Active High

A digital signal that represents active, on, or true when its voltage is higher than the other logic state (low). Active-high signals can range from a few volts DC to as high as 24V DC, depending on the logic family or devices in use.

Active Low

A digital signal that represents active, on, or true when its voltage is lower than the other logic state (high). Active-low signals can range from digital ground to a few tenths of a volt.


1. A number that specifies where data is stored (e.g., memory or I/O address). 2. A name or number identifying a unique device or group of devices on a network (e.g., Modbus or MAC address).


A formula with a specific set of instructions for performing a series of computational procedures or calculations.


1. Using a standard baud rate that the operating system understands, which is substituted with a different baud rate at the device driver level. Common in serial adapters with non-standard oscillators. 2. Assigning an additional name to a string or object.


Commonly shortened from ampere. A base unit for measuring electrical current. One amp is equal to 6.242 x 10^18 electrons, or one Coulomb of electric charge, passing a given point in one second (1 A = 1 C/s). See: Ampere, Coulomb


(Symbol: A) An SI base unit for measuring electrical current, commonly shortened to amp. Named after André-Marie Ampère (1775–1836). See: Amp


A type of signal that varies continuously (lighter to darker, 4 to 20 mA, and so on), as opposed to a digital signal that can exist in only one of two possible states.

Analog Ground

The location in a system that serves as a reference ground for all analog signals. Some circuits may combine analog ground and digital ground, but most circuits separate them to reduce noise and ground currents.

AND Gate

A circuit that performs an AND operation based on the state of its inputs.


American Standard Code for Information Interchange – An 8-bit binary code that represents characters and symbols in the Roman (English) alphabet. ASCII includes codes that controlled older communication devices, thus the CRTL key on computer keyboards.


In serial communications, refers to the transmission of data without an encoded or shared clock signal.



(Symbol: Bd) An SI unit of measure named after Emile Baudot. Synonymous with Baud Rate. See: Baud Rate

Baud Rate

The number of symbols or pulses that are transmitted per second. See: Baud

Baud Rate Generator

An internal circuit (on Zilog Z8523x ESCC, Zilog Z16C32 IUSC, etc.) that divides an external oscillator by an integer value to produce a clock that is used to generate or decode a synchronous serial data stream. Abbreviated BRG.


Binary-Coded Decimal – The encoding of decimal numbers as four-bit binary values from 00002 for 0, to 10012 for 9. BCD uses only 10 of the 16 4-bit combinations.


Acronym for Bit Error Rate Test. A utility that transmits a known bit pattern and compares it to the bit pattern received to determine if any bits were changed. The Sealevel WinSSD utility includes a BERT that is useful for verifying the operation and performance of a serial port.


A numbering system that allows for only two states, usually 1 and 0.


The most basic unit of information or data in computing or serial communications. Bit is a contraction of ‘binary digit’ and is denoted by the numerical digits 0 or 1.

Bit Wise

An operation, usually between two bytes or words, in which corresponding bits take part in an operation.

Blocking Diode

A diode, also called an isolation diode, that stops, or blocks, current from flowing through a circuit. Typically used in a battery circuit to prevent the reverse biasing of a battery by a more positive power supply.

Boolean Logic

A form of mathematics named after George Boole (1815-1864) who devised formal expressions for AND, OR, and INVERT operations.


Baud Rate Generator


An output device that operates high-current or high-voltage devices. Some manufacturers produce drivers specifically to control devices such as stepper motors or displays. See: Driver.


A signal that has passed through a buffer. See: Buffer.


A group of related electrical signals. 1. A control bus, an address, bus, a data bus, and so on. Some buses carry specific names, such as PCI Bus and Universal Serial Bus (USB). 2. A group of conductors that distribute power.


Acronym for Binary Term. In computing or serial communications, a byte of data describes a single character unit. Commonly confused with Octet to describe an 8-bit block of data. See Octet.



An electronic component that stores a charge and provides a reserve of power in a circuit. Typically used to smooth variations in a power-supply’s output voltage, and to provide power in the event of brief power failures.

Carry Current

The amount of current a relay’s contacts can safely conduct after the contacts close.


Acronym for Charging Downstream Port. Refers to a USB port defined in the Battery Charging Specification (BC v1.2) that supports data transfer as well as a minimum of 100mA and a maximum of 1.5A @ 5V DC to connected USB devices.

Chassis Ground

The ground point in a system, typically on a metal chassis, where signals connect to an earth ground. In most cases, a grounded chassis helps shield circuits from EMI and RFI, and provides a safety connection to ground. This type of ground should not carry current. See also: Analog Ground and Digital Ground.


