Bryan L. Programmable controllers. Theory and implementation

Подождите немного. Документ загружается.

259

CHAPTER

Special Function I/O and

Serial Communication Interfacing

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

SECTION

Components

and Systems

Table 8-2. Omron’s FLU space requirements.

• Like the first word, the first four bits (0–3) of the third word (word

112) contain the number of outputs in BCD.

• The fourth word (word 113) contains the starting address for the

storage of the output data, which is the result of the fuzzy logic

computations. The length of the data block is the BCD number from

word 112.

Because fuzzy logic modules work through other I/O interfaces, their input/

output data must be transferred from/to the word address locations of the I/O

modules working with them. Figure 8-41 illustrates the memory addresses

(words) used by the Omron FLU in the previous example, along with the

Figure 8-41. Memory addresses used by example FLU.

:stupnIdaerebotstupniforebmunehtyficeps011drowfo3–0stib

)8=I,.g.e()xam8(

:111droW)Ifohtgnel(detacolsiatadtupnierehwsserddagnitrats

)021=sserdda,.g.e(

:stuptuOebotstuptuoforebmunehtyficeps211drowfo3–0stib

)4=O,.g.e()xam4(nettirw

:311droW)Ofohtgnel(detacolsiatadtuptuoerehwsserddagnitrats

)031=sserdda,.g.e(

:411droWsgnittesdnasgalfrofdesu

:911–511sdroWsesserddadrowgnikrowsaelbaliava

8 Inputs

Input data

to fuzzy unit

120

121

122

123

124

125

126

127

Contents of words

120–127 will be used

as inputs for fuzzy

computing if word 110

contains 8 in BCD

Fuzzy Unit

bit 15 = 0 fuzzy processing OFF

bit 15 = 1 fuzzy processing ON

110

111

112

113

114

115

116

117

118

119

4 Outputs

Output data

from fuzzy unit

130

131

132

133

fuzzy logic

flags and setting

starting output

address

starting input

address

3210765411 1 0 8915 14 13 12

Contents of words

130–133 will contain

output results from

fuzzy computing if word

112 contains 4 in BCD

260

SECTION

Components

and Systems

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

CHAPTER

Special Function I/O and

Serial Communication Interfacing

Figure 8-42. Data transfer between I/O modules and fuzzy module.

Block transfer instructions can be used to transfer data between the I/O

modules and the fuzzy module (see Figure 8-42). Chapter 17 explains more

about fuzzy logic control.

Analog

Input

Module

Fuzzy

Input

Data

Fuzzy

Output

Data

Analog

Output

Module

Fuzzy

Configuration

Data

Input 2

Input 1

Input 3

Chan 1

Block

Transfer

Block

Transfer

8 Analog Input Module

Single-Ended

4 Analog Output Module

Differential

8 words max

4 words max

Analog input information

of 8 channels is stored

Analog output

information of 4

channels is stored

8-7 PERIPHERAL INTERFACING

Regardless of the type of peripheral used, the user must properly connect

the peripheral device to the PLC or intelligent module to achieve correct

communication. Typical peripherals communicate in serial form at speeds

ranging from 110 to 19,200 bits per second (baud), with parity and nonparity,

asynchronicity, and various communication interface standards.

COMMUNICATION STANDARDS

Communication standards fall into two categories: proclaimed and de facto.

Proclaimed standards are officially established standards set by various

261

CHAPTER

Special Function I/O and

Serial Communication Interfacing

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

SECTION

Components

and Systems

electronics organizations, such as the Institute of Electrical and Electronics

Engineers (IEEE) and the Electronic Industries Association (EIA). These

institutions define public specifications through which manufacturers can

establish communication schemes that allow compatibility among different

manufacturers’ products. Proclaimed standards, such as the IEEE 488 instru-

ment bus, the EIA RS-232C, the EIA RS-422, and the EIA RS-485, are

examples of well-defined proclaimed standards.

De facto standards are interface methods that have gained popularity through

widespread use. Although these popular standards have been adopted

throughout the industry, they have no official definition. Because they are not

properly defined, some de facto standards cause interface problems; however,

other standards, such as the 20 mA current loop, are good, well-defined de

facto standards.

