The glossary below appears thanks to the generosity of Jon Stenerson, author of
Industrial Automation and Process Control.
- 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
- 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.
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 high.
- 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 combinations.
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
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
- 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.)
Programming technique that is used to extend the range of timers and
- CENELEC: European
Committee for Electrotechnical Standardization. It develops standards
which cover dimensional and operating characteristics of control
- 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.
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.
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.
Devices normally have software routines that aid in identifying and
finding problems in the device. They identify fault conditions in a
- Digital output: An
output that can have two states: on or off. These are also called
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.
- Downtime:The time a system is not available for production or operation is
called downtime. Downtime can be caused by breakdowns in systems.
Electrically erasable programmable read only memory.
Instruction that causes a bit to be a one. This turns an output on.
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
- 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
- Full duplex:
Communication scheme where data flows in both directions
- 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.
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 the
- 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
- 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.
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,
- 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
- Ladder diagram:
Programmable controller language that uses contacts and coils to define
a control sequence.
- LAN: See Local area
- 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.
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.
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
- 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
- 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
- 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.
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
- 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.
communication: A method of communications where data is transferred on
several wires simultaneously.
- 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
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
- 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.
- Repeatability: 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
- 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 be
- 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
- 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.
Instruction type that is used to program a sequential operation.
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.
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.