PLC Fundamentals: CompactLogix, Studio 5000, and Ladder Logic M1–M4

[M1] What does the PLC link to I/O instructions in its internal program through the use of addresses?

Physical inputs and outputs - Addresses tie the program's I/O instructions to the physical I/O terminals.

[M1] The status of which of these determines the sequence in which output devices (solenoid valves, motor contactors) are turned on and off?

Input devices - The PLC reads input device status, and the program uses it to decide which outputs to turn on and off.

[M1] What is the toolbar called that contains the instructions you can add to a Ladder Logic program in the Ladder Editor?

Ladder Instruction toolbar - The Ladder Instruction toolbar holds instructions such as XIC, XIO, and OTE.

[M1] A PLC controls a machine by monitoring the status of the input sensors and using that status to control which type of devices?

Output - Inputs are monitored and outputs are controlled.

[M1] Which PLC programming language is graphical, resembles the rungs of a ladder, and operates like hard-wired relay circuits?

Ladder Logic - Ladder Logic mimics relay control circuits.

[M1] Which PLCs are smaller versions of ControlLogix PLCs and are also programmed with Studio 5000?

CompactLogix - CompactLogix is the smaller family. Micro800 uses Connected Components Workbench, and S7 PLCs are Siemens.

[M1] What is the hard-wired safety seal-in relay circuit that turns output power on and off using pushbuttons called?

MCR - MCR is the Master Control Relay circuit.

[M1] What is the name of the switch that, in the REM position, allows an online PLC's mode to be changed from Studio 5000?

Mode switch - It is the controller's Mode switch (RUN / REM / PROG).

[M1] In what position must the PLC mode switch be for the mode to be changed using the programming software?

REM - REM (remote) lets Studio 5000 change the mode. RUN and PROG lock it at the switch.

[M1] The PLC does not care what devices are connected to its I/O. It checks the on/off status of each input terminal, then turns on outputs according to this:

PLC Program - The program defines how input status turns outputs on and off.

[M1] If voltage exists at an input terminal, what is the status of the input?

On - Voltage present means the input is On (a 1).

[M1] What term describes continuously examining input conditions, making decisions from the program logic, then updating the output terminals?

PLC Scan Cycle - That repeating read-decide-update process is the scan cycle.

[M1] Which type of PLC includes a processor and a fixed number of digital and analog I/O in one enclosure?

Brick-style - Brick-style PLCs are all-in-one units. Chassis-style PLCs use separate modules.

[M1] Which part of a machine is monitored by a PLC to control the output devices?

Input sensors - Input sensors report machine conditions to the PLC.

[M1] Which type of control devices were replaced by PLCs?

Electromechanical relays - PLC logic replaced relay panels. Pushbuttons and limit switches are still used as inputs.

[M1] What is the CompactLogix USB port primarily used for?

Configuring the controller's initial IP address - USB is typically used for initial setup, such as setting the IP address, before using Ethernet.

[M1] What programming software is used with ControlLogix and CompactLogix PLCs?

Studio 5000 - Studio 5000 Logix Designer. TIA Portal is Siemens, and CCW is for Micro800.

[M1] What does the PLC processor (CPU) do?

Executes the program, manages memory, and handles communications - The processor runs the user program, manages memory and I/O data, and handles communications.

[M1] Which is an analog input device?

Temperature sensor with a 4-20 mA output - Analog signals vary continuously. Pushbuttons and limit switches are discrete, and a lamp is an output.

[M1] Which of these is a discrete OUTPUT device?

Solenoid valve - Outputs are loads the PLC drives, such as solenoids, lamps, and motor starter coils.

[M1] What is scan time?

The time one full pass of the scan cycle takes - Scan time is how long one pass takes, usually milliseconds or less.

[M1] In Logix, what does RPI stand for?

Requested Packet Interval - RPI sets how often an I/O module's data is updated, independent of the program scan.