The wire-wound electromagnetic core of a relay or solenoid. See also: Relay and Solenoid.


1. Abbreviation of "Communication" (e.g., COM Port). 2. Acronym for "Computer on Module", an integrated computing form factor (i.e., COM Express).

Common Reference

A single reference point for one or more signals.


In logic, an operation in which a logic 1 becomes a logic 0, and vice versa. In binary numbers, the complement of 101100 = 010011.


(Symbol: C) A unit for measuring the electrical charge accumulated over one second of continuously receiving one amp of current (1 C = 1 A * 1 s). Named after Charles-Augustin de Coulomb (1736-1806). See: Amp


Central Processing Unit – The decision-making part of a computer, usually found within a computer’s microprocessor.


Cyclic Redundancy Check


Comma Separated Values format. Actually, semicolon is more commonly used as separator than comma.

Current (Electrical)

A measure of the amount of electron flow in a circuit, typically measured in amperes (A) or milliamperes (mA).


Darlington Output

A configuration of output transistors that can handle high currents. Usually found on the outputs of sensors or buffers that drive relays or solenoids.


Data Communication Equipment. A communication device that controls the communication link. In serial communication, the DCE device is generally a modem or other serial device. See also DTE.


Acronym for Dedicated Charging Port. Refers to a USB port defined in the Battery Charging Specification (BC v1.2) that is used strictly for power, requires no host connection and provides no data transfer capabilities.


Acronym for Digital Display Interface. Depending on electrical interface and pin out, DDI can refer to DisplayPort, DVI, HDMI, SDVO or other digital video interfaces.


A decrease in the rating of device characteristics, depending on operating conditions.

Differential Manchester Encoding

A method of encoding serial data where a 1 is represented by a transition at the center of the bit cell with the opposite polarity from the transition at the center of the preceding bit cell. A 0 is represented by a transition at the center of the bit cell with the same polarity as the transition at the center of the preceding bit cell. In both cases, transitions at the beginning of the bit cell set up the level required to make the correct center transition. Also referred to as conditioned diphase or differential bi-phase encoding.


A system that uses discrete states to represent information.

Digital Ground

A common 0V ground reference for all digital signals. Digital ground and analog ground systems are usually wired separately to avoid introducing digital noise into the analog circuit.


An electronic component that lets current flow only in one direction.


A VESA standard digital video interface that can transmit video, audio, USB and other forms of data.


1. A driver circuit, or buffer, that operates high-current or high-voltage devices. 2. Driver software links application programs and specific I/O devices.

Dry Contact

1. Metallic contacts in a relay or switch that mechanically touch to make a contact. 2. Contacts through which no current flows. See: Wet Contact.


Data Terminal Equipment. A communication device that generates or receives data. In serial communication, the DTE device is generally a computer or host. See also DCE.


Acronym for Digital Visual Interface. The DVI was developed to provide a digital video transmission method to replace the analog VGA interface.


Earth Ground

The ground point in a system that provides the lowest voltage-reference point, or "earth." An earth ground usually connects to a power-line ground, a ground rod, or in some cases, cold-water plumbing. An earth ground should not carry current.


Electro Magnetic Interference – Energy induced into a circuit by radiated emissions. EMI may cause unpredictable results. See: RFI.


A network switch (PSE) that provides power to PoE-powered devices. See: PoE, PSE, Midspan


To provide power to a device or circuit. Typically to power a relay coil, thus forcing it to change the state of its contacts.

Excitation Voltage

A voltage that powers a sensor or transducer.



An acronym for First In, First Out. Describes the order data is manipulated in a buffer.


1. An electronic device, usually with two possible states, that signals an external event to a computer. 2. An internal CPU indicator that signals a condition such as register overflow or error. Sensed with software.

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Flip Flop

A bistable logic circuit that changes state due to an input event, generally a clock or pulse signal. A flip-flop remains in that state until the next input event causes it to "flip" or "flop" to its other state.

Floating Ground

An isolated Ground reference point that is not connected to Earth Ground. A battery is an example of a power source with a floating ground.

Floating Signal

An analog signal that has an isolated Ground reference point instead of an Earth Ground reference point. Examples include outputs from transformers, thermistors, battery-powered devices and optical isolators.

Form A

Form-A Relay – A relay that supplies normally-open (NO) SPST contacts.

Form B

Form-B Relay – A relay that supplies normally-closed (NC) SPST contacts.

Form C

Form-C Relay – A relay that supplies normally-open and normally-closed SPDT contacts.

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File Transfer Protocol

Full Duplex

Refers to the simultaneous transmission of data in two directions, each over a discrete signal path. In RS-485 networks, commonly called four-wire communication.



