Overview of PLC Registers 305

address

The unique location assigned to each I/O module so that it may be correctly referenced and signaled by the PLC program.

binary

A number system used for PLCs that has either 0 or 1 as a value. Using binary, a function is either on or off.

bit

A single numerical unit in the binary number system. Bit is short for BInary digiT.

bit number

The numbered location of the specific individual bit being addressed. Bit numbers are typically the last number found in an address.

byte

A series of eight bits.

common

The screw terminal on a PLC that connects to the power source.

counter

A set of PLC instructions that counts, calculates, or keeps a record of the number of times something happens.

digital

Consisting of information that is input or output electronically as a series of pulses or signals often resulting in binary strings of 0s and 1s.

examine off

A basic PLC logic instruction symbol. Also referred to as Examine If Open. Examine Off becomes True if an input is absent.

examine on

A basic PLC logic instruction symbol. Also referred to as Examine If Closed. Examine On becomes TRUE if an input is present.

file

A data register in a PLC that stores bits of information. Files are often referred to as registers.

file identifier

A letter code given to each type of data file to make it easier for the programmer to identify the files.

file number

The specific bit number assigned to each type of data file so that it can be easily found within the ladder logic program.

holding register

The PLC equivalent of traditional relays. Holding registers hold data that will later be executed by the processor.

I/O module

The part of a modular PLC to which input and output devices are connected.

input group register

A type of input register that can receive data from 16 consecutive input terminals. Grouping registers is an efficient way to control multiple inputs.

input register

A register that receives and stores data signaled from a hardwired input.

nibble

Four bits in series, or half an 8-bit byte.

output register

A register that stores data on the status of a hardwired output.

read from

The process of a PLC scanning a register and using the data found within that register in a meaningful way.

real-time value

The exact current value of a bit of data. PLC register data is not considered real-time because values can change between PLC scans.

register

A data file in a PLC that stores bits of information. Some PLCs refer to registers as files.

remote I/O

An input/output module that is located far away from the CPU of the PLC.

scan

One complete cycle of a PLC checking inputs, executing its programs, and updating the status of its outputs.

sequential

Doing one and only one step at a time, followed by the next step in a logical order.

shift register

Shifting bits forward or backward within a word to vary how the outputs respond.

slot number

The numbered location of the specific PLC module being addressed.

terminal block

A device that joins wires or cables. Terminal blocks typically snap into a metal rail or are screw-mounted on the panel of a control enclosure.

timer

A set of PLC instructions that automatically starts or stops machines and other devices when a preset time period has been exceeded.

word

A defined and specific number of digits grouped together. For most PLCs, a word consists of 16 digits.

word number

The numbered location of the specific data word being addressed. Word numbers are found only on advanced PLCs.

written to

The process of a PLC storing data within a register for later use.