[M1] What is firmware?

The controller's built-in operating software - The firmware revision must be compatible with the Studio 5000 version and the project.

[M1] What is the order of the conceptual PLC scan cycle?

Read inputs, execute program, update outputs, communications/housekeeping - Inputs are read, the program runs top to bottom, outputs are updated, and then the cycle repeats.

[M1] In which controller mode does the program execute and outputs follow the logic?

Run mode - Program mode stops execution, and that is when you normally download.

[M3] What links the PLC program's I/O instructions with the physical input and output terminals?

Addresses - Each I/O instruction is assigned an address that corresponds to a physical terminal.

[M3] What type of diagram shows the I/O devices and how they are wired to the power supply and PLC I/O terminals?

I/O diagram - The I/O diagram identifies the input and output devices and their terminal connections.

[M3] What kind of diagram shows how control devices interact with each other through the power components?

Power diagram - Per your course sheet, power diagrams show interaction through the power components. I/O diagrams show the PLC wiring.

[M3] What are common names assigned to instruction addresses in addition to the absolute address?

Symbolic addresses - Symbolic addresses are readable names such as Start_PB.

[M3] Which type of PLC program address does not change?

Absolute address - The absolute address is fixed by the hardware location. Symbolic names can be changed.

[M3] Which type of input instruction is FALSE when its assigned input terminal is energized by a voltage?

NC contact instruction - An NC contact (XIO) is true when the input is off and false when it is on.

[M3] What is the discrete output instruction on the far right of the rung called?

Output coil - The output coil (OTE) sits at the right end of the rung.

[M3] Contacts in series on a rung represent which logic?

AND - All series conditions must be true for the rung to be true.

[M3] A parallel branch represents which logic?

OR - Any one true path through the branch makes the rung true.

[M3] What does a One Shot (ONS) do?

Makes the rung true for one scan on a false-to-true transition - It triggers an action once per event.

[M3] What is an alias tag?

Another name that points to an existing base tag - Aliases give I/O points readable names, like Start_PB for Local:1:I.Data.0.

[M3] Which is the default integer data type in Logix?

- DINT is the native, most efficient integer type in Logix.

[M3] Why avoid using OTE on the same bit in two rungs?

The last rung scanned overrides the other, giving confusing results - Both rungs write the bit every scan, so the later one wins.

[M3] What does Verify check? connected

For program errors such as syntax and undefined tags, not whether the logic is right - A verified project can still behave incorrectly. You must test it.

[M3] What is the difference between OTE and OTL?

OTE follows the rung state; OTL sets the bit until an OTU clears it - OTL and OTU are a pair. A latched bit stays on until it is unlatched.

[M3] When is an XIC instruction true?

When its bit is 1 - XIC (Examine If Closed, the NO contact) is true when its bit is 1. XIO is true when the bit is 0.

[M4] Which type of tags must be added to the project before they are assigned to instructions?

Non-module-defined tags - Module-defined tags are created automatically when you add an I/O module. Non-module-defined tags you create yourself.

[M4] What are the alphanumeric names assigned to memory locations in CompactLogix PLCs called?

Tags - Logix uses tag-based memory instead of fixed numeric addresses.

[M4] A Studio 5000 project is a collection of files that includes hardware configurations, tags, communication settings, and what type of program?

Ladder Logic - The project contains the Ladder Logic program along with the configuration.

[M4] What is the text called in a PLC project that explains the function or operation of an item?

Documentation - Documentation includes tag descriptions and rung comments.

[M4] After edits to the ladder logic program, what is the process of checking for errors called?

Verifying - Verify checks for errors. Download sends the program to the controller.

[M4] What is the continuous task?

A task that runs repeatedly with no set interval - The continuous task restarts as soon as it finishes. Periodic tasks run at set intervals and event tasks on triggers.

[M4] What triggers a periodic task?

A fixed time interval - Periodic tasks run at a set interval and take priority over the continuous task.

