Page 101 - Programmable Logic Controllers, Fifth Edition - Mobile version

manufacturer. Figure 5-15c shows how the original hard-
PL 1
wired circuit could be programmed using the instruction Sensor 1 Sensor 2 Caution
list programming language. Note that the instructional
list consists of a series of instructions that refer to the
basic AND, OR, and NOT logic gate functions. Ladder diagram
Functional block diagram programming uses instructions
that are programmed as blocks wired together on screen BAND_01
to accomplish certain functions. Typical types of func- BAND
tion blocks include logic, timers, and counters. Functional Boolean And Caution
block diagrams are similar in layout to electrical/electronic Out 0 PL 1
block diagrams used to simplify complex systems by show- 0
ing blocks of functionality. The primary concept behind a Sensor 1 0 In1
functional block diagram is data flow. Function blocks are Sensor 2 In2
linked together to complete a circuit that satisfies a control
requirement. Data flow on a path from inputs, through func- Function block diagram
tion blocks or instructions, and then to outputs. Figure 5-17 PLC ladder and equivalent function block
The use of function blocks for programming of pro- diagram.
grammable logic controllers (PLCs) is gaining wider
acceptance. Rather than the classic contact and coil repre- The function block solution consists of a logic Boolean
sentation of ladder diagram or relay ladder logic program- And function block with two input references tags for the
ming, function blocks present a graphical image to the sensors and a single output reference tag for the pilot light.
programmer with underlying algorithms already defined. Note there are no power rails in the function block diagram.
The programmer simply completes needed information Sequential function chart programming language is
within the block to complete that phase of the program. similar to a flowchart of your process. SFC programming
Figure 5-16 shows function block diagram equivalents to is designed to accommodate the programming of more
ladder logic contacts. advanced processes. This type of program can be split
Figure 5-17 illustrates how ladder diagram and func- into steps with multiple operations happening in paral-
tional block diagram programming could be used to pro- lel branches. The basic elements of a sequential function
duce the same logical output. For this application, the chart program are shown in Figure 5-18.
objective is to turn on caution pilot light PL 1 whenever
both sensor switch 1 and sensor switch 2 are closed. The
ladder logic consists of a single rung across the power rails.
This rung contains the two input sensor instructions pro-
grammed in series with the pilot light output instruction. Initial Action
Step 1
Wire
Functional block loop Transition
Ladder logic diagram equivalent
Wire
AND_BOOL
A B Step 2 Action
A
B
A Transition
OR_BOOL
A
B
B Step 3 Action

AND_BOOL
A B Transition
A
B Stop

Figure 5-16 Function block diagram equivalents to ladder Figure 5-18 Major elements of a sequential function chart
logic contacts. program.

82 Chapter 5 Basics of PLC Programming

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