| 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. |