PLC Hardware
This "brain" of the PLC system executes the control program and processes data.
What is the Central Processing Unit (CPU)?
This is the most common graphical programming language used for PLCs, designed to look like electrical circuit diagrams.
What is Ladder Logic (LAD)?
This type of input device only has two states: on or off.
What is a discrete (or digital) input?
This is the continuous three-step process a PLC follows: read inputs, execute program, and update outputs.
What is the Scan Cycle?
These small lights on the front of PLC modules are the first thing a technician should check to verify power and signal status.
What are Status Indicators (or LEDs)?
PLCs were originally designed to replace this type of hard-wired control system.
What are relay logic systems? (Hard-wired relay panels).
These are the two vertical lines in a ladder logic diagram that represent the "power rails."
What are rails?
Pushbuttons, limit switches, and proximity sensors are all examples of this.
What are input devices?
This PLC mode allows the processor to execute the program and control the outputs.
What is RUN mode?
This software tool allows a technician to view the PLC program in real-time to see which instructions are "highlighted" or true.
What is Online Monitoring?
This hardware component converts incoming AC line voltage to the low-voltage DC power required by the PLC’s internal circuitry.
What is the Power Supply?
In ladder logic, this instruction is true if the physical input device is "on" or closed.
What is Examine if Closed (XIC)? (Also known as a Normally Open contact).
This type of I/O module can handle varying signals, such as 0-10V or 4-20mA.
What is an Analog I/O module?
This is the amount of time it takes for a PLC to complete one full internal cycle.
What is the scan time?
To safely work on field devices connected to a PLC, you must follow this safety procedure to ensure power cannot be accidentally restored.
What is Lockout/Tagout (LOTO)?
In a modular PLC, this is the physical assembly that holds the various modules and provides the communication bus between them.
What is the rack (or chassis)?
This programming element allows a PLC to perform an action only after a specific duration has passed.
What is a Timer? (Acceptable: TON or TOF).
This component inside an I/O module provides electrical separation between the field wiring and the PLC's sensitive internal CPU.
What is an optical isolator (or opto-isolator)?
This specific memory area in the PLC stores the current status of all hardware input and output points.
What is the I/O Image Table?
This software feature allows a technician to manually turn a PLC bit "on" or "off" regardless of the physical input or program logic.
What is a Force (or Forcing)?
This type of memory is used to store the user program and remains intact even when power is lost.
What is Non-Volatile Memory? (Acceptable: EEPROM or Flash memory).
This type of logic gate output is "true" only if all of its inputs are "true."
What is an AND gate?
This term describes the maximum current an output module can "sink" or "source" to a field device.
What is the current rating?
In this mode, the PLC is powered on but the program is not executing and outputs are typically disabled for safety during programming.
What is PROGRAM mode?
If the "FAULT" or "ERR" light is flashing on the CPU, this is the first place you should look within the programming software to identify the specific error.
What is the Processor Status (or Error Log)?