Serial communication, as the name implies, occurs in serial form through

simple, twisted-pair cables. Serial data transmission is used for most periph-

eral communication devices, since these devices are slow in nature and

require long cable connections. Serial communication allows peripheral

equipment, such as terminals, modems, operator interface panels, and line

printers, to receive ASCII information.

Two of the most popular standards for serial communication are the RS-

232C and the 20 mA current loop. Other PLC standards are the RS-422 and

RS-485, which improve performance and give greater flexibility in data

communication interfaces.

The data communication links used with peripheral equipment can be

unidirectional or bidirectional. If a peripheral is strictly either an input or an

output device, then data transmission occurs in only one direction. In this

case, a unidirectional serial signal line is all that is required to complete the

link. Devices that serve as both input and output devices (e.g., video

terminals) require bidirectional links. There are two ways to achieve this

bidirectional communication. First, a single data line can be used as a shared

communication line. The data can be sent in either direction, but only in one

direction at a time. This operation is known as half duplex. If simultaneous

bidirectional communication is required, two lines can connect the PLC to

the peripheral. One line would be assigned permanently as an input, while the

other would be a permanent output. This mode is known as full duplex. Figure

8-43 illustrates the unidirectional, half-duplex, and full-duplex communica-

tion methods.

SERIAL COMMUNICATION

262

SECTION

Components

and Systems

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

CHAPTER

Special Function I/O and

Serial Communication Interfacing

EIA RS-232C. The EIA RS-232C is a proclaimed standard that defines the

interfacing between data equipment and communication equipment that

employs serial binary data interchange. This standard defines both the

electrical signals and the mechanical details of the interface. A complete RS-

232C interface consists of 25 data lines, which encompass all of the possible

signals for simple and complex communication interfaces. Although several

of these lines are specialized and a few are undefined, most peripherals

require only three to five lines to operate properly. Table 8-3 describes the 25

data lines as specified by the EIA.

Figure 8-44a illustrates an RS-232C data communication system using a

telephone modem, while Figure 8-44b shows the RS-232C wiring connec-

tions from a computer to a smart EIA PLC interface module. Figure 8-44c

illustrates a typical RS-232C interface to a printer. Note that the communi-

cation between a computer and a PLC has few lines swapped if no modem

or other data communication equipment is used. This configuration is

Figure 8-43. (a) Unidirectional, (b) half-duplex, and (c) full-duplex data commu-

nication formats.

Line

Printer

Terminal

Equipment

PLC

Terminal

Equipment

Output

Driver

Input

Receiver

Output

Driver

Input

Receiver

Output

Driver

Input

Receiver

Input

Receiver

Output

Driver

Input

Receiver

Output

Driver

Two-direction

data transfer

One-direction

data transfer

Simultaneous two-direction

data transfers

(a)

(b)

(c)

263

CHAPTER

Special Function I/O and

Serial Communication Interfacing

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

SECTION

Components

and Systems

Table 8-3. EIA RS-232C data line descriptions.

rebmuNniPnoitpircseD

1dnuorgevitcetorP

2ataddettimsnarT

3ataddevieceR

4dnesottseuqeR

5dnesotraelC

6ydaertesataD

7 )nruternommoc(dnuorglangiS

8 rotcetedlangisenildevieceR

9 )gnitsettesatadrofdevreseR(

01 )gnitsettesatadrofdevreseR(

11dengissanU

21 rotcetedlangisenildevieceryradnoceS

31 dnesotraelcyradnoceS

41 ataddettimsnartyradnoceS

51 )ECD(gnimittnemelelangisnoissimsnarT

61 ataddevieceryradnoceS

71 )ECD(gnimittnemelelangisrevieceR

81dengissanU

91 dnesottseuqeryradnoceS

02ydaerlanimretataD

12rotcetedytilauqlangiS

22rotacidnigniR

32 )ECD/ETD(rotcelesetarlangisataD

42 )ETD(gnimittnemelelangistimsnarT

52dengissanU

called a null modem cable. The connection between a PLC and an RS-232C

peripheral (printer, etc.) usually requires four wires; however, the user

should refer to the connection specifications for both devices for specific

details.

The RS-232C standard calls for certain electrical characteristics. Some of

these specifications are as follow:

• The signal voltages at the interface point should be a minimum of

+5 V and a maximum of +15 V for logic 0; for logic 1, the minimum

is –15 V and the maximum is –5 V.

