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

FBD uses graphical function blocks (representing algorithms like PID, timers, comparators) with their inputs and outputs connected by lines showing the flow of data — ideal for signal processing and continuous control applications.

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

The watchdog timer is a safety mechanism — if the scan cycle doesn't reset the timer within the configured period (e.g., 150ms), it indicates a program fault or infinite loop, triggering a CPU fault and transition to safe state.

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

XIC (Examine If Closed) passes power flow (logical continuity) only when the referenced bit is TRUE (1). When the bit is FALSE (0), XIC acts as an open contact, blocking power flow in that rung branch.

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

A retentive timer (RTO/TONR) preserves its accumulated count when de-energized — useful for tracking total running time. A non-retentive timer (TON/TOF) resets to zero whenever the enabling condition goes false.

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

EtherNet/IP uses the Common Industrial Protocol (CIP) over standard Ethernet/TCP-UDP — it is the native protocol of Allen-Bradley PLCs (ControlLogix, CompactLogix) for explicit messaging and I/O connections.

SACA - Smart Automation Certification Alliance Practice Test

SACA - Smart Automation Certification Alliance Practice Test

SACA Programmable Logic Controllers 2