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

Downloading a new program to the PLC from a remote engineering workstation over the internet

Answer

Editing a program in simulation mode before downloading; no machine risk involved

Answer

Making program changes that only take effect after the next scheduled maintenance shutdown

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

Replace the output module immediately, then test the field device

Answer

Check only the field device wiring and replace if corroded

Answer

Reset the PLC to default configuration and reload the program

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

It latches an output ON indefinitely until a separate reset instruction clears it

Answer

It generates one pulse per second regardless of rung conditions

Answer

It compares the current scan value to the previous scan value and outputs their difference

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 backup PLC program that runs when the primary program encounters an error

Answer

A password-protected section of the PLC program that prevents unauthorized modifications

Answer

A maintenance logging routine that records fault codes and timestamps to a data file

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

The input sensor is faulty and sending incorrect signals

Answer

The PLC scan cycle is too fast for the sensor's switching frequency

Answer

The input card needs replacement because the LED circuit and logic circuit have failed independently

Question 6

What is 'structured programming' in PLC design, and how do routines/subroutines improve maintainability?

Accepted Answer

Organizing PLC code into separate routines by function (Startup, Running, Fault, Safety), called from a main routine — making programs easier to understand, test, and modify

Answer

Writing all logic in a single flat ladder program without branching to external routines

Answer

Using only Structured Text language (no ladder) in all PLC programs for readability

Answer

Dividing the program into sections that run on different CPUs in a redundant PLC pair