glossary below appears thanks to the generosity of Jon Stenerson, author
of Fundamentals of Programmable Logic Controllers, Sensors, and Communications.
| B | C | D | E
| F | G | H | I
| J | K | L | M
| N | O | P | Q
| R | S | T | U
| V | W | X | Y
- AC input module:
This is a module that converts a real-world AC input signal to the logic
level required by the PLC processor.
- AC output module:
Module that converts the processor logic level to an AC output signal
to control a real-world device.
- 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.
- Actuator: Output
device normally connected to an output module. An example would be an
air valve and cylinder.
- Address: Number
used to specify a storage location in memory.
- Ambient temperature:
Temperature that naturally exists in the environment. For example, the
ambient temperature of a PLC in a cabinet near a steel furnace is very
- 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.
- ANSI: American
National Standards Institute.
- ASCII: American
Standard Code for Information Interchange. A coding system used to represent
letters and characters. Seven-bit ASCII can represent 128 different
combinations. Eight-bit ASCII (extended ASCII) can represent 256 different
- Asynchronous communications:
Method of communications that uses a series of bits to send data between
devices. There is a start bit, data bits (7 or 8), a parity bit (odd,
even none, mark, or space), and stop bits (1, 1.5, or 2). One character
is transmitted at a time. RS-232 is the most common.
- 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.
- BEUG (BITBUS European
Users Group): BEUG is a nonprofit organization devoted to spreading
the BITBUS technology and organizing a basic platform where people using
BITBUS can share application experiences.
- Binary: Base two
number system. Binary is a system in which ones and zeros are used to
- 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
- BITBUS: It is one
of the most widely used fieldbuses. It was promoted as a standard in
1990 by a special committee of the IEEE (standard IEEE-1118 1990).
- Boolean: Logic
system that uses operators such as AND, OR, NOR, and NAND. This is the
system that is utilized by PLCs, although it is usually made invisible
by the programming software for the ease of the programmer.
- Bounce: This is
an undesirable effect. It is the erratic make and break of electrical
- Branch: Parallel
logic path in a ladder diagram.
- Byte: Eight bits
or two nibbles. (A nibble is 4 bits.)
- Cascade: Programming
technique that is used to extend the range of timers and counters.
- CENELEC: European
Committee for Electrotechnical Standardization. It develops standards
which cover dimensional and operating characteristics of control components.
- Central processing
unit (CPU): Microprocessor portion of the PLC. It is the portion of
the PLC that handles the logic.
- Color mark sensor:
Sensor that was designed to differentiate between two different colors.
They actually differentiate on the basis of contrast between the two
- Complement: The
complement is the inverse of a digital signal.
- CMOS (complementary
metal-oxide semiconductor): Integrated circuits that consume very little
power and also have good noise immunity.
- 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. Used 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.
- CSA (Canadian Standards
Organization): Develops standards, tests products and provides certification
for a wide variety of products.
- 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.
- Cyclic Redundancy
Check (CRC): A calculated value, based on the content of a communication
frame. It is inserted in the frame to enable a check of data accuracy
after receiving the frame across a network. BITBUS uses the standard
- Dark-on: Refers
to a photosensor's output. If the sensor output is on when no object
is sensed, it is called a dark-on sensor.
- Data highway: This
is a communications network that allows devices such as PLCs to communicate.
They are normally proprietary, which means that only like devices of
the same brand can communicate over the highway.
- 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.
- Diagnostics: Devices
normally have software routines that aid in identifying and finding
problems in the device. They identify fault conditions in a system.
- Digital output:
An output that can have two states: on or off. These are also called
- Distributed processing:
The concept of distributed processing allows individual discrete devices
to control their area and still communicate to the others via a network.
The distributed control takes the processing load off the "host" system.
Documentation is descriptive paperwork that explains a system or program.
It describes the system so that the technician can understand, install,
troubleshoot, maintain, or change the system.
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 it is 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.
- Expansion rack:
A rack added to a PLC system when the application requires more modules
than the main rack can contain. A remote rack is sometimes used to permit
I/O to be remotely located from the main rack.
- False: Disabled
logic state (off).
- Fault: Failure
in a system that prevents normal operation of a system.
- 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.
- Flowchart: Used
to make program design easier.