• The maximum recommended cable distance is 50 feet, or 15 meters;

however, longer distances are permissible provided that the resulting

load capacitance, measured at the interface point and including the

signal terminator, does not exceed 2500 picofarads.

264

SECTION

Components

and Systems

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

CHAPTER

Special Function I/O and

Serial Communication Interfacing

Figure 8-44. RS-232C communication connections for (a) a PLC to a modem, (b) a PLC to

a computer, and (c) a PLC to a printer.

Telephone lines

Signal ground

Transmit data

Request to send

Receive data

Data, set, ready

Transmit data

Receive data

Request to send

Clear to send

Carrier detect

Data set ready

Ring indicator

Data terminal ready

Programmable

Controller

Signal ground

Transmit data

Request to send

Receive data

Data, set, ready

Modem Modem

(a)

(b)

(c)

Transmit data

Receive data

Request to send

Clear to send

Carrier detect

Data set ready

Ring indicator

Data terminal ready

Transmit

Receive

Transmit

Receive

–V

Ground

+V –V Com

User

DC Supply

Computer

Printer

RS-232

Connector

Computer

Signal ground

PLC’s

Smart

EIA

Interface

PLC’s

EIA

Interface

Data set ready

265

CHAPTER

Special Function I/O and

Serial Communication Interfacing

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

SECTION

Components

and Systems

• The drivers used must be able to withstand open or short circuits

between pins in the interface.

• The load impedance at the terminator side must be between 3000 and

7000 ohms, with no more than 2500 picofarads capacitance.

• Voltages under –3 V (logic 1) are called mark potentials (signal

conditions); voltages above +3 V (logic 0) are called space voltages.

The area between –3 V and +3 V is not defined.

Figure 8-45 illustrates a typical RS-232C serial ASCII pulse train. The

transmission begins with a START bit (0) and ends with either one or two

STOP bits (1). The transmission also includes parity, which can be even or

odd (see Chapter 4 for parity).

Figure 8-45. RS-232C serial ASCII pulse train.

EIA RS-422. The RS-422 standard overcomes some of the RS-232C short-

comings, including an upper data rate of 20K baud, a maximum cable

distance of 50 feet, and an insufficient capacity to control additional loop-test

functions for fault isolation. Like the RS-232C, the RS-422 standard still

deals with the traditional serial/binary switch signals of two voltage levels

across the interface. The RS-449 standard, which meets new operational

requirements, defines the physical and mechanical specifications for the RS-

422 electrical interface standard.

The RS-232C is an unbalanced link communication method, meaning that

it specifies a primary station that is always in control (master/slave relation-

ship). This primary station is responsible for setting logical states and

operational modes of each secondary station, thereby controlling the entire

data communication process. The RS-422, however, is a balanced link in

which either station can configure itself and initiate transmission when both

stations have identical data transfer and link control capabilities. The RS-422

specifies electrically balanced receivers and generators that tolerate and

produce less noise. These provide superior performance up to 10 megabaud

(10,000 K baud).

01100101011

MSB PAR StopStop

Start

LSB

EIA DATA

1 (–V) Mark

0 (+V) Space

Character

=123

110 Baud

2 Stop Bits

0110010101

Start

1 (–V) Mark

0 (+V) Space

All Other

Baud Rates

1 Stop Bit

266

SECTION

Components

and Systems

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

CHAPTER

Special Function I/O and

Serial Communication Interfacing

A balanced circuit in an RS-422 configuration employs differential signaling

over a pair of wires for each circuit, while an unbalanced configuration signal

(RS-232C) uses one wire for each circuit and a common return circuit. Figure

8-46 illustrates configurations for both RS-422 and RS-232C circuits.

The RS-422 standard may be required when interconnecting cables are too

long for effective unbalanced operation and noise in excess of 1 V can be

measured across the signal conductors. The driver circuits for an RS-422

configuration are capable of furnishing the DC signal necessary to drive up

to 10 parallel, connected RS-422 receivers. However, this capability involves

considerations such as stub line lengths, data rate, grounding, fail-safe

networks, etc. The standard does not specify cable characteristics, but to

ensure proper operation, paired cables with metallic conductors should be

employed and, if necessary, shielded.

