Bryan L. Programmable controllers. Theory and implementation
Подождите немного. Документ загружается.
179
CHAPTER
6
The Discrete
Input/Output System
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
SECTION
2
Components
and Systems
Table 6-6. Summary of I/O specifications.
E
LACIRTCEL
.gnitaRegatloVtupnI dnaedutingamehtseificepstahteulavCDroCAnA
.tpeccalliwtiucricalangisfoepyt
.gnitaRtnerruCtupnI tupninaegatlovdetarehttatnerrucmuminimehT
.gnivirdfoelbapacebtsumecived
.egatloVdlohserhTtupnI dezingocersilangistupninahcihwtaegatlovehT
.NOgniebsa
.yaleDtupnI ebotNOebtsumlangistupninahcihwrofnoitarudehT
.tupnidilavasadezingocer
.gnitaRegatloVtuptuO edutingamehtseificepstahteulavCDroCAnA
.lortnocnaceludomO/Inatahtegatlovfoepytdna
.gnitaRtnerruCtuptuO tiucrictuptuoelgnisatahttnerrucmumixamehT
.daolrednuyrracylefasnac
.gnitaRrewoPtuptuO etapissidnaceludomtuptuonarewopmumixamehT
.dezigrenestiucricllahtiw
.stnemeriuqeRtnerruC nosecalpeludomO/InatahtdnamedtnerrucehT
.ylppusrewopmetsyseht
.)xaM(tnerruCegruS nahcihwrofnoitaruddnatnerrucmumixamehT
.gnitartnerrucetats-NOmumixamstideecxenactiucrictuptuo
.tnerruCegakaeLetatS-FFO swolftahttnerrucegakaelmumixamehT
.etatsFFOstignirudrotsisnart/cairtehthguorht
.yaleD-NOtuptuO NOotFFOmorfnrutottuptuonarofemitesnopserehT
.dnammocNOnaseviecertiretfa
.yaleD-FFOtuptuO FFOotNOmorfnrutottuptuonarofemitesnopserehT
.dnammocFFOnaseviecertiretfa
A.noitalosIlacirtcelE noitalosiehtgninifedstlovnieulavmumixam
.cigolrellortnocehtdnatiucricO/Iehtneewteb
.segnaRtnerruC/egatloVtuptuO fogniwstnerruc/egatlovehtfoeulavehT
.retrevnocgolana-ot-latigideht
.segnaRtnerruC/egatloVtupnI fogniwstnerruc/egatlovehtfoeulavehT
.retrevnoclatigid-ot-golanaeht
.noituloseRlatigiD O/IgolanadetrevnocehtylesolcwohfoerusaemA
.eulavgolanalautcaehtsetamixorppalangisegatlovrotnerruc
.gnitaResuFtuptuO nidesuebdluohstahtsesuffognitardnaepytehT
.ecafretnieht
M
LACINAHCE
.eludoMrePstnioP anoeratahtstiucrictuptuorotupniforebmunehT
.eludomelgnis
.eziSeriW O/IehteriweguagtsegralehtdnasrotcudnocforebmunehT
.tpeccalliwstniopnoitanimret
E
LATNEMNORIVN
.gnitaRerutarepmeTtneibmA gnidnuorruserutarepmetriamumixamehT
.snoitidnocgnitarepolaedirofmetsysO/Ieht
.ytidimuH .metsysO/IehtgnidnuorrusytidimuhriamumixamehT
180
SECTION
2
Components
and Systems
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
CHAPTER
6
The Discrete
Input/Output System
specification may also be stated as the output voltage range, in which case
both the minimum and maximum operating voltages are given. An output
circuit rated at 48 VDC, for example, can have an absolute working range
of 42 to 56 VDC.
Output Current Rating. This specification is also known as the ON-state
continuous current rating, a value that defines the maximum current that a
single output circuit can safely carry under load. The output current rating is
a function of the electrical and heat dissipation characteristics of the compo-
nent. This rating is generally specified at an ambient temperature (typically
0–60°C). As the ambient temperature increases, the output current decreases.
Exceeding the output current rating or oversizing the manufacturer’s fuse
rating can result in a permanent short-circuit failure or other damage.