- 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.
- Frame: Packet of
bits that will be transmitted across a network. A frame contains a header,
user data and an end of frame. The frame must contain all the necessary
information to enable the sender and receiver(s) of the communication
to decode the user's data and to ensure that this data is right.
- Full duplex: Communication
scheme where data flows in both directions simultaneously.
- Ground: Direct
connection between equipment (chassis) and earth ground.
- Half duplex: Communication
scheme where data flows in both directions but in only one direction
at a time.
- Hard contacts:
Physical switch connections.
- Hard copy: Printed
copy of computer information.
- HDLC (High-level
Data Link Control): Standard protocol of communication oriented in message
transmission (frames). The user's data field in an HDLC-frame can be
of a free number of bits. The SLDC is a subset of the HDLC that defines
the whole protocol in more detail and is byte-oriented.
- Hexadecimal: Numbering
system that utilizes base 16.
- Host computer:
One to which devices communicate. The host may download or upload programs,
or the host might be used to program the device. An example would be
a PLC connected to a microcomputer. The host (microcomputer) "controls"
the PLC by sending programs, variables, and commands. The PLC controls
the actual process but at the direction and to the specifications of
- 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
- IEC (International
Electrotechnical Commission): Develops and distributes recommended safety
and performance standards.
- IEEE: Institute
of Electrical and Electronic Engineers.
- Image table: Area
used to store the status of input and output bits.
- Incremental: This
term typically refers to encoders. Encoders provide logic states of
0 and 1 for each successive cycle of resolution.
- Instruction set:
Instructions that are available to program the PLC.
- Intelligent I/O:
PLC modules that have a microprocessor built in. An example would be
a module that would control closed-loop positioning.
- Interfacing: Connection
of a PLC to external devices.
- 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 that a 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.
- Isolation: Used
to segregate real-world inputs and outputs from the central processing
unit. Isolation assures that even if there is a major problem with real-world
inputs or outputs (such as a short), the CPU will be protected. This
isolation is normally provided by optical isolation.
- K: Abbreviation
for the number 1000. In computer language it is equal to two to the
tenth, or 1024.
- Keying: Technique
to ensure that modules are not put in the wrong slots of a PLC. The
user sets up the system with modules in the desired slots. The user
then keys the slots to assure that only a module of the correct type
can be physically installed.
- Ladder diagram:
Programmable controller language that uses contacts and coils to define
a control sequence.
- LAN: See Local
- Latch: An instruction
used in ladder diagram programming to represent an element that retains
its state during controlled toggle and power outage.
- 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.
- Light-on sensor:
This refers to a photosensor's output. If the output is on when an object
is sensed, the sensor is a light-on sensor.
- Linear output:
- 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.
- Load resistor:
A resistor connected in parallel with a high-impedance load to enable
the output circuit to output enough current to ensure proper operation.
- Local area network
(LAN): A system of hardware and software designed to allow a group of
intelligent devices to communicate within a fairly close proximity.
- Lockout: The placement
of a lockout device on an energy isolating device, in accordance with
an established procedure, to ensure that the energy isolating device
and the equipment being controlled cannot be operated until the lockout
device is removed.
- Lockout device:
A device that utilizes a positive means such as a lock, either key or
combination type, to hold an energy isolating device in the safe position
and prevent the energizing of a machine or equipment.
- LSB: Least significant
- Machine language:
Control program reduced to binary form.
- MAP (manufacturing
automation protocol): "Standard" developed to make industrial devices
communicate more easily. Based on a seven-layer model of communications.
- 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
- Memory map: Drawing
showing the areas, sizes, and uses of memory in a particular PLC.
- Microsecond: A
microsecond is one millionth (0.000001) of a second.
- Millisecond: A
millisecond is one thousandth (.001) of a second.
- Mnemonic codes:
Symbols designated to represent a specific set of instructions for use
in a control program. An abbreviation given to an instruction: usually
an acronym that is made by combining the initial letters or parts of
- MSB: Most significant
- NEMA (National
Electrical Manufacturers Association): Develops standards that define
a product, process, or procedure. The standards consider construction,
dimensions, tolerances, safety, operating characteristics, electrical
rating and so on. They are probably best known for their rating system
for electrical cabinets.
