexibility and readability. It's recognized globally by manufacturers and users of programmable controller technologies. The official documentation can be accessed through the IEC's website or purchased from national standards organizations worldwide.

Ask the right questions secure the right IEC 61131-3 ST (Structured Text) talent among an increasingly shrinking pool of talent.

First 20 minutes

General IEC 61131-3 ST (Structured Text) app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in IEC 61131-3 ST (Structured Text) application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

What are the basic data types used in IEC 61131-3 ST?

The basic data types used in IEC 61131-3 ST include BOOL, BYTE, WORD, DWORD, LWORD, SINT, INT, DINT, LINT, USINT, UINT, UDINT, ULINT, REAL, LREAL, TIME, DATE, TIME_OF_DAY, DATE_AND_TIME, STRING, WCHAR, WSTRING.

How would you write a simple IF statement in Structured Text?

A simple IF statement in Structured Text can be written as follows: IF condition THEN statements; END_IF;

Describe the difference between a FUNCTION and a FUNCTION_BLOCK in IEC 61131-3 ST.

A FUNCTION in IEC 61131-3 ST is a reusable block of code that returns a single value, whereas a FUNCTION_BLOCK is a reusable block of code that can return multiple output values and retain its internal state between calls.

What are the five programming languages defined by IEC 61131-3 standard?

The five programming languages defined by IEC 61131-3 standard are Structured Text (ST), Instruction List (IL), Ladder Diagram (LD), Function Block Diagram (FBD), and Sequential Function Chart (SFC).

How would you declare an array in Structured Text?

In Structured Text, an array can be declared as follows: VAR arrayName: ARRAY [startIndex..endIndex] OF dataType; END_VAR

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What you’re looking for early on

Does the candidate have a solid understanding of IEC 61131-3 ST programming language?

Has the candidate demonstrated problem-solving skills?

Can the candidate communicate effectively?

Does the candidate have experience with PLC (Programmable Logic Controller) systems?

Next 20 minutes

Specific IEC 61131-3 ST (Structured Text) development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

What is the purpose of the 'AT' keyword in IEC 61131-3 ST?

The 'AT' keyword in IEC 61131-3 ST is used to specify the physical address of a variable in the memory.

Describe the difference between SINT and INT data types in IEC 61131-3 ST.

SINT and INT are both integer data types in IEC 61131-3 ST. The difference lies in their size and range. SINT (Short INTeger) is an 8-bit integer with a range from -128 to 127, whereas INT is a 16-bit integer with a range from -32768 to 32767.

How would you implement a FOR loop in Structured Text?

A FOR loop in Structured Text can be implemented as follows: FOR variable := start TO end DO statements; END_FOR;

What are the standard comparison operators used in IEC 61131-3 ST?

The standard comparison operators used in IEC 61131-3 ST are = (equal), <> (not equal), < (less than), > (greater than), <= (less than or equal), >= (greater than or equal).

How would you handle errors or exceptions in Structured Text?

Errors or exceptions in Structured Text can be handled using the TRY...EXCEPT...END_TRY construct. The code that may cause an error is placed within the TRY block, and the code to handle the error is placed within the EXCEPT block.

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At this point, a skilled IEC 61131-3 ST (Structured Text) engineer should demonstrate strong problem-solving abilities, proficiency in IEC 61131-3 ST (Structured Text) programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with IEC 61131-3 ST (Structured Text).

What does this simple Structured Text code do?
PROGRAM Prg
VAR
  x : INT;
END_VAR
x := 5;
IF x > 3 THEN
  x := x * 2;
END_IF
END_PROGRAM
This code declares an integer variable 'x', assigns it a value of 5, and then checks if 'x' is greater than 3. If 'x' is indeed greater than 3, it multiplies 'x' by 2. So, the final value of 'x' will be 10.
What does the following Structured Text code do?
PROGRAM Prg
VAR
  y : BOOL;
  z : INT;
END_VAR
y := TRUE;
z := y ? 10 : 20;
END_PROGRAM
This code declares a boolean variable 'y' and an integer variable 'z'. It assigns TRUE to 'y' and then uses a ternary operator to assign a value to 'z'. If 'y' is TRUE, 'z' gets the value 10, otherwise 'z' gets the value 20. In this case, 'z' will be 10.
What will be the output of the following Structured Text code that manipulates an array?
PROGRAM Prg
VAR
  arr : ARRAY [1..5] OF INT := [1, 2, 3, 4, 5];
  i : INT;
  sum : INT;
END_VAR
FOR i := 1 TO 5 DO
  sum := sum + arr[i];
END_FOR
END_PROGRAM
This code declares an array 'arr' of 5 integers and initializes it with values from 1 to 5. It then declares an integer 'i' for the loop counter and 'sum' for the total sum. The FOR loop iterates over the array and adds each element to 'sum'. The final value of 'sum' will be 15.
What does the following Structured Text code do that involves concurrency?
PROGRAM Prg
VAR
  x : INT;
END_VAR
x := 1;
PARALLEL
  x := x + 1;
  x := x * 2;
END_PARALLEL
END_PROGRAM
This code declares an integer 'x' and assigns it a value of 1. Then, it executes two statements in parallel. One statement increments 'x' by 1 and the other multiplies 'x' by 2. However, the final value of 'x' cannot be determined due to the non-deterministic nature of parallel execution.

Wrap-up questions

Final candidate for IEC 61131-3 ST (Structured Text) Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle IEC 61131-3 ST (Structured Text) application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

Describe the difference between CONSTANT and RETAIN variables in IEC 61131-3 ST.

CONSTANT variables in IEC 61131-3 ST are variables that cannot be changed during the execution of the program. RETAIN variables, on the other hand, are variables that retain their values even after a power cycle or program restart.

What are the standard logical operators used in IEC 61131-3 ST?

The standard logical operators used in IEC 61131-3 ST are AND, OR, XOR, NOT.

How would you implement a CASE statement in Structured Text?

A CASE statement in Structured Text can be implemented as follows: CASE variable OF value1: statements1; value2: statements2; ELSE statements_default; END_CASE;

IEC 61131-3 ST (Structured Text) application related

Product Perfect's IEC 61131-3 ST (Structured Text) development capabilities

Beyond hiring for your IEC 61131-3 ST (Structured Text) engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in IEC 61131-3 ST (Structured Text) projects, and can engage in multiple capacities.

IEC 61131-3 ST (Structured Text) Consultive assessments

IEC 61131-3 ST (Structured Text) Application Conversion Assessment IEC 61131-3 ST (Structured Text) Application Integration Assessment IEC 61131-3 ST (Structured Text) Data & Database Integration Assessment

Custom IEC 61131-3 ST (Structured Text) development projects

Custom IEC 61131-3 ST (Structured Text) App Conversion IEC 61131-3 ST (Structured Text) Custom App Development Process App UI Design for IEC 61131-3 ST (Structured Text) Facelifts

Hiring guide for IEC 61131-3 ST (Structured Text) Engineers

IEC 61131-3 ST (Structured Text) Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

What you’re looking to see on the IEC 61131-3 ST (Structured Text) engineer at this point.

IEC 61131-3 ST (Structured Text) Consultive assessments

Custom IEC 61131-3 ST (Structured Text) development projects