absolute operand |
An operand that can be used by any code block in a program. In STEP 7 (TIA Portal), absolute operands are preceded by the % symbol. |
AND |
A Boolean logic function for which the result is 1 if only when all inputs to the function are 1. |
assign instruction |
An FBD instruction that sets the status bit specified by the operand to 1 when the result of logic operation (RLO) to the instruction is 1, and resets the status bit to 0 when the RLO to the instruction is 0. The assign instruction is the FBD equivalent of the LAD output coil instruction. |
binary |
A number system used for PLCs that has either 0 to 1 as a value. |
bit |
A single numerical unit in the binary number system. Bit is the smallest unit of information on a computer. |
Bit logic instructions |
A category of instructions in the FBD programming language that performs logical operations using single bit memory locations. |
bit memory |
M. A part of the global memory of a Siemens PLC used to store the immediate results of program operations. |
Boolean logic |
A branch of mathematics that deals with the use of logical operators that define relationships between two or more entities. Examples of Boolean functions include AND, OR, and Exclusive OR. |
byte |
8 consecutive binary bits. |
code blocks |
Program blocks in a Siemens PLC program. Code blocks store information in global and instance data blocks and are allocated local memory when being executed. |
control circuit |
A type of circuit that uses control devices to determine when loads are energized or de-energized by controlling current flow. Control circuits usually operate with lower voltages and currents. |
double words |
32 consecutive binary bits. |
forward-reverse control |
An application that controls the forward and reverse motion of a motor. The forward-reverse control of a three-phase AC motor is one of the most basic applications for PLC bit logic instructions. |
function block |
One of the allowed code block types in a STEP 7 PLC program. A function block can be programmed using any allowable PLC programming language. |
function block diagram programming |
FBD programming. A graphical PLC programming language defined in part 3 of the IEC 61131 standard.” |
global memory |
A type of PLC memory that is available to all code blocks. Global memory includes the input (I) and output (Q) status bits of the process image and bit memory (M). |
IEC 61131 |
A standard of the International Electrotechical Commission that provides common approaches and concepts for PLCs. Part three of the standard describes the FBD programming language for PLCs. |
input signal module |
A Siemens PLC input module. An input signal module converts the signal from an input device to the low voltage DC used by the CPU. |
instance data blocks |
Data blocks (DBs) that store data for a specific function block. |
inversion |
An operation that changes a logic 1 to a logic 0 or vice versa. |
invert instruction |
An instruction that changes a 1 to a 0 and a 0 to a 1. An invert instruction is represented by a small circle adjacent to another FBD instruction. |
LAD |
Ladder logic or ladder diagram programming. A graphical programming language that is defined in IEC 61131-3 and derived from the diagrams used in electromechanical control circuits, though it has expanded to include more advanced capabilities. |
linear program |
A PLC program whose instructions are contained in one main program block, called organization block 1, or OB1. Linear programs are executed with each PLC scan. |
local memory |
L. A type of PLC memory that temporarily holds information but does not store it. Local memory is allocated to code blocks when they are executed. |
negate assign instruction |
An FBD instruction that sets the status bit specified by the operand to 1 when the result of logic operation (RLO) to the instruction is 0, and resets the status bit to 0 when the RLO to the instruction is 1. The negate assign instruction is the FBD equivalent of the LAD inverted output coil instruction. |
operands |
A part of a PLC instruction that identifies a memory location associated with the instruction. |
OR |
A Boolean logic function for which the result is 1 when one or more of the inputs to the function are 1. |
output signal module |
A Siemens PLC output module. An output signal module controls an output device in response to the signal provided from the CPU. |
overload |
A level of current that exceeds the recommended level for a device or circuit, but is less than the level of a short circuit. |
overload relay |
A device that disconnects a motor from its power circuit when the overload relay senses and overload for a specified time. |
PLC tag |
A symbolic identifier used in a PLC program to associate an instruction with a CPU memory area. |
power circuit |
A type of circuit that carries power to electrical loads. |
process image |
The area of CPU memory where input and output status information is stored. |
reset set flip-flop |
An instruction with reset and set inputs that determine the state of the instruction output. The set input is dominant over the reset input.” |
result of logic operation |
RLO. The solution to a configuration of instructions. The RLO at any point in a network affects the operation of instructions that come after that point in the network. |
scans |
One complete cycle of a PLC's cyclical operation that includes checking inputs, executing its programs, updating the status of its outputs, and performing various other required processes. |
set reset flip-flop |
An instruction with set and reset inputs that determine the state of the instruction output. The reset input is dominant over the set input. |
STEP 7 |
Software used for the configuration, programming, testing, and diagnosis of Siemens SIMATIC S7 PLCs. |
structured program |
A PLC program with instructions divided into multiple program blocks. Program blocks in a structured program can be executed with each PLC scan or on a conditional basis. |
three-phase AC motor |
A type of electric motor that runs on alternating current. Three-phase AC motors are the most common type of industrial motor. |
TIA Portal |
Siemens Totally Integrated Automation Portal. Software used for designing, commissioning, operating, maintaining, and upgrading automation systems. |
truth table |
A table that shows all the possible true and false combinations for a logic function or configuration and the corresponding state of the output for each combination. |
words |
16 consecutive binary bits. |
XOR |
Exclusive OR. A Boolean logic function for which the result is 1 when an odd number of the inputs to the function are 1. |