- Network: System
that is connected to devices or computers for communication purposes.
- Node: Point on
the network that allows access.
- Noise: Unwanted
electrical interference in a programmable controller or network. It
can be caused by motors, coils, high voltages, welders, and so on. It
can disrupt communications and control.
- 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.
- NOR: The logic
gate that results in zero unless both inputs are zero.
- NOT: The logic
gate that results in the complement of the input.
- 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
- One-shot contact:
Contact that is only on for one scan when activated.
- Operating system:
The fundamental software for a system that defines how it will store
and transmit information.
- Optical isolation:
Technique used in I/O module design that provides logic separation from
- OR: Logic gate
that results in 1 unless both inputs are 0.
- Parallel communication:
A method of communications where data is transferred on several wires
- 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
- Programmable controller:
A special-purpose computer. Programmed in ladder logic. It was also
designed so that devices could be easily interfaced with it.
- Pulse modulated:
Turning a light source on and off at a very high frequency. In sensors
the sending unit pulse modulates the light source. The receiver only
responds to that frequency. This helps make photo-sensors immune to
- 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.
- Rack: PLC chassis.
Modules are installed in the rack to meet the user's need.
- Radio frequency
(RF): Communications technology in which there is a transmitter/receiver
and tags. The transmitter/ receiver can read or write to the tags. There
are active and passive tags available. Active tags are battery powered.
Passive tags are powered from the RF emitted from the transmitter. Active
tags have a much wider range of communication. Either tag can have several
K of memory.
- RAM (random access
memory): Normally considered user memory.
- Register: Storage
area. It is typically used to store bit states or values of items such
as timers and counters.
The ability to repeat movements or readings. For a robot it would be
how accurately it would return to a position time after time. Repeatability
is unrelated to resolution and is usually 3 to 10 times better than
- Resolution: A measure
of how closely a device can measure or divide a quantity. For example,
in an encoder resolution would be defined as counts per turn. For an
analog to digital card it would be the number of bits of resolution.
For example, for a 12-bit card the resolution would be 4096.
- 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.
Photosensor that sends out a light which is reflected from a reflector
back to the receiver (the receiver and emitter are in the same housing).
When an object passes through it breaks the beam.
- RF (radio frequency):
See radio frequency.
- 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
- RS-422 and RS-423:
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-423 uses the unbalanced mode. Its speeds
and transmission distances are much greater than RS-232 but less than
- RS-449: Electrical
standard for RS-422/RS-423. It is a more complete standard than the
RS-232. It specifies the connectors to be used also. 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.
- Rung: Group of
contacts that control one or more outputs. In a ladder diagram it is
the horizontal lines on the diagram.
- 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
- SDLC: Serial Data
Link Control, subset of the HDLC used in a large number of communication
systems like Ethernet, ISDN, BITBUS, and others. This protocol defines
the structure of the frames and the values of a number of specific fields
in these frames.
- Sensitivity: Refers
to a device's ability to discriminate between levels. If it's a sensor
it would relate to the finest difference it could detect. If it were
an analog module for a PLC, it would be the smallest change it could
- Sensor: Device
used to detect change. Normally it is a digital device. The outputs
of sensors change state when they detect the correct change. Sensors
can be analog or digital in nature. They can also be purchased with
normally closed or normally open outputs.
- 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.
- Speech modules:
Used by a PLC to output spoken messages to operators. The sound is typically
digitized human speech stored in the module's memory. The PLC requests
the message number to play it.
- Tagout: The placement
of a tagout device on an energy isolating device, in accordance with
an established procedure, to indicate that the energy isolating device
and the equipment being controlled may not be operated until the tagout
device is removed.
- Tagout device:
A prominent warning device, such as a tag and a means of attachment,
which can be securely fastened to an energy isolating device in accordance
with an established procedure, to indicate that the energy isolating
device and the equipment being controlled may not be operated until
the tagout device is removed.
- 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.
- TOP (technical
and office protocol): Communication standard that was developed by Boeing.
Based on the contention access method. The MAP standard is meant for
the factory floor and TOP is meant for the office and technical areas.
- Transitional contact:
Contact that changes state for one scan when activated.
- True: This is the
enabling logic state. Generally associated with a "one" or "high" 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-bit word.