A Guide to Common Automation Terms

The glossary below is an excerpt from Jon Stenerson’s “Fundamentals of Programmable Logic Controllers, Sensors, and Communications”. For the complete list, go to the Technical Support section of the AutomationDirect Web site.

Common Automation Terms

• Accumulated value: Applies to the use of timers and counters. The accumulated value is the present count or time.

• Accuracy: The deviation between the actual position and the theoretical position.

• Address: Number used to specify a storage location in memory.

• Analog: Signal with a smooth range of possible values. For example, a temperature that could vary between 60 and 300 degrees would be analog in nature.

• Backplane: Bus in the back of a PLC chassis. It is a printed circuit board with sockets that accept various modules.

• Baud rate: Speed of serial communications. The number of bits per second transmitted. For example, RS-232 is normally used with a baud rate of 9600. This would be about 9600 bits per second. It takes about 10 bits in serial to send an ASCII character so that a baud rate of 9600 would transmit about 960 characters per second.

• Binary: Base-two number system. Binary is a system in which ones and zeros are used to represent numbers.

• Binary-coded decimal (BCD): A number system. Each decimal number is represented by four binary bits. For example, the decimal number 967 would be represented by 1001 0110 0111 in BCD.

• Bit: Binary digit. The smallest element of binary data. A bit will be either a zero or a one.

• Byte: Eight bits or two nibbles. (A nibble is 4 bits.)

• Central processing unit (CPU): Microprocessor portion of the PLC. It is the portion of the PLC that handles the logic.

• Compare instruction: PLC instruction that is used to test numerical values for equal, greater than, or less than relationships.

• Contact: Symbol used in programming PLCs to represent inputs. There are normally open and normally closed contacts. Contacts are also the conductors in electrical devices such as starters.

• Contactor: Special-purpose relay that is used to control large electrical current.

• Current sinking: Refers to an output device (typically an NPN transistor) that allows current flow from the load through the output to ground.

 Current sourcing: Output device (typically a PNP transistor) that allows current flow from the output through the load and then to ground.

• Data table: A consecutive group of user references (data) of the same size that can be accessed with table read/write functions.

• Debugging: Process of finding problems (bugs) in any system.

• Digital output: An output that can have two states: on or off. These are also called discrete outputs.

• Downtime: The time a system is not available for production or operation is called downtime. Downtime can be caused by breakdowns in systems.

• EEPROM: Electrically erasable programmable read only memory.

• Energize: Instruction that causes a bit to be a one. This turns an output on.

• Examine-off: Contact used in ladder logic. It is a normally closed contact. The contact is true (or closed) if the real-world input associated with itis off.

• Examine-on: Contact used in ladder logic programming. Called a normally open contact. This type of contact is true (or closed) if the real- world input associated with it is on.

• Firmware: A series of instructions contained in read-only memory (ROM) that are used for the operating system functions. Some manufacturers offer upgrades for PLCs. This is often done by replacing a ROM chip. Thus the combination of software and hardware lead to it being called firmware.

• Force: Refers to changing the state of actual I/O by changing the bit status in the PLC. In other words, a person can force an output on by changing the bit associated with the real-world output to a 1. Forcing is normally used to troubleshoot a system.

• Ground: Direct connection between equipment (chassis) and earth ground.

• Hexadecimal: Numbering system that utilizes base 16.

• Hysteresis: A dead band that is purposely introduced to eliminate false reads in the case of a sensor. In an encoder hysteresis would be introduced in the electronics to prevent ambiguities if the system happens to dither on a transition.

• Image table: Area used to store the status of input and output bits.

• Instruction set: Instructions that are available to program the PLC.

• I/O (input/output): Used to speak about the number of inputs and outputs that are needed for a system, or the number of inputs and outputs thata particular programmable logic controller can handle.

• IP rating: Rating system established by the IEC that defines the protection offered by electrical enclosures. It is similar to the NEMA rating system.

• K: Abbreviation for the number 1000. In computer language it is equal to two to the tenth, or 1024.

• Ladder diagram: Programmable controller language that uses contacts and coils to define a control sequence.

• LAN: See Local area network.

• Leakage current: Small amount of current that flows through load-powered sensors. The small current is necessary for the operation of the sensor. The small amount of current flow is normally not sensed by the PLC input. If the leakage is too great a bleeder resistor must be used to avoid false inputs at the PLC.

• LED (light-emitting diode): A solid-state semiconductor that emits red, green, or yellow light or invisible infrared radiation.

• Line driver: A line driver is a differential output driver intended for use with a differential receiver. These are usually used where long lines and high frequency are required and noise may be a problem.

• Line-powered sensor: Normally, three-wire sensors, although four-wire sensors also exist. The line-powered sensor is powered from the power supply. A separate wire (the third) is used for the output line.

 Load: Any device that current flows through and produces a voltage drop.