Output Power Rating. This maximum value defines the total power that an
output module can dissipate with all circuits energized. The output power
rating for a single energized output is the product of the output voltage rating
and the output current rating expressed in volt-amperes or watts (e.g., 120 V
× 2 A = 240 VA). This value for a given I/O module may or may not be the
same if all outputs on the module are energized simultaneously. The rating for
an individual output when all other outputs are energized should be verified
with the manufacturer.
Current Requirements. The current requirement specification defines the
current demand that a particular I/O module’s logic circuitry places on the
system power supply. To determine whether the power supply is adequate,
add the current requirements of all the installed modules that the power
supply supports, and compare the total with the maximum current the power
supply can provide. The current requirement specification will provide a
typical rating and a maximum rating (all I/O activated). An insufficient power
supply current can result in an undercurrent condition, causing intermittent
operation of field input and output interfaces.
Surge Current (Max). The surge current, also called the inrush current,
defines the maximum current and duration (e.g., 20 amps for 0.1 sec) for
which an output circuit can exceed its maximum ON-state continuous current
rating. Heavy surge currents are usually a result of either transients on the
output load or power supply line or the switching of inductive loads.
Freewheeling diodes, Zener diodes, or RC networks across the load terminals
normally provide output circuits with internal protection. If not, protection
should be provided externally.
OFF-State Leakage Current. Typically, this is a maximum value that
measures the small leakage current that flows through the triac/transistor
during its OFF state. This value normally ranges from a few microamperes to
a few milliamperes and presents little problem. It can present problems when
switching very low currents or can give false indications when using a
sensitive instrument, such as a volt-ohm meter, to check contact continuity.
181
CHAPTER
6
The Discrete
Input/Output System
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
SECTION
2
Components
and Systems
Output-ON Delay. This specification defines the response time for the output
to go from OFF to ON once the logic circuitry has received the command to
turn ON. The ON response time of the output circuit affects the total time
required to activate an output device. The worst-case time required to turn an
output device ON after the control logic goes TRUE is the total of the two
program scan times plus the I/O update, output-ON delay, and device-ON
response times.
Output-OFF Delay. The output-OFF delay specification defines the re-
sponse time for the output to go from ON to OFF once the logic circuitry has
received the command to turn OFF. The OFF response time of the output
circuit affects the total time required to deactivate an output device. The
worst-case time required to turn an output device OFF after the control logic
goes FALSE is the total of the two program scan times plus the I/O update,
output-OFF delay, and device-OFF response times.
Electrical Isolation. This maximum value in volts defines the isolation
between the I/O circuit and the controller logic circuitry. Although this
isolation protects the logic side of the module from excessive input/output
voltages or currents, the power circuitry of the module can still be damaged.
Output Voltage/Current Ranges. This specification is a nominal expression
of the voltage/current swing of the D/A converter in analog outputs. This
output will always be a proportional current or voltage within the output range.
A given analog output module may have several hardware- or software-
selectable, unipolar or bipolar ranges (e.g., 0 to 10 V, –10 to +10 V, 4 to 20 mA).
Input Voltage/Current Ranges. This specification defines the voltage/
current swing of the A/D converter in analog inputs. This specification will
always be a proportional current or voltage within the input range. A given
analog input module may have several hardware- or software-selectable,
unipolar or bipolar ranges (e.g., 0 to 10 V, –10 to +10 V, 4 to 20 mA).
Digital Resolution. This specification defines how closely the converted
analog input/output current or voltage signal approximates the actual analog
value within a specified voltage or current range. Resolution is a function of
the number of bits used by the A/D or D/A converter. An 8-bit converter has
a resolution of 1 part in 2
8
or 1 part in 256. If the range is 0 to 10 V, then the
resolution is 10 divided by 256, or 40 mV/bit.
Output Fuse Rating. Fuses are often supplied as a part of the output circuit,
but only to protect the semiconductor output device (triac or transistor). The
manufacturer carefully selects the fuse that is employed or recommended for
the interface based on the fusing current rating of the output switching device.
Fuse rating incorporates a fuse opening time along with a current overload
rating, which allows opening within a time frame that will avoid damage to
the triac or transistor. The recommended specifications should be followed
when replacing fuses or when adding fuses to the interface.
