| AC |
Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz. |
| actions |
An output instruction in a PLC program. An action is the result of an input condition being met. |
| address |
The unique identification of the location of an input device, output device, or saved data within the CPU. Addresses may be labeled on input/output modules for connected input/output devices. |
| alternating current |
AC. Electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction 60 times per second, or 60 hertz. |
| amperage |
The strength or intensity of an electrical current. Amperage is measured in amperes. |
| analog |
An electrical signal that continuously varies within a specific range. Analog signals communicate variable conditions, such as pressure or temperature. |
| analog to digital converter |
ADC. A component within an analog input module that changes variable signals to binary numbers. An analog to digital converter changes the continuous fluctuations from an analog input device to a digital, or binary, signal for the CPU. |
| backplane |
The portion of a modular PLC where I/O modules, the CPU, and power supply are connected. The backplane provides communication paths for electrical signals. |
| binary digits |
A 1 or a 0 in the binary numbering system. Binary digits are what a CPU uses to process information. |
| central processing unit |
CPU. The microprocessor device inside a computer that controls system activities. The central processing unit executes a PLC program and performs all logic activities for the system. |
| chassis |
An enclosure with slots in it that is used to connect and hold the multiple units of a modular PLC. A chassis typically allows room for additional units. |
| commands |
A specific instruction given to a computer to perform a task or function. Commands can be used to override specific instructions for a CPU. |
| common terminal |
A shared connection point on an input/output module that connects to a power source. A common terminal is at the bottom of the screw terminal. |
| condition |
An input instruction in a PLC program. A condition must be met by PLC components in order to result in an action. |
| CPU |
Central processing unit. The microprocessor device inside a computer that controls system activities. The CPU executes a PLC program and performs all logic activities for the system. |
| data file |
The area of a CPU's memory that contains the status of input devices, output devices, and other program instructions. Data files store information, such as addresses or conditions for associated devices. |
| DC |
Direct current. Electricity that forms when electrons flow in one continuous direction. DC is used in many PLC circuits. |
| digital multimeter |
DMM. A device that can measure voltage, current, or resistance. A digital multimeter is used for electrical maintenance and troubleshooting. |
| digital to analog converter |
DAC. A component within an analog output module that changes binary numbers to analog signals. A digital to analog converter changes the binary numbers from the CPU into a continuous variable signal for an analog output device. |
| direct current |
DC. Electricity that forms when electrons flow in one continuous direction. Direct current is used in many PLC circuits. |
| discrete |
An electrical signal that has only two states. Discrete signals are either on or off. |
| electrical noise |
A power line disturbance caused by sudden changes in the load. Electrical noise is problematic to a PLC because the components cannot differentiate between an intended electrical spike and an unintended electrical spike. |
| electrical signal |
A current that carries data from one device to another. Electrical signals are either analog or discrete. |
| electromagnetic |
Electrical noise in a circuit that can interrupt or degrade the performance of the circuit. Electromagnetic interference can be reduced by using shielded cable or fiber optic cables. |
| field device |
A component that connects to an input/output module on a PLC and performs an action. Each field device, or input/output device, has its own electrical requirement. |
| fixed PLC |
A type of PLC in which the central processing unit, input/output module, and power supply are housed in a self-contained unit. A fixed PLC is used for applications requiring basic process control. |
| force command |
A troubleshooting command on a CPU used to override the input/output status of a device in order to test or debug a program. The force command is activated and removed using a programming device. |
| I/O module |
Input/output module. The portion of a PLC that contains connection points for input/output devices and circuitry to convert electrical current. An I/O module may be input only, output only, or a combination. |
| I/O points |
Input/output points. A connection point on an input/output module for a field device wire. I/O points allow an electrical connection between field devices, or input/output devices, and the input/output module. |
| inductive |
A load that uses a magnetic field generated by electricity. Inductive devices are likely to cause excess electrical noise. |
| input module |
The portion of a PLC that receives high-voltage electrical signals from input devices. The input module contains electrical conversion circuitry to reduce the voltage so it can safely send a signal to the CPU. |
| input/output devices |
I/O devices. A component that connects to an input/output module on a PLC and performs an action. Each input/output device, or field device, has its own electrical requirement. |
| input/output module |
I/O module. The portion of a PLC that contains connection points for input/output devices and circuitry to convert electrical current. An input/output module may be input only, output only, or a combination. |
| input/output points |
I/O points. A connection point on an input/output module for a field device wire. Input/output points allow an electrical connection between field devices and the input/output module. |
| isolated |
An electrical circuit connection that protects components from electrical damage. Electrical isolation blocks DC signals between circuits but allows AC signals to pass. |
| limit switches |
A type of contact that detects the presence or absence of an object. Limit switches may be used as discrete input devices. |
| line filters |
