Bryan L. Programmable controllers. Theory and implementation
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CHAPTER
8
Special Function I/O and
Serial Communication Interfacing
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voltage-oriented interface, thus allowing the current loop interface to drive
signals longer distances. To avoid this voltage drop, a current loop uses a
constant source to generate the 20 mA current.
Converting a 20 mA current loop to an RS-232C interface can be done simply
by employing an RS-232C-level receiver. The receiver drives a switching
transistor on the transmission end, and an optical isolator and load resistor
drive the RS-232C driver on the receiving end.
Figure 8-49. (a) RS-232C–to–RS-485 and (b) RS-422–to–RS-485 converters.
INTERFACE USES AND APPLICATIONS
Communications standards are used extensively in applications with a host
PLC or with a computer in a network where one or more interfaces are used.
Sometimes a PLC with an RS-232C or RS-422 communication interface
must communicate with an RS-485 device. In this case, an RS-232C–to–
RS-485 converter (or an RS-422–to–RS-485 converter) can provide this
communication (see Figure 8-49). These converters provide electrical isola-
tion, in addition to longer distance. Figure 8-50a illustrates one of B&R
Industrial Automation’s interface converters, which can be used for com-
municating between two PLCs (see Figure 8-50b) over a long distance
(maximum of 500 m, or 16,500 ft). Each PLC starts its interfacing via RS-
232 (or RS-422) and transfers to RS-485 to achieve the required distance.
RS-232C
Interface
RS-232–to–RS-485
Converter
RS-485
Device
232
232
TX
RX
485
232
232
TX
RX
RX
TX
485
485
485
RS-422
Interface
RS-422–to–RS-485
Converter
RS-485
Device
422
422
485
422
422
TX
RX
TX
RX
RX
TX
485
485
485
(a)
(b)
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CHAPTER
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Special Function I/O and
Serial Communication Interfacing
Figure 8-51 shows the relationship between transmission distance and data
rate for the RS-485 interface converter. This diagram is based on a cable with
an impedance of 110 Ω, a capacitance of 41 picofarads/m, and a cable
ohmic resistance of 0.094 Ω/m. The converter is capable of driving a signal
at rates of 115.2K baud at a distance of 1500 m (5000 ft). It is also capable of
operating at a distance of 5000 m (16,500 ft) at a rate of 9.6K baud.
Figure 8-52 shows another application of serial communication. In this
example, an isolated link coupler (1747-AIC) interface connects several
Allen-Bradley SLC-500 PLC processors to a DH-485 network (RS-485–
based). This link coupler provides a connection for each of the SLC-500
Figure 8-50. (a) B&R Industrial Automation’s interface converter and (b) an example
of the convertor communicating between two PLCs.
32
4
5
1
9
6
7
8
Installation/Grounding
(DIN rail mount)
1–DIN rail (grounded)
2–RS-232 cable
3–Cable holder
4–Cable shield
5–Grounding clamp
6–Voltage supply cable
7–Cable holder
8–RS-485 cable
(e.g., twisted pair)
9–Grounding the
negative supply
RS-232C or RS-422
Interface
RS-232C or RS-422
Interface
RS-232C or RS-422
to RS-485
RS-485 to
RS-232C or RS-422
(16,500 ft)
5000 m
Max
(a)
(b)
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Special Function I/O and
Serial Communication Interfacing
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Figure 8-51. Baud rates for transmission distances in a RS-485 converter.
Figure 8-52. PLC processors connected to an isolated link coupler interface.
CPUs in the DH-485 network. The DH-485 network also interfaces with a
personal computer through an RS-232–to–DH-485 communication inter-
face. The maximum length of the main trunk of the DH-485 network is 4000
feet at a rate of 19.2K baud. This type of subnetwork is very useful for remote
programming and data acquisition links of up to 32 devices.
110
115.2
38.4
19.2
9.6
120
100
90
80
70
60
50
40
30
20
10
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
Distance (m)
Baud Rate (Kbits/sec)
SLC 500SLC 500SLC 500
DH-485 network
RS-232/DH-485
Interface Connector
1747-AIC
Isolated Link Coupler
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ASCII I/O interfaces
BASIC module
cold junction compensation
direct action I/O interface
distributed I/O processing
encoder/counter module
fast-input interface
fast-response interface
fuzzy logic interface
intelligent I/O interface
lead resistance compensation
network interface module
proportional-integral-derivative (PID) interface
resistance temperature detector (RTD) interface
serial communication
servo motor interface
stepper motor interface
thermocouple input module
weight input module
wire input fault module
KEY
TERMS
PLC
PROGRAMMING
SECTION THREE
• Programming Languages
•
The IEC 1131 Standard and Programming Language
• System Programming and Implementation
• PLC System Documentation
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PROGRAMMING
LANGUAGES
CHAPTER
NINE
Language is only the instrument of science,
and words are but the signs of ideas.