182
SECTION
2
Components
and Systems
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
CHAPTER
6
The Discrete
Input/Output System
ENVIRONMENTAL
MECHANICAL
Points Per Module. This specification defines the number of input/output
circuits that are on a single module (encasement). Typically, a module will
have 1, 2, 4, 8, or 16 points per module. The number of points per module has
two implications that may be of importance to the user. First, the less dense
(fewer the number of points) a module is, the greater the space requirements
are; second, the higher the density, the lower the likelihood that the I/O count
requirements can be closely matched with the hardware. For example, if a
module contains 16 points and the user requires 17 points, two modules must
be purchased. Thus, the user must purchase 15 extra inputs or outputs.
Wire Size. This specification defines the number of conductors and the
largest gauge wire that the I/O termination points will accept (e.g., two #14
AWG). The manufacturer does not always provide wire size specifications,
but the user should still verify it.
Ambient Temperature Rating. This value is the maximum temperature of
the air surrounding the input/output system for best operating conditions.
This specification considers the heat dissipation characteristics of the circuit
components, which are considerably higher than the ambient temperature
rating itself. The ambient temperature rating is much less than the heat
dissipation factors so that the surrounding air does not contribute to the heat
already generated by internal power dissipation. The ambient temperature
rating should never be exceeded.
Humidity Rating. The humidity rating for PLCs is typically 0–95%
noncondensing. Special consideration should be given to ensure that the
humidity is properly controlled in the area where the input/output system is
installed. Humidity is a major atmospheric contaminant that can cause circuit
failure if moisture is allowed to condense on printed circuit boards.
Proper observance of the specifications provided on the manufacturer’s data
sheets will help to ensure correct, safe operation of control equipment.
Chapter 20 discusses other considerations for properly installing and main-
taining input/output systems.
6-10 SUMMARY OF DISCRETE I/O
For the most part, all PLC system applications require the types of discrete
I/O interfaces covered in this chapter. In addition to discrete interfaces, some
PLC applications require analog and special I/O modules (covered in the next
two chapters) to implement the required control.
183
CHAPTER
6
The Discrete
Input/Output System
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
SECTION
2
Components
and Systems
All I/O interfaces accept input status data for the input table and accept
processed data from the output table. This information is placed in or written
from the I/O table (in word locations) according to the location or address of
the modules. This address depends on the module’s placement in the I/O rack
enclosure; therefore, the placement of I/O interfaces is an important detail to
keep in mind.
The software instructions that are generally used with discrete-type interfaces
are basic relay instructions (ladder type), although multibit modules use
functional block instructions as well as some advanced ladder functions.
Chapter 9 explains these software instructions. Figure 6-58 shows several
programmable controller input and output modules and enclosures.
Figure 6-58. PLC families sharing the use of I/O modules and enclosures.
Top: courtesy of B & R Industrial Automation, Roswell, GA; bottom: courtesy of Giddings & Lewis, Fond du Lac, WI
184
SECTION
2
Components
and Systems
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
CHAPTER
6
The Discrete
Input/Output System
KEY
TERMS
AC/DC I/O interface
bypass/control station
contact output interface
DC I/O interface
digital signal
discrete input interface
discrete output interface
I/O address
I/O module
isolated I/O interface
local rack
master rack
multiplexing
rack enclosure
register/BCD I/O interface
remote I/O subsystem
remote rack
sinking configuration
sourcing configuration
TTL I/O interface
THE ANALOG
INPUT/OUTPUT SYSTEM
One line alone has no meaning; a second one
is needed to give it expression.
—Eugène Delacroix
CHAPTER
SEVEN
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
186
SECTION
2
Components
and Systems
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
CHAPTER
7
The Analog
Input/Output System
CHAPTER
HIGHLIGHTS
Although discrete I/O systems are invaluable tools for PLC controls, they
cannot meet all the demands of new technological and application advances.
Because they can interpret continuous signals, analog I/O interfaces are used
in applications, such as batching and temperature control, where the simple
two-state capabilities of discrete I/O systems are insufficient. This chapter
explains the function and application of analog I/O interfaces, including a
discussion of analog connections and instructions. In the next chapter, you
will learn about another type of I/O system—special function interfaces—
which are used to accomplish specific control tasks.