An electronic filter placed between an electrical component and an external power line. Line filters can reduce electrical noise and interference in a power line. |
| micro PLC |
A type of PLC with 32 or fewer input/output points. Micro PLCs are small, but they are still able to offer process control capabilities. |
| milliamperes |
mA. A unit of current equal to one thousandth of an ampere. Milliamperes may be used to measure current of analog input/output devices. |
| milliseconds |
ms. One thousandth of a second. Milliseconds can be used to measure the response times of programmable logic controllers. |
| modular PLC |
A type of PLC that has removable and interchangeable units, including the central processing unit, power supply, and input/output modules. A modular PLC can process a larger variety of input/output signal levels to meet the needs of a more complex application. |
| motor starter |
A mechanical device that turns on or off to start or stop a motor. Motor starters may be used as discrete output devices. |
| negative terminal |
A connection point on a battery or other voltage source that has more electrons than the positive connection point. Electrons flow from the negative terminal through the external circuit to the positive terminal. |
| output module |
The portion of a PLC that receives low voltage output signals from the CPU. The output module contains electrical conversion circuitry to increase the voltage it sends to output devices. |
| overcurrent |
Excess current flowing through a circuit. Overcurrent in an output module can be prevented by using a fuse or circuit breaker. |
| override |
Suspends the normal functionality of a device. An override can be activated using a force command. |
| PLCs |
Programmable logic controllers. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. A PLC can replace many physical relays and hard-wired connections in a process. |
| positive terminal |
A connection point on a battery or other power source toward which electrons flow through the external circuit. The positive terminal's position on a PLC power source determines the direction of current flow. |
| power supply |
The component that provides power to the central processing unit and input/output modules on a PLC. The power supply sends low voltage, direct current to the CPU and input/output modules. |
| pressure transducers |
A device that converts variations in pressure into an analog electrical signal. Pressure transducers are a type of analog input device. |
| process control |
An approach to manufacturing that attempts to closely monitor and control all aspects of a process. Process control is provided by PLCs. |
| program |
The instructions entered into a CPU that direct the input/output devices to perform actions. A program contains lines of instruction. |
| programmable logic controller |
PLC. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. A programmable logic controller can replace many physical relays and hard-wired connections in a process. |
| programming device |
A peripheral device connected to a PLC that someone uses to enter a program into a CPU's memory. Programming devices include personal computers and handheld programmers. |
| proportional control valves |
A control valve used in fluid systems that can change its output in response to an input value. A proportional control valve is a type of analog output device. |
| proximity switches |
A type of sensor that can detect the presence or absence of nearby objects without any physical contact. Proximity switches may be used as discrete input devices. |
| push buttons |
A type of contact that turns on or off to control some aspect of a machine or process. Push buttons are common discrete input devices. |
| relays |
A device that controls an electrical circuit by opening and closing contacts in another circuit. Relays are common discrete output devices. |
| resistors |
A device that controls current by opposing the flow of current. Resistors also help to reduce electrical noise in a power line. |
| run mode |
A CPU operating mode that is active when the CPU is executing the steps of a program. Run mode is required when using force commands. |
| sensors |
A device that detects a change in a physical condition and turns it into an electrical signal. Sensors are common analog input devices that respond to variable conditions, such as pressure, temperature, or flow. |
| shielded cable |
Cable that has an outer layer of insulating material covering an inner layer of conducting material. Shielded cable is used to reduce electrical noise in power lines. |
| sink/source |
A wiring configuration that allows current to flow in both directions. In a sink/source circuit, all shared common terminals are wired as sinking only or sourcing only. |
| sinking |
A wiring arrangement on a DC circuit in which the I/O device provides current to the I/O module. In a sinking configuration, the I/O device is the source of current. |
| snubber circuit |
An electrical circuit that absorbs energy and suppresses voltage spikes. A snubber circuit is used with inductive devices. |
| solenoids |
A device that converts electrical energy into linear mechanical force. Solenoids are common discrete output devices. |
| sourcing |
A wiring arrangement on a DC circuit in which the I/O module provides current to the I/O device. In a sourcing configuration, the I/O module is the source of current. |
| status LEDs |
An indicator light made of a semiconductive material that indicates a device's status or mode. Status LEDs on the input/output module show the condition of all connected input/output devices. |
| terminal block |
An electrical connector in which the device wires are clamped down to connection points. Terminal blocks allow many input/output devices to connect to an input/output module. |
| transducer |
A device that converts variable physical quantities into analog electrical signals. Transducers are used with analog input devices. |
| two-state |
Existing in one of two states at any given time. Two-state devices for a PLC send discrete signals that the CPU processes using binary digits. |
| variable frequency drive |
VFD. A device that converts incoming AC power into other desired frequencies to allow for AC motor speed control. A variable frequency drive is a common analog output device. |
| volts |
V. The unit of measure for electromagnetic force or pressure. Volts are an indication of the amount of current flowing through a circuit. |