[M4] What does JSR do?

Jumps to another routine and returns afterward - Jump to Subroutine calls another routine.

[M4] What is the difference between download and upload? changes the mode

Download copies the project from the PC to the controller; upload copies it back - Download sends your project to the controller. Upload retrieves it.

[M4] What must you do before an online edit takes effect?

Test and then accept the edits - Pending edits are tested, then accepted so they become part of the running program.

[M4] What is an array?

A group of tags of the same data type accessed by index - For example, a DINT array of 10 elements accessed as MyArray[3].

[M4] What does adding a module to the I/O Configuration do?

Creates the module's I/O tags and tells the controller what to expect - The configuration must match the hardware or you can get faults.

[M4] What is the project hierarchy from largest to smallest?

Controller, task, program, routine - A controller holds tasks, tasks hold programs, and programs hold routines.

[M4] What does an active FORCE indicator mean, and why be careful?

An I/O value is overridden regardless of logic or the real signal - Forces can start equipment unexpectedly or hide a problem. Remove them when done.

[M4] What is a controller-scoped tag?

A tag visible to all programs in the controller - Program-scoped tags are visible only within their own program.

[M5] What part of a magnetic motor starter interfaces the PLC output to the starter contacts?

Solenoid or coil - The PLC output energizes the starter's coil, which closes the power contacts.

[M5] PLCs also control fluid power motors, both hydraulic and this type:

Pneumatic - Fluid power means hydraulic and pneumatic.

[M5] An external output instruction can control NO and NC input contact instructions by assigning what to the input instructions?

Output instruction address - The contacts use the same address as the output, which is how seal-in works.

[M5] What programming technique lets an operator press Start momentarily to make the motor run continuously?

Seal-in logic - A contact of the output is wired in parallel with Start to hold the rung true.

[M5] What are the internal memory bits that can be set by PLC output instructions called?

Internal outputs - Internal outputs are memory bits that are not tied to a physical terminal.

[M5] What is the electromagnetically operated switch used in PLC motor control called?

Magnetic motor starter - A DCV is a fluid power valve. The MCR is the safety circuit.

[M5] What types of input instructions can be controlled by internal output addresses?

NO and NC contact instructions - Contacts assigned an internal output's address follow its state.

[M5] How is a Start pushbutton normally wired?

Normally open - Start is normally open, so it sends a 1 only while pressed. Stop is normally closed.

[M5] A normally closed Stop button is not pressed. What is its input bit?

1 - The contact is closed, so the input is on. Use an XIC in the program so pressing Stop breaks the rung.

[M5] How do you require Motor 1 to be running before Motor 2 can start?

XIC of Motor 1's run bit in series in Motor 2's start rung - The series XIC permits Motor 2 only while Motor 1 is running.

[M5] What does an overload relay protect against?

Sustained motor overcurrent that can overheat the motor - Its contact opens on overcurrent so the motor stops.

[M5] How does a PLC output run a motor?

It energizes a motor starter or contactor coil that closes the power contacts - PLC outputs carry low power. The starter switches the motor's circuit.

[M5] Can a software interlock replace the hardwired emergency stop?

No, safety functions need hardwired protection independent of the program - Hardware safety circuits like the MCR and e-stop remain essential.

[M5] Why are Stop and overload devices usually wired normally closed?

A broken wire or lost power looks like a Stop, so the motor shuts off safely - This is fail-safe wiring.

[M5] What does an interlock in a reversing motor circuit prevent?

Both directions energized at once, causing a short circuit - Each direction's rung includes an XIO of the opposite direction's output.

[M6] What is the amount of time that a TON timer has been timing called?

Elapsed Time (ET) - ET is the time elapsed so far. In Logix tags this is the ACC value.

[M6] Timer instructions are often used in machine controls to provide this between two events in a sequence:

Time delay - Timers add a time delay between events.

