Bryan L. Programmable controllers. Theory and implementation
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Figure 7-14. Bits within a register indicating the status of each channel.
7-5 ANALOG INPUT CONNECTIONS
Analog input modules usually provide a high input impedance (in the
megaohm range) for voltage-type input signals. This allows the module to
interface with high source-resistance outputs from input-sensing devices
(e.g., transmitters or transducers). Current-type input modules provide low
input impedance (between 250 and 500 ohms), which is necessary to properly
interface with their compatible field sensing devices.
Analog input interfaces can receive either single-ended or differential
inputs. The commons in single-ended inputs are electrically tied together,
whereas differential inputs have individual return or common lines for each
channel. Single-ended modules offer more points per module than their
differential counterparts. Depending on the manufacturer, a module may be
set to either single-ended or differential mode during software setup using
rocker switches. Figure 7-15 illustrates typical analog connections for single-
ended and differential inputs.
Module’s
Status
Register
(2 Bits per
Channel)
1010101010101010
101010101010
010101010101
101010101010
010101010101
101010101010
010101010101
101010101010
010101010101
Register Channel
76543210
Status of
Channel
Register Bits
12-Bit Value in Binary
Analog
Counts
0
1
2
3
4
5
6
7
200
201
202
203
204
205
206
207
Code
00
01
10
11
Channel Fault
Overflow
Channel OK
Signal Lost
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
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1
2
3
4
5
6
7
8
C
1
1C
2
2C
3
3C
4
4C
Analog
Signal
Transmitter
+
–
Analog
Signal
Transmitter
+
–
(a) Single-ended inputs
Analog
Signal
Transmitter
+
–
Analog
Signal
Transmitter
+
–
(b) Differential inputs
Each channel in an analog interface provides signal filtering and isolation
circuits to protect the module from field noise. In addition to the noise
precautions resident in the module, the user should consider protection from
other electrical noise during the installation of the module (see Chapter 20).
Shielded conductor cables should be used to connect both the input module
and the transducer. These cables lower line impedance imbalances and
maintain a good common mode rejection ratio of noise levels, such as power
line frequencies.
Analog input interfaces seldom require external power supply sources be-
cause they receive their required power from the back plane of the rack or
enclosure. These interfaces, however, draw more current than their discrete
counterparts; therefore, loading considerations should be kept in mind during
PLC system configuration and power supply selection.
Figure 7-15. Connection diagrams for (a) single-ended and (b) differential analog
input modules.
(a) Single-ended inputs
(b) Differential inputs
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Analog output interfaces are used in applications requiring the control of field
devices that respond to continuous voltage or current levels. An example of
this type of field device is a volume adjust valve (see Figure 7-16). This type
of valve, which is used in hydraulic-based punch presses, requires a 0–10
VDC signal to vary the volume of oil being pumped to the press cylinders,
thereby changing the speed of the ram or platen. Table 7-4 lists some other
common analog output devices.
7-6 OVERVIEW OF ANALOG OUTPUT SIGNALS
Figure 7-16. Representation of a volume adjust valve.
Table 7-4. Typical analog output field devices.
Multibit output instructions, which are similar to those used with multibit
discrete outputs, are used to send analog information to field devices. The
controller transfers the contents of a register, generally specified by 12 bits,
to the output module upon the execution of the instruction (see Figure 7-17).
The module then transforms this value, whether BCD or binary, from digital
stuptuOgolanA
sevlavgolanA
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sevirdrotomcirtcelE
sretemgolanA
srecudsnarterusserP
Analog Outputs
1
1C
2
2C
3
3C
4
4C
Voltage or Current
Output Signal
Signal Common
+
–
Transducer
(Voltage Pressure)
100 to 800 psi
Pressure
psi
Analog
Pressure
Signal
Pump
from
oil resevoir
Volume
Adjust
Valve
To hydraulic
platen or ram
press cylinder system
From
oil reservoir
Multibit analog output instructions, which are similar to those used with
multibit discrete outputs, are used to send analog information to field devices.
The controller transfers the contents of a register, generally specified by 12
bits, to the output module upon execution of the instruction (see Figure 7-17).
7-7 INSTRUCTIONS FOR ANALOG OUTPUT MODULES
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to analog and passes it to the field control device. Figure 7-18 illustrates a
multibit instruction transferring 12 bits of data from register 2000 to an analog
output module that is connected to a control valve. These 12 bits of informa-
tion, which are transferred to the field device for control, may be the result of
other computations in the PLC program. Chapter 9 explains these instructions
in more detail.
EXAMPLE 7-4
Transforms
data from PLC
to analog signal
Continuous Signal
(voltage or current)
for analog
control actuator
PLC
Register/Word
Memory Location
Analog
Output
Module
Figure 7-17. Conversion of register data to an analog signal.
Figure 7-18. Steps in converting a binary value into an analog signal.
EXAMPLE 7-4
Figure 7-19 illustrates the binary transfer of information to an analog
output module via a multibit instruction. Assume that the module
converts a digital signal equal to the binary value 0000 0000 0000 (0
decimal) to an analog value that makes the control valve be completely
closed, while it converts a value of 1111 1111 1111 (4095 decimal) to
an analog value that makes the valve be fully open. What will the state
of the valve be according to the contents of register 2000?