 Load-powered sensor: A load-powered sensor has two wires. A small leakage current flows through the sensor even when the output is off. The current is required to operate the sensor electronics.

 LSB: Least significant bit.

• Master: The master on a network is the device that controls communication traffic. The master of a network usually polls every slave to check if it has something to transmit. In a master-slave configuration, only the active master can place a message on the bus. The slave can reply only if it receives a frame from the master that contains a logical token that explicitly enables the slave to reply.

• Master control relay (MCR): Hardwired relay that can be deenergized by any hardwired series-connected switch. Used to deenergize all devices. If one emergency switch is hit it must cause the master control relay to drop power to all devices. There is also a master control relay available in most PLCs. The master control relay in the PLC is not sufficient to meet safety requirements.

• Microsecond: A microsecond is one millionth (0.000001) of a second.

• Millisecond: A millisecond is one thousandth (.001) of a second.

• MSB: Most significant bit.

• Network: System that is connected to devices or computers for communication purposes.

• Nonretentive coil: A coil that will turn off upon removal of applied power to the CPU.

• Nonretentive timer: Timer that loses the time if the input enable signal is lost.

• Nonvolatile memory: Memory in a controller that does not require power to retain its contents.

• Octal: Number system based on the number 8, utilizing numbers 0 through 7.

• Off-delay timer: This is a type of timer that is on immediately when it receives its input enable. It turns off after it reaches its preset time.

• Off-line programming: Programming that is done while not attached to the actual device. For example, a PLC program can be written for a PLC without being attached. The program can then be downloaded to the PLC.

• On-delay timer: Timer that does not turn on until its time has reached the preset time value.

• One-shot contact: Contact that is only on for one scan when activated.

• Parity: Bit used to help check for data integrity during a data communication.

• Peer-to-peer: This is communication that occurs between similar devices. For example, two PLCs communicating would be peer-to-peer. A PLC communicating to a computer would be device-to-host.

• PID (Proportional, integral, derivative) control: Control algorithm that is used to closely control processes such as temperature, mixture, position, and velocity. The proportional portion takes care of the magnitude of the error. The integral takes care of small errors over time. The derivative compensates for the rate of error change.

• PLC: Programmable logic controller.

• Programmable controller: A special-purpose computer. Programmed in ladder logic. It was also designed so that devices could be easily interfaced with it.

• PPR (Pulses per revolution): This refers to the number of pulses an encoder produces in one revolution.

• Quadrature: Two output channels out of phase with each other by 90 degrees.

• Retentive coil: A coil that will remain in its last state, even though power was removed.

• Retentive timer: Timer that retains the present count even if the input enable signal is lost. When the input enable is active again, the timer begins to count again from where it left off.

• ROM (read-only memory): This is operating system memory. ROM is nonvolatile. It is not lost when the power is turned off.

• RS-232: Common serial communications standard. This standard specifies the purpose of each of 25 pins. It does not specify connectors or which pins must be used.

• RS-422: Standards for two types of serial communication. RS-422 is a balanced serial mode. This means that the transmit and receive lines have their own common instead of sharing one like RS-232. Balanced mode is more noise immune. This allows for higher data transmission rates and longer transmission distances.

• RS-485: Similar to the RS-422 standard. Receivers have additional sensitivity which allows for longer distances and more communication drops. Includes some extra protection for receiver circuits.

• Scan time: Amount of time it takes a programmable controller to evaluate a ladder diagram. The PLC continuously scans the ladder diagram. The time it takes to evaluate it once is the scan time. It is typically in the low-millisecond range.

• Sequencer: Instruction type that is used to program a sequential operation.

• Serial communication: Sending of data one bit at a time. The data is represented by a coding system such as ASCII.

• Slave: On a master-slave configured network, there is usually one master and several slaves. The slaves are nodes of the network that can transmit informations to the master only when they are polled (called) from it. The rest of the time a slave never transmits anything.

• Thermocouple: A thermocouple is a sensing transducer. It changes a temperature to a current. The current can then be measured and converted to a binary equivalent that the PLC can understand.

• Thumbwheel: Device used by an operator to enter a number between 0 and 9. Thumbwheels are combined to enter larger numbers. Thumbwheels typically output BCD numbers to a device.

• Timer: Instruction used to accumulate time until a certain value is achieved. The timer then changes its output state.

• UL (Underwriters Laboratory): Organization that operates laboratories to investigate systems with respect to safety.

• User memory: Memory used to store user information. The user’s program, timer/counter values, input/output status, and so on, are all stored in user memory.

• Volatile memory: Memory that is lost when power is lost.

• Watchdog timer: Timer that can be used for safety. For example, if there is an event or sequence that must occur within a certain amount of time, a watchdog timer can be set to shut the system down in case the time is exceeded.

• Word: Length of data in bits that a microprocessor can handle. For example, a word for a 16-bit computer would be 16 bits long, or two bytes. A 32-bit computer would have a 32-bi


Originally Published: June 1, 2006