SACA Programmable Logic Controllers 2

Question 1

In IEC 61131-3, what is the key characteristic of a Function Block Diagram (FBD) program?

Accepted Answer

Graphical blocks with inputs and outputs connected by signal flow lines

Answer

Text-based sequential instructions similar to assembly language

Answer

Flowchart-style diagrams showing process steps and decisions

Answer

Relay-equivalent contact and coil symbols on rungs

Question 2

What is the purpose of the PLC's watchdog timer?

Accepted Answer

To reset or halt the CPU if the scan cycle takes longer than a programmed time limit

Answer

To automatically schedule preventive maintenance alerts

Answer

To track the total operating hours of the PLC

Answer

To synchronize multiple PLC clocks across a network

Question 3

In Ladder Diagram programming, what does an 'XIC' (Examine If Closed) instruction do when the referenced bit is FALSE (0)?

Accepted Answer

It blocks continuity (prevents power flow), acting like an open switch

Answer

It provides continuity (passes power flow) to the next element in the rung

Answer

It forces the bit to TRUE and continues scanning

Answer

It generates a fault and halts program execution

Question 4

What is the difference between retentive and non-retentive timer instructions in a PLC?

Accepted Answer

Retentive timers maintain their accumulated value when the rung goes false; non-retentive timers reset to zero

Answer

Retentive timers count in seconds; non-retentive timers count in milliseconds

Answer

Retentive timers output a pulse; non-retentive timers hold their output continuously

Answer

Retentive timers require battery backup; non-retentive timers work without battery

Question 5

Which PLC communication protocol operates at the application layer and is specifically designed for reading/writing data in Rockwell/Allen-Bradley controllers?

Accepted Answer

EtherNet/IP with CIP

Answer

Modbus RTU

Answer

PROFIBUS DP

Answer

DeviceNet

Question 6

What is 'indirect addressing' in PLC programming and when is it used?

Accepted Answer

Using a variable as a pointer to specify the memory address of the actual data

Answer

Accessing data in a remote PLC over a network connection

Answer

Programming offline without connecting to a physical PLC

Answer

Accessing data from a previous scan cycle's register values