The module then transforms this value, whether BCD or binary, from digital
to analog and passes it to the field control device. Figure 7-18 illustrates a
multibit instruction transferring 12 bits of data from register 2000 to an analog
output module that is connected to a control valve. These 12 bits of informa-
tion, which are transferred to the field device for control, may be the result of
other computations in the PLC program. Chapter 9 explains PLC instructions
in more detail.
Analog Output Module
12
Block
transfer out
instruction
Output
Transducer/Actuator
(e.g., control valve)
D/A
1110 789 6543210
Storage Area
Word/
Register
1000
171615141312111076543210
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Figure 7-19 illustrates the binary transfer of information to an analog
output module via a multibit instruction. Assume that the output
module converts a digital signal equaling the binary value 0000 0000
0000 (0 decimal) to an analog value that makes the control valve be
completely closed, while it converts a value of 1111 1111 1111 (4095
decimal) to an analog value that makes the valve be fully open. What
will the state of the valve be according to the contents of register 2000?
S
OLUTION
The value stored in register 2000 is 0000 0011 1111, equivalent to
decimal 63. The valve is open approximately 1/64th (or 1.56%) of its
fully open position. Note that the position of the valve is determined by
the decimal equivalent of the binary value, not the number of 1s—a
binary number with half 1s and half 0s does not mean that the valve is
half open. If the value in the register had been in BCD, the module
would have converted that number to determine the valve position.
7-8 ANALOG OUTPUT DATA REPRESENTATION
Like analog inputs, analog output interfaces are usually connected to
controlling devices through transducers (see Figure 7-20). These transducers
amplify, reduce, or change the discrete voltage signal into an analog signal,
which in turn, controls the output device. Since there are many types of
controlling devices, transducers are available in several standard voltage and
current ratings. Table 7-5 lists some of the standard ratings used in program-
mable controllers with analog output capabilities.
Figure 7-20. Analog output device connected to a transducer.
Transducer
Analog
Output
Module
Binary data
to module
from processor
Transforms
digital value to
voltage or current
Takes voltage or
current and affects or
controls the process
Example:
Voltage to pressure
Effect:
Increase or decrease
psi in process
Process
Figure 7-19. Block transfer of register contents to an analog output module.
SOLUTION
The value stored in register 2000 is 0000 0011 1111, which is
equivalent to decimal 63. Thus, the valve is open approximately
1/65th, or 1.53%, of its fully open position (63 ÷ 4095 = 1.53%). Note
that the position of the valve is determined by the decimal equivalent
of the binary value, not the number of 1s and 0s—a binary number with
half 1s and half 0s does not indicate that the valve is half open. If the
value in the register had been in BCD, the output module would have
converted the value to decimal to determine the valve position.
Analog Output Module
12
Word/
Register
2000
Block transfer
in instruction
Decimal
0
4095
Binary
0000 0000 0000 Valve Closed
1111 1111 1111 Full Open
Control
Valve
000000111111
0000000000111111
D/A
Storage Area
171615141312111076543210
1110 789 6543210
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An analog output interface operates much like an analog input module, except
that the data direction is reversed. As mentioned earlier, a PLC processor can
only interpret digital binary numbers, so it assumes that all other devices
operate in the same manner. An analog output module’s responsibility, then,
is to change the PLC’s data from a binary value to an analog real-world signal
that can be understood by field devices.
The data transformation that occurs in an output interface is exactly opposite
of the transformation in an analog input interface (see Figure 7-21). A digital-
to-analog converter (D/A or DAC) transforms the numerical data (BCD or
binary) sent from the processor into an analog signal. This analog output value
is proportional to the digital numerical value received by the module. Thus,
the D/A converter creates a continuous analog signal with a magnitude
proportional to the minimum and maximum capable analog voltages or
currents of the field device (e.g., 0 to 10 VDC).
Table 7-5. Analog ouput ratings.
Figure 7-21. Digital-to-analog conversion of numerical data in a PLC register.
Output Interfaces
4–20 mA
10–50 mA
0 to +5 volts DC
0 to +10 volts DC
± 2.5 volts DC
± 5 volts DC
± 10 volts DC
171615141312111076543210
Bit 11
Bit 10
Bit 9
Bit 8
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
16-Bit PLC Register
Digital-to
-Analog
Converter
12
Bits
DC Voltage
or
Current
Output
D/A
D/A
Not Used
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The resolution of a digital-to-analog converter is defined by the number of bits
that it uses for the analog conversion. For example, a D/A with a 12-bit
resolution creates an analog signal ranging from 0 to 4095 counts (4096 total
values), which is proportional to a 12-bit digital signal (2
12
= 4096). Therefore,
the analog value 2047 in a 12-bit resolution is equal to half of the full range.
For an analog field device with a range of 0 VDC (closed) to 10 VDC (fully
open), a 2047 analog value would be equal to a 5 VDC signal. Table 7-6 shows
the current, voltage, and psi output values from a D/A with a 12-bit resolution.