Signal Wires

Generator

Circuit Ground

Load

Circuit Ground

Signal Wires

A A'

Signal Wires

A' A

Signal Wires

Signal Ground

Figure 8-46. Circuit configurations for (a) RS-422 and (b) RS-232C connections (G =

generator; R = receiver; R

= optional cable termination; A, B, A', B' =

interface points).

(a) EIA RS-422 circuit

(b) EIA RS-232C circuit

267

CHAPTER

Special Function I/O and

Serial Communication Interfacing

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

SECTION

Components

and Systems

The maximum allowable cable distance for the RS-422 standard is a function

of the data transmission rate. Figure 8-47 illustrates the relationship between

distance and data rate. The graph describes empirical measures using a 24

AWG copper conductor and a twisted-pair cable with a shunt capacitance

of 52.5 pF/meter (16 pF/foot) terminated in a 100 ohm resistive load. The

balanced electrical characteristics of RS-422 perform even better with an

optimal cable termination of approximately 120 ohms in the receiver load.

Figure 8-47. Cable distance versus data rate relationship for the RS-422 and RS-

232C communications standards.

In reality, the curves in Figure 8-47 are conservative for RS-422 balanced

operation. A cable can perform effectively, at lower data rates, at a distance

of several miles with good engineering practice. However, if longer

distances are required, the user should perform an analysis of the absolute

loop resistance and the capacitance of the cable. In general, longer distances

are possible when using 19 AWG cable, but the type and length of cable

used must be capable of maintaining the necessary signal quality for the

particular application.

The RS-449 mechanical standard, which supports the RS-422 electrical

standard, offers several extra circuits (signals) that provide greater flexibility

to the interface and accommodate new common return circuits. These

additional functions and wires were beyond the capacity of an RS-232C 25-

pin connector; therefore, the EIA selected a 37-pin connector for the RS-422

standard, because it satisfies interface channel requirements. If secondary

channel operation is to be used as a low-speed TTY or acknowledgments

channel, a separate 9-pin connector is also needed.

EIA RS-485. The RS-485 standard, like the RS-422, has dual transmitting and

receiving lines (differential signals). This type of interface is best suited for

industrial applications, because it provides better electrical isolation from

the PLC or host than the RS-422 standard. It is also capable of being used in

1200

1000

100

1K 2.4K 4.8K 10K 20K 56K 100K 1M 2M 10M

Data Rate (bits/sec.)

Cable Distance (meters)

RS-422

RS-232C

RS-422 Without Cable Termination

RS-422 With Cable Terminations

268

SECTION

Components

and Systems

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

CHAPTER

Special Function I/O and

Serial Communication Interfacing

a network (e.g., multiple transmitters and receivers operated on a common

media, such as twisted-pair cable). Distances of up to 4000 feet (1200 meters)

can be attained with this standard.

20 mA Current Loop. The 20 mA current loop de facto standard consists of

four basic wires: transmit plus, transmit minus, receive plus, and receive

minus. Figure 8-48 illustrates the four lines used to form the 20 mA current

loop. This de facto standard is also referred to as a TTY serial interface.

Figure 8-48. 20 mA current loop operation diagram.

In the 20 mA current loop standard, the opening and closing of current loops

signifies 0s and 1s, respectively. When the current loop standard was first

used in teletypewriters, rotating switch contacts in the sending teletypewriter

connected and broke the loop; the corresponding 20 mA signal drove a print

magnet in the receiving teletypewriter. Today, most 20 mA current loops

electronically operate the opening switch and printer magnet arrangement.

To generate a current, the voltage in a 20 mA current loop is applied to a

current limiting resistor at the data-sending end. This voltage is dropped

across both the current limiting resistor (R

) and across the load resistor (R

The R values and the positive voltage applied to them must generate a flow

current of 20 mA. Typically, a high voltage and high resistance (R

) are

chosen, even though a low voltage and low resistance can be used. Current

loop communications provide an advantage over other methods, since the

wire resistance has no effect on the constant current loop. Voltage does not

drop across the wire in current loop communications as it does in an RS-232C

Transmit

20 mA

Data

20 mA

Receive

Amplifier

Receive

Amplifier

or Sensor

PLC

Terminal

Transmit

Receive +

Receive –

–