A logic device that performs Boolean-logic operations.


A signal that is enabled, allowed to operate, or allowed to pass through a circuit depending on the state of a separate logic condition or signal.


An acronym for Global Positioning System. Uses an array of orbiting satellites to provide accurate navigation around the world.


A zero-volt reference point in a system. Provides the reference for all other voltages.


Half Duplex

Refers to the transmission of data in one direction at a time, typically over the same signal path. In RS-485 networks, commonly called two-wire communication.


Acronym for High-Definition Multimedia Interface. A compact audio/video interface for transmitting a video signal, up to eight audio channels, a consumer-electronics control signal, and an Ethernet data connection.

Hex Digit

Contraction of hexadecimal digit. The term specifies four bits, or half an Octet. See Nibble.


Slang term for Hexadecimal Digit. The term specifies four bits, or half an Octet. See Nibble.

High Impedance

1. A high resistance that reduces current flow. 2. A third state in special logic devices that "disconnects" them from a bus.

High Side Switch

A switch that makes a connection directly to power at a higher voltage than that at the controlled load.


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Input/output, as in I/O port. See: Port.


Similar to resistance, an impedance represents the total opposition to the flow of current offered by a circuit. Impedance equals the vector sum of resistance and reactance, which is the complex resistance resulting from inductance and capacitance, not just pure resistance. Measured in ohms, and given the symbol, Z.

Inrush Current

A large charging current that flows into a capacitor or circuit when power is first applied.

Interposing Relay

A relay that isolates the circuit driving it, and switches a higher current or voltage than the driving circuit could provide. See: Relay.


A logic device that complements the logic state of its input. See: Complement.


Industry Standard Architecture. An 8-bit or 16-bit communication bus standard developed to provide expansion slots on motherboards of IBM compatible computers.


A condition that separates circuits so no current can flow between them. Special devices such as opto-couplers provide a signal path between two circuits, but without current flow between them.


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A logic circuit that takes a "snapshot" of information and saves it. Latches operate using an edge-triggered or a level-triggered control signal.


Light-Emitting Diode. A diode that emits light when current passes through it (forward biased). LEDs provide white light as well as most colors. LEDs usually require an external current-limiting resistor.

Logic Ground

A ground-reference point in a circuit for all logic signals. Usually kept separate from other grounds in a system due to noise concerns.

Logical AND

A Boolean-logic operation that produces a true output only when all the function or circuit inputs exist in the true state.

Logical NOT

The equivalent of an inversion operation, usually applied as part of another logic element or operation. See: Inverter.

Logical OR

A Boolean-logic operation that produces a false output only when all the function or circuit inputs exist in the false state.

Low Impedance

A low-resistance circuit that usually requires high current to drive it, as opposed to a high-impedance circuit.

Low Side Switch

A switch that makes a connection directly to ground.


Acronym for Low Voltage, Differential Signaling. A low power, high-speed differential signaling system primarily used to transmit video, but can be used for transmitting other forms of data.


Macromedia Flash

Former name. Now it is called Adobe Flash.

Manchester Encoding

A method of encoding serial data that always produces a transition at the center of the bit cell. If the transition is Low to High, the bit is 0. If the transition is High to Low, the bit is 1. Also referred to as split phase or bi-phase level encoding.

Mask Byte

A combination of 1’s and 0’s used in a bit-wise logical operation to set or clear individual bits. Masks can exist as any integer value, such as byte, word, long word, and so on.


The Multifunction Enumerator is a driver component of Microsoft Windows. The mf.sys driver handles the PnP and enumeration functions for multifunction devices such as Sealevel multiport serial adapters.


A PSE device that sits between a non-PoE switch or hub and a PoE-powered device. A midspan is typically an external PoE injector. See: PoE, PSE, Endspan


Nagle’s algorithm

Reduces the number of network packets required to transmit a block of data by buffering the network MTU (up to 1500 bytes of data) or waiting up to 200 msec. When either condition is met, it sends the buffered data in one Ethernet packet. Also referred to as "nagling" and detailed by John Nagle in RFC 896.


A circuit that performs a NOT-AND operation based on the state of its inputs. This gate performs an AND operation and inverts (NOTs) its output.

Negative Logic

A notation that indicates a logic 0 represents the active state for a signal.


In computing or serial communications, a nibble designates four binary digits, or half an Octet. Also corresponds to a single hexadecimal digit and referred to as a Hex Digit. Sometimes spelled ‘Nybble’.

Non Buffered

An unbuffered signal that should not drive more than a few inputs within its logic family. See: Buffered.