7-1 OVERVIEW OF ANALOG INPUT SIGNALS
Analog input modules, like the ones shown in Figure 7-1, are used in
applications where the field device’s signal is continuous (see Figure 7-2).
Unlike discrete signals, which possess only two states (ON and OFF), analog
signals have an infinite number of states. Temperature, for example, is an
analog signal because it continuously changes by infinitesimal amounts.
Consequently, a change from 70°F to 71°F is not just one change of 1°F, but
rather an infinite number of smaller changes of a fraction of a degree.
Figure 7-2. Representation of a continuous analog signal.
Figure 7-1. Analog input modules.
Continuous
Signal
Time
Measured
Signal
Courtesy of Allen-Bradley, Highland Heights, OH
187
CHAPTER
7
The Analog
Input/Output System
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
SECTION
2
Components
and Systems
PLCs, like other digital computers, are discrete systems that only understand
1s and 0s. Therefore, they cannot interpret analog signals in their continuous
form. Analog input interfaces translate continuous analog signals into
discrete values that can be interpreted by PLC processors. These discrete
values are subsequently used in the control program. Table 7-1 lists some
devices that are typically interfaced with analog input modules.
Table 7-1. Devices used with analog input interfaces.
Figure 7-3. Digitization of an analog signal.
Figure 7-4 illustrates the sequence of events that occurs while reading an
analog input signal. The module transforms the analog signal, through an
analog-to-digital converter (A/D), into 12 bits of digital information that will
be stored in register 1000 after the instruction is executed. After the PLC reads
this information, the control program can reference the register address for
comparisons, arithmetic calculations, etc. The analog value stored in the
register will be in either BCD or binary format.
stupnIgolanA
srecudsnartwolF
srecudsnartytidimuH
srecudsnartllecdaoL
sretemoitnetoP
srecudsnarterusserP
srecudsnartnoitarbiV
srecudsnarterutarepmeT
Analog Inputs
Module transforms input
by digitizing signal
Continuous
Signal
Binary value
stored in registers
PLC
Analog
Input
Module
7-2 INSTRUCTIONS FOR ANALOG INPUT MODULES
Analog input modules digitize analog input signals, thereby bringing analog
information into the PLC (see Figure 7-3). The modules store this multibit
information in register locations inside the PLC. The analog instructions used
with analog input modules are similar to, if not the same as, the instructions
used with multibit discrete inputs. The only difference between them is that
analog multibit instructions are the result of a digital transformation of the
analog signal, while discrete multibit instructions are the result of many
multibit devices (or separate signals) connected to the same number of
discrete input connections.
188
SECTION
2
Components
and Systems
Industrial Text & Video Company 1-800-752-8398
www.industrialtext.com
CHAPTER
7
The Analog
Input/Output System
Figure 7-4. Steps in converting an analog signal to binary format.
EXAMPLE 7-1
What will the contents of register 1000 be after the multibit instruction
shown in Figure 7-5 is executed? Note that the digitized value
corresponding to the analog transformation shown in the figure is
represented by 12 bits in binary format.
Figure 7-5. Multibit instruction.
SOLUTION
After the instruction is executed, the contents of register 1000 will be:
0000 1010 1100 1111
This number corresponds to the digitized value generated by the
module. Since the value is represented in 12 bits, the preceding bits
are filled with 0s. Note that the value stored in register 1000 is in binary.
Its decimal equivalent, for computational purposes, is 2767.
➀ The transducer detects the
process signal (e.g.,
temperature).
➁ The transducer transforms
the process signal into an
electrical signal that the
analog input can recognize.
➂ The analog input transforms the signal into a 12-bit value proportional to the
electrical input to the module.
√ A block transfer in instruction, or another analog input instruction, transfers the
12-bit value to the PLC.
➄ The PLC stores the 12-bit digital value in a memory location for future use.
Transducer
1 2
4
5
3
Analog Input Module
A/D
12 Bits
To
PLC
Storage Area
Word/
Register
1000
171615141312111076543210
1110 789 6543210
4
150
˚
C +5.7
VDC
T
R
A
N
S
D
U
C
E
R
Analog Input Module
A/D
12 Bits
Storage Area
Word/
Register
1000
1110 789 6543210
10 101 1001111
Block transfer in
instruction
Process
171615141312111076543210