PLCs & Automation — Industrial Electrician (442A) Practice Questions

Free PLCs & Automation practice questions

1. 1.A PLC scan cycle consists of:

   • A.Only running the program
   • B.Input scan → Program execution → Output scan → Housekeeping/communications✓
   • C.Reading sensors only
   • D.Writing outputs only

   Why: Each PLC scan: reads all inputs, executes the entire program, updates all outputs, then handles communications/diagnostics. Scan time is typically 1-100ms depending on program size.

2. 2.What is the difference between OTL (Output Latch) and OTE?

   • A.No difference
   • B.OTL remains ON even when rung goes FALSE — requires OTU to unlatch✓
   • C.OTL is faster
   • D.OTE is permanent

   Why: OTL (latch) stays ON once energized, even if the rung goes FALSE. It remains latched through power cycles (retentive). Only an OTU (unlatch) instruction on the same address turns it OFF.

3. 3.A safety PLC differs from a standard PLC by:

   • A.Colour only
   • B.Dual-channel redundant processing and diagnostics meeting SIL/Category standards for safety functions✓
   • C.Higher speed
   • D.More memory

   Why: Safety PLCs have redundant processors that cross-check each other, diagnostic coverage, and certified safety integrity levels (SIL) for E-stop, light curtain, and safety interlock functions.

4. 4.A PLC forcing an output ON bypasses normal program logic. When is this acceptable?

   • A.Anytime during production
   • B.Only during commissioning/troubleshooting with proper safety procedures and documentation✓
   • C.Never
   • D.Whenever convenient

   Why: Forcing bypasses program safety logic — extremely dangerous in production. Only during commissioning/troubleshooting with proper lockout, a spotter on the equipment, and forces must be removed after.

5. 5.What is a PLC's I/O addressing system?

   • A.Email addresses
   • B.A structured numbering scheme that maps physical I/O points to memory locations in the program✓
   • C.IP addresses
   • D.Phone numbers

   Why: I/O addressing maps each physical input and output terminal to a specific memory location. Example: I:1/0 = Input module slot 1, terminal 0. Addressing varies by PLC manufacturer.

6. 6.What is the purpose of a watchdog timer in a PLC?

   • A.Time process events
   • B.Detect if the PLC scan takes too long (indicating a fault) and trigger a safe shutdown✓
   • C.Count parts
   • D.Set the clock

   Why: The watchdog timer monitors scan time. If a scan exceeds the configured limit (program hang, hardware fault), the PLC faults to a safe state to prevent uncontrolled operation.

7. 7.What is the purpose of PLC battery backup?

   • A.Power the outputs
   • B.Maintain program and data memory during power loss for non-flash-based PLCs✓
   • C.Run the motor
   • D.Charge the HMI

   Why: Battery maintains volatile RAM contents (program, data, retentive values) during power off. Flash-based PLCs retain programs without battery. Low battery warnings must be addressed promptly.

8. 8.What is the benefit of remote I/O in a large facility?

   • A.Eliminates the PLC
   • B.Reduces wiring by placing I/O modules near field devices and communicating digitally to the PLC✓
   • C.Increases wire count
   • D.Slows down the system

   Why: Remote I/O eliminates long home-run wiring. I/O racks near the equipment connect to the PLC over a single network cable, reducing installation cost, troubleshooting time, and wire tray fill.

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