Table 7-6. Output values for a 12-bit analog output module.
An analog output module ensures that the value provided by the processor is
proportional to the signal or variable that is being controlled by the field
device. For instance, if an output device provides pressure control ranging
from 100 to 800 psi, the values from the processor, in counts, will be
proportional to this range. Output modules can have both unipolar and bipolar
configurations, which provide control voltages with either all positive values
or negative and positive values, respectively.
EXAMPLE 7-5
A transducer connects an analog output module with a flow control
valve capable of opening from 0 to 100% of total flow. The percentage
of opening is proportional to a –10 to +10 VDC signal at the
transducer’s input. Tabulate the relationship between percentage
opening, output voltage, and counts for the output module in incre-
ments of 10% (i.e., 10%, 20%, etc.). The bipolar output module has a
12-bit D/A (binary) with an additional sign bit that provides polarity to
the output swing.
S
OLUTION
Since the analog output module has a sign bit, it receives counts
ranging from –4095 to +4095, which are proportional to the –10 to +10
VDC signal required by the transducer. Figure 7-22 graphically
illustrates the relationship between the module’s counts, the output
voltage, and the percentage opening.
PLC Register Output
Decimal Binary
0–10 VDC
4–20 mA
0
2047
4095
0 VDC
5 VDC
10 VDC
Pressure
(psi)
0 psi
1000 psi
2000 psi
4 mA
12 mA
20 mA
0000 0000 0000 0000
0000 0111 1111 1111
0000 1111 1111 1111
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Figure 7-22. Relationship between counts, voltage, and percentage.
To formulate the desired table, first determine the equivalent values for
each variable. Since the solution should be expressed in increments
as a function of percentage, the percentage changes are calculated
as follows:
∆Percentage ∆Voltage(−10to+ 10) ∆Counts(−4095to+ 4095)
100 20 8190
1% change as function of voltage =
20 VDC
100
= 0.2 VDC
1% change as function of counts =
8190
100
= 81.90 counts
Note that these computations are magnitude changes. To implement
the table, the offset values for the voltage and counts must be added,
taking into consideration the bipolar effect of the module and the
negative-to-positive changes in counts. Therefore, to obtain the volt-
age and count equivalents per percentage change, add the offset
voltage and count values when the percentage is at 0%. Thus:
Percentage as function of voltage = (0.2 ×
P
) −10 VDC
Percentage as function of counts = (81.9 ×
P
) − 4095 counts
where
P
is the percentage to be used in the table. Therefore, to
calculate the required table, multiply each voltage and count relation-
ship by the desired percentage of opening (see Table 7-7).
The PLC’s software program calculates output counts according to a
predetermined algorithm. Sometimes, the output computations are
expressed in engineering units that indicate a 0000 to 9999 (binary
value or BCD) change in output value. These values must be ultimately
converted to counts—in this case, –4095 to +4095 counts.
–4095 +4095
100%
0%
+10 VDC
–10 VDC
Flow
Control Voltage
-10 VDC to
+10 VDC
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Table 7-7. Equivalent counts, voltages, and percentages.
7-9 ANALOG OUTPUT DATA HANDLING
In the previous section, we explained how a module transfers a signal to the
transducer, which sends it to the controlling output device. Now, we will
discuss how the processor handles this data, along with some common
methods of linearizing output data to reflect engineering units.
The storage or I/O table section of a PLC’s data table area holds the data to
be sent to an analog output module (see Figure 7-23). This data comes from
program computations that, when sent to the module, will control an analog
output device. During the execution of the output update, the processor sends
the register/word contents to the analog module specified by the address in the
instruction. The module transforms the register/word’s binary or BCD value
into an analog output voltage or current. Since the program calculates the
register/word value, the user should take precautions during programming to
avoid computing or sending nonvalid ranges to the module. For example, if
a word location containing a binary value of +5173 is sent to a 12-bit
resolution module without checking for range validity, the module will be
unable to interpret the data, thus emitting an incorrect analog output signal
(5173 in binary uses more than 12 bits).
Like their input counterparts, analog output modules can handle more than
one channel at a time, so one module can control several devices. The
instructions that are used with these output interfaces provide the capability
of transferring several words or register locations to the module. These
instructions are called block transfer out, analog out, block out, or location
out instructions (see Chapter 9). It is possible, however, to find PLCs that use
arithmetic or other instructions to send data to the analog module address,
using the destination register of the instruction.
gninepOegatnecrePegatloVtuptuOstnuoC
%0CDV01–5904–
018–6723–
026–7542–
034–8361–
042–918–
050 0
062+918+
074+8361+
086+7542+
098+6723+
00101+5904+
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Figure 7-23. Transformation of binary storage table data into an analog signal.
Voltage Signal
Return Line
Analog
Output
Module
Analog Output
Instruction Used
D/A
12 Bits
001 11 1 110000
0000001111110000
12-Bit Register
Holding Analog Value
Analog Variable
not used
16-Bit Data Table
Word Location
or
Register Storage
Location
Analog
Transducer