SACA PLC Programming and Troubleshooting 2

Question 1

What does 'online editing' mean in PLC programming, and what precautions are necessary?

Accepted Answer

Editing a PLC program while the PLC is in RUN mode, with changes taking effect immediately — requiring careful review since incorrect logic can immediately affect the controlled machine

Answer

Online editing (make changes while the PLC scans) allows modifying a running production program without halting the process. Changes take effect in the next scan — requiring thorough review, proper change management, and awareness that incorrect rungs can immediately command machine actions.

Question 2

When troubleshooting a PLC output that is 'on' in the program but the field device is not operating, what is the correct diagnostic sequence?

Accepted Answer

Verify the output bit is TRUE in the PLC data table, measure voltage at the output terminal, check output fuse, check field device wiring, then verify field device operation

Answer

Systematic fault isolation: confirm the PLC bit is forced/set (logic works), check the output card's LED indicator, measure voltage at the terminal (output card working?), trace wiring to field device (continuity/break?), then verify field device itself — narrowing the fault zone at each step.

Question 3

In ladder logic, what does the 'one-shot rising' (OSR or ONS) instruction accomplish?

Accepted Answer

It creates a pulse output that stays high for exactly one scan cycle when the preceding rung logic transitions from false-to-true

Answer

OSR (One Shot Rising) monitors the rung condition — when it transitions from FALSE to TRUE, it sets its output bit for exactly one scan cycle, then clears it even if the enabling condition remains TRUE. Used to trigger a single action (counter increment, subroutine call) on the leading edge of an event.

Question 4

What is a 'PLC fault routine' and why is it important for safe machine operation?

Accepted Answer

A special program section that executes automatically when certain faults occur, allowing graceful fault handling rather than an abrupt CPU fault that leaves outputs in an indeterminate state

Answer

A fault routine (major fault routine in Allen-Bradley, OB80-OB87 in Siemens) lets the programmer define what happens when a fault is detected — logging the fault, safely ramping down outputs, sending alarms — instead of the default behavior of immediately faulting the CPU and leaving outputs uncontrolled.

Question 5

When a PLC input card shows its LED indicator lit but the PLC program shows the input as FALSE, what is the likely cause?

Accepted Answer

There is likely a wiring error or the sensor output type mismatches the input card type (e.g., PNP sensor connected to NPN-configured input), preventing correct signal interpretation

Answer

LED lit (hardware sees a signal) but program bit = FALSE indicates the signal is present but not recognized correctly — common causes: PNP vs NPN wiring mismatch (signal is sinking when card expects sourcing), threshold voltage issue, or card configured for AC when DC input is applied.

SACA - Smart Automation Certification Alliance Practice Test

SACA - Smart Automation Certification Alliance Practice Test

SACA PLC Programming and Troubleshooting 2