[M6] Which non-retentive timer only turns on its Q output after the elapsed time equals the preset time?

TON - TON is an on-delay timer. Q is the done output, DN in Logix.

[M6] Which input does a retentive RTO use that the non-retentive TON and TOF do not?

Reset - An RTO keeps its elapsed time, so it needs a reset input.

[M6] Which type of timer has a reset input?

Retentive - Retentive timers (RTO) use a reset input.

[M6] Which non-retentive timer turns on its output when its input turns true?

TOF - A TOF output comes on immediately and then delays turning off once the input goes false.

[M6] What happens to a TON's ACC and DN when its rung goes false?

They reset to zero and off - A TON is non-retentive. It needs a continuous true rung for the full preset.

[M6] What does the TT bit indicate?

The timer is actively timing - TT is on while the timer is timing. EN follows the rung, and DN means done.

[M6] What is PRE = 1000 for a Logix timer?

1 second - Logix timer values are in milliseconds.

[M6] How do you clear an RTO?

Use a RES with the same timer tag - The RTO retains ACC, and only a RES clears it.

[M6] How can you build a flasher with timers?

Two timers that alternately start and reset each other - One timer's DN starts the other, and the cycle repeats.

[M6] When is a TOF's TT bit on?

While the rung is false and it is timing out - A TOF times after the rung goes false.

[M6] What goes wrong if two TON instructions share one tag?

The instructions interfere, giving erratic timing - Each timing instruction needs its own timer tag.

[M6] A TOF rung goes true and then false. How does DN behave?

DN turns on immediately and turns off after the preset once the rung goes false - A TOF delays the turn-off, not the turn-on.

[M6] An RTO reaches its preset, and then the rung goes false. What happens to DN?

It stays on until a RES clears the timer - An RTO keeps ACC and DN until it is reset.

[M7] A basic two-step reciprocating cycle sequences how many times after the Start pushbutton is pressed?

Once - This is a single cycle. Continuous cycling needs added logic.

[M7] What is the normal operating mode of most machines?

Automatic - Manual and jog modes are mainly for setup and maintenance.

[M7] Which type of stop causes the machine to stop after it finishes the last step in its cycle?

Cycle-Stop - A cycle-stop lets the machine complete its cycle. An e-stop stops it immediately.

[M7] A single-cycle reciprocating cylinder system can be changed to cycle continuously by changing the logic and adding this to detect when the cylinder has retracted:

Second limit switch - The retract limit switch signals the end of the cycle so it can restart.

[M7] Which type of sequence uses input sensors to signal the controller that each step is complete?

Event-driven sequence - Each step is confirmed by an event, such as a limit switch.

[M7] What advances an event-driven sequence?

A confirming event, such as a limit switch or sensor - For example, a cylinder reaching its extended limit switch advances to the next step.

[M7] What is the purpose of return-to-home or reset logic?

To put the machine in a known state before it starts again - After a stop or fault the machine may be mid-cycle.

[M7] What does continuous cycle mean?

The sequence restarts automatically after the last step - Single cycle runs once and waits for another start.

[M7] A limit switch fails and never closes. What happens to an event-driven sequence?

It hangs at that step - The transition condition is never true.

[M7] Why must the previous step be unlatched when the next step is latched?

So only one step is active at a time - Two active steps can run conflicting outputs.

[M7] A sequence stalls at Step 3. What do you check first?

The condition that should advance Step 3 (sensor, timer, interlock) - Find the active step, then check the condition that advances it.

[M8] What is sequencing with timer instructions instead of limit switches called?

Time-driven sequencing - Each step advances when a timer finishes.

[M8] What type of process should be used to design complex PLC applications?

Structured - A structured design process breaks the application into planned, documented steps.

[M8] What type of application is suitable for time-driven sequencing?

Paint mixing application - Paint mixing runs on set times. Clamp-and-drill and cylinder applications need position sensors.