Basic Function Block Diagram Programming for Siemens PLCs 290

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.