—Samuel Johnson
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Programming
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9
Programming
Languages
The programming languages used in programmable controllers have been
evolving since the inception of the PLC in the late 1960s. In this chapter, we
will introduce the three types of languages used in PLCs today—ladder,
Boolean, and Grafcet. During our discussion of these languages, we will
explain some of the versatile, powerful instructions associated with them.
These instructions expand programming possibilities in areas such as data
manipulation, network communication, data transfer, and program/flow
controls, just to name a very few. After you gain a knowledge of these
languages and instructions, you will be ready to explore the IEC 1131-3
standard for PLC programming languages, which includes ladder diagrams
and the implementation of Boolean programming in an IEC 1131 environ-
ment. This programming language standard holds powerful capabilities for
the future of PLC programming.
9-1 INTRODUCTION TO PROGRAMMING LANGUAGES
As PLCs have developed and expanded, programming languages have
developed with them. Programming languages allow the user to enter a
control program into a PLC using an established syntax. Today’s advanced
languages have new, more versatile instructions, which initiate control
program actions. These new instructions provide more computing power for
single operations performed by the instruction itself. For instance, PLCs can
now transfer blocks of data from one memory location to another while, at
the same time, performing a logic or arithmetic operation on another block.
As a result of these new, expanded instructions, control programs can now
handle data more easily.
In addition to new programming instructions, the development of powerful
I/O modules has also changed existing instructions. These changes include
the ability to send data to and obtain data from modules by addressing the
modules’ locations. For example, PLCs can now read and write data to and
from analog modules. All of these advances, in conjunction with projected
industry needs, have created a demand for more powerful instructions that
allow easier, more compact, function-oriented PLC programs.
9-2 TYPES OF PLC LANGUAGES
The three types of programming languages used in PLCs are:
• ladder
• Boolean
• Grafcet
CHAPTER
HIGHLIGHTS
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CHAPTER
9
Programming
Languages
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The ladder and Boolean languages essentially implement operations in the
same way, but they differ in the way their instructions are represented and
how they are entered into the PLC. The Grafcet language implements control
instructions in a different manner, based on steps and actions in a graphic-
oriented program.
The programmable controller was developed for ease of programming using
existing relay ladder symbols and expressions to represent the program logic
needed to control the machine or process. The resulting programming
language, which used these original basic relay ladder symbols, was given the
name ladder language. Figure 9-1 illustrates a relay ladder logic circuit and
the PLC ladder language representation of the same circuit.
Figure 9-1. Hardwired logic circuit and its PLC ladder language implementation.
The evolution of the original ladder language has turned ladder programming
into a more powerful instruction set. New functions have been added to the
basic relay, timing, and counting operations. The term function is used to
describe instructions that, as the name implies, perform a function on data—
that is, handle and transfer data within the programmable controller. These
instructions are still based on the simple principles of basic relay logic,
although they allow complex operations to be implemented and performed.
LADDER LANGUAGE
L1 L2
FS
PB*
PB*
LS
PL
Hardwired Ladder Circuit
PLC Ladder Circuit
FS
LS PL
*Note: The PLC will know the elements PB, LS, FS, and PL by their
addresses once the address assignment has been performed.
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CHAPTER
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Programming
Languages
New additions to the basic ladder logic also include function blocks, which
use a set of instructions to operate on a block of data. The use of function
blocks increases the power of the basic ladder language, forming what is
known as enhanced ladder language. Figure 9-2 shows enhanced functions
driven by basic relay ladder instructions. As shown in the figure, a block or
a functional instruction between two contact symbols represents an enhanced
functional block.
Figure 9-2. Enhanced functional block format.
The format representation of an enhanced ladder function depends on the
programmable controller manufacturer; however, regardless of their format,
all similar enhanced and basic ladder functions operate the same way.
Throughout this chapter, we will refer to enhanced ladder instructions as
block format instructions.
As indicated earlier, the ladder languages available in PLCs can be divided
into two groups:
• basic ladder language
• enhanced ladder language
Each of these groups consists of many PLC instructions that form the
language. The classification of which instructions fall into which categories
differs among manufacturers and users, since a definite classification does
not exist. However, a de facto standard has been created throughout the years
that sorts the instructions into either the basic or enhanced ladder language.
Table 9-1 shows a typical classification of basic and enhanced instructions.
Sometimes, basic ladder instructions are referred to as low-level language,
while enhanced ladder functions are referred to as high-level language. The
A B
Output
A B
A B
Enable
Reset
MOVE
Register-to-Table
Function Block
MOVE register to table
Functional Instruction