NOR Gate

A circuit that performs a NOT-OR operation based on the state of its inputs. This gate performs an OR operation and inverts (NOTs) its output.

Normally Closed

(NC). Relay or switch contacts that normally form a complete low-resistance path for current flow. In an unenergized relay, a set of closed contacts.

Normally Open

(NO). Relay or switch contacts that normally do not make contact. In an unenergized relay, a set of open contacts.


A type of transistor often used as an on-off switch in electronic devices. An NPN switch usually sinks current from a higher potential through a device to ground.


See Nibble.



In computing or serial communications, an octet designates eight binary digits of data. Standardized as a unit of information by IEC 80000-13. See also: Byte.

Open Collector

A logic device or sensor that provides an output transistor with an unconnected collector. When turned on, this transistor sinks current to ground, but it cannot source any current. An open-collector output usually serves as a switch to ground.

Optical Coupler

See: Opto Coupler

Optical Isolation

The use of a light path to transfer a signal from a transmitter, usually a light emitting diode (LED), to a receiver, usually a phototransistor. This technique provides electrical isolation as a signal passes from one circuit to another.

Optical Isolator

See: Opto Coupler

Opto Coupler

A device that uses light emissions to cause an isolated output stage to turn on. This device allows detection and sensing of potentially dangerous or high voltage signals, while providing isolation and protection to the circuitry sensing them.

Opto Isolator

See: Opto Coupler

OR Gate

A circuit that performs an OR operation based on the state of its inputs.

Overload Protection

The capability to protect a circuit when current exceeds a predetermined value. Devices such as fuses or circuit breakers automatically disconnect a load when they sense an overcurrent.



Printed Circuit Board.


Acronym for PCI Express Graphics. A PCI-SIG standard port definition for computer graphics adapters.


PHP: Hypertext Preprocessor


A type of transistor often used as an on-off switch in electronic devices. A PNP switch usually sources current from a positive supply to a device at a lower potential.


Acronym for Power over Ethernet. The technology for transmitting electrical power and data on the same Ethernet cable. Governed by IEEE 802.3 standards, which specify the maximum power allowed for connected devices.


A collection of signals that go to or from a computer for the input or output of information. For example, an 8-bit input port or a serial port.

Positive Logic

A notation that indicates a logic 1 represents the active state for a signal.


Acronym for Power Sourcing Equipment. A device that provides power on Ethernet cabling to PoE devices. See: PoE, Midspan, Endspan

Pull Down Resistor

A resistor used to pull a logic input "down" to the low state, or logic-0 state, thus preventing a disconnected input from floating into an undetermined state.

Pull Up Resistor

A resistor used to pull a logic input "up" to the high state, or logic-1 state, thus preventing a disconnected input from floating into an undetermined state.


Reed Relay

A small relay comprising two magnetic contacts within a sealed glass envelope. When energized, a coil around the envelope moves the contacts to make a low-resistance connection. See: Relay.


A device that opens or closes a circuit under control of a separate and isolated circuit. A mechanical relay uses a coil to actuate mechanical contacts. A solid state relay uses electronic devices to open or close circuit paths. Both types of relays isolate the controlling circuit from the circuit the relay controls.


The total amount of opposition to current in a circuit. Resistance carries the units of ohms and the Greek symbol omega, Ω. Resistance values may have units of kilohms, kΩ or megohms MΩ. See: Resistor.


A device that opposes or limits current flow. Usually noted in schematic diagrams as R. See: Resistance.


A subprotocol of Telnet that added the abilities to (1) send com port configuration information, (2) send modem control signal updates, and (3) manage flow control.


Radio Frequency Interference – Unwanted high-frequency signals, often generated by switching circuits, power supplies, computer cables, and oscillators. RFI may interfere with the proper operation of other circuits.


Resistance Temperature Detector – A stable, linear temperature detector that provides a varying resistance in direct proportion to temperature changes.



Acronym for Standard Downstream Port. Refers to a traditional USB port that supports data transfer capabilities while providing a minimum of 100mA and a maximum of 500mA @ 5V DC to connected USB devices.


Acronym for Serial Digital Video Output. A proprietary Intel technology that additional video signals over a PCI Express slot that contains an Intel 9xx-series chipset.


A device that monitors or measures phenomena such as temperature, pressure, light intensity, weight, conductivity, and so on. Sensors may provide digital or analog output proportional to the phenomenon measured.


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A top level filter that enables plug-and-play functionality for serial ports in Microsoft Windows by automatically determining the type of serial device connected. When the serial port enumerator (serenum.sys) loads, it probes the serial port by toggling RTS and DTR while monitoring the incoming data to determine the device type.

SI Unit

From the French Système international d’unités. The International System of Units, abbreviated SI, defines seven base units of measurement involving the number ten and is commonly referred to as the metric system. The seven base units are ampere, candela, kelvin, kilogram, meter, mole, and second.

Silent Install

A software installation that does not display dialog windows or system messages during the installation progress. Also referred to as an unattended installation.


The ability to allow current to flow through the circuit, usually to ground.


Simple Mail Transfer Protocol


A circuit that suppresses inductive "kickback" that may result when inductive loads switch off. Unless snubbed, the kickback voltage can harm the device that drives the load. See: Suppression Diode.


An electrical coil equipped with a magnetic core. Energizing the coil moves the core. Removing the current lets the solenoid core return to its normal position. Solenoids move levers, open valves, and so on.

Source (Electrical)

The ability to provide current flow.


Single-Pole Double-Throw – A three-terminal switch or relay in which one central terminal connects to either one of the other two terminals. This type of switch can alternately connect a signal to one of two devices.


Single-Pole Single-Throw – A two-terminal switch or relay that can open or close one circuit.


Solid State Relay – A solid state circuit that employs devices such as opto-couplers, transistors, and triacs to perform the function of a mechanical relay. See: Relay.

Supply Current

The total current that a circuit requires from a power supply.

Suppression Diode

A reverse-biased diode placed across a relay or solenoid coil. When the coil loses power, the diode provides a short circuit that quickly dissipates energy stored in the coil.

Surge Current

A high charging current that flows into a power supply filter capacitor or similar circuit as the power is first turned on. Similar to inrush current.

Surge Suppressor

A circuit that limits the effects of power surges. Devices such as metal-oxide varistors (MOVs), zener diodes, and fuses provide this function.


An electronic or mechanical device that can connect one signal to a series of connections. Switches ideally have zero impedance when closed and infinite impedance when open.



Abbreviated from Teletype Network. A networked terminal emulation protocol that provides transparent remote access to a server or device as though it were a local hardwired connection.


A temperature transducer made of two dissimilar metals welded together at one point to form a junction that, when heated in a complete circuit, generates a small voltage proportional to the junction temperature.

Three State

An output from a logic device that can exist in one of three states; logic 0, logic 1, or a high-impedance (disconnected) state. This latter state allows multiple outputs to connect to one signal, effectively providing a "bus" that many signals can share. Three-state devices will provide an output-enable signal that either connects logic signals to the device’s outputs, or places the outputs in a high-impedance state. (National Semiconductor owns the trademark, "tristate™", although the term finds common use among designers.)

Transparent Latch

A latch that passes signals from its inputs to its outputs as long as its Enable signal remains active—usually logic 1. When the Enable signal changes to its inactive state—usually a logic 0—the latch closes and then the outputs remain as they were when the Enable signal changed from logic 1 to logic 0. In effect, this IC acts like a small memory.


A semiconductor switch that can control devices powered by AC current.

Truth Table

A table that shows all possible input and output conditions for a logic element such as a gate or flip-flop. This table may show binary states as well as clock and signal transitions.


Transistor-Transistor-Logic – The type of circuit used in the popular 7400 logic-device families.



Universal Asynchronous Receiver Transmitter – the chip that enables serial communications between the COM port and other serial devices.



Volt-Ampere – The unit of apparent power in an AC circuit containing capacitive or inductive reactance. The apparent power is the product of source voltage and current.


The symbol for the positive supply voltage in a circuit. Also noted as Vcc.


Acronym for Video Graphics Array. 1) An analog video display standard. 2) A DB-15 video connector. 3) An analog video resolution of 640 x 480 pixels.


The unit of potential difference or electromotive force, abbreviated V. One volt represents the potential difference needed to produce one ampere of current through a resistance of one ohm.


The unit of apparent power in an AC circuit containing capacitive or inductive reactance. The apparent power is the product of source voltage and current. Abbreviated VA.


The term used to designate electrical potential that causes current to flow.



The unit of electrical power required to do work at the rate of one joule per second. One watt of power is expended when one ampere of direct current flows through a resistance of one ohm. Abbreviated W.

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Wet Contact

1. Mercury-wetted contacts in sealed reed relays. When the contacts meet, the surface tension of the mercury draws the contacts together and forms a low-resistance path for low-level signals. In effect, the small amount of mercury ensures low-resistance contacts for low-level signals that don’t clean the contacts. 2. Contacts through which current flows. See: Dry Contact.



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Zero Crossing Detector

A circuit that detects when an AC voltage signal has reached zero volts. Switching a circuit at this time reduces inrush currents and minimizes any EMI or RFI produced during switching.