214
◾
Introduction to Industrial Automation
4. The nominal current of the module’s outputs, which denotes the maximum electric current
value that a digital output can hold, during the operation of the corresponding load
5. The presence or absence of an optical isolation in the circuits of the I/O digital modules
Apart from the previous fundamental technical specifications, there is also a set of secondary
technical characteristics that can be very useful during the application stage. Especially in cases of
operational problems of PLCs, only knowledge of these characteristics can lead to an explanation
of the problem, and hence to specific and straightforward solutions. Every manufacturer of a PLC
provides these specific technical characteristics in the corresponding operational manuals, where,
based on this information, the engineer can understand if all the corresponding hardware is uti-
lized properly and safely, and also to denote the restrictions that exist in these I/O modules that
DIGITAL
OUTPUTS
16Χ24 V DC
0,5 A
4
5
6
7
00
11
22
33
C
L1
4
5
6
7
0
1
2
3
C
L1
4
5
6
7
00
11
22
33
C
L1
4
5
6
7
0
1
2
3
C
L1
DIGITAL
OUTPUTS
8Χ24 V DC
0,5 A
00
11
22
33
C
L1
44
55
6
77
C
L1
4
5
6
7
0
1
2
3
C
L1
4
5
6
7
0
1
2
3
C
L1
0
1
2
3
C
L1
4
5
6
7
C
L1
Digital
outputs
8Χ24 V DC
0,5 A
Digital
inputs
16Χ24 V DC
0,5 A
Figure 6.20 Modules of digital outputs with DC power supply in groups of four and eight out-
puts. Each group contains non-isolated outputs.
Basic Operating Principles of PLCs
◾
215
limit specific types of electrical connections. Some typical examples of these secondary technical
characteristics are the following:
◾
Range of voltage for the digital input, where the input signal is recognized as a logical “0”. As
an example, in a digital input module, with a nominal operation voltage of 24 V DC, every
input voltage from
−
30 V until +5 V is recognized as a logical “0”.
◾
Range of voltage for the digital input, where the input signal is recognized as a logical “1”. For
example, in a digital input module, with a nominal operation voltage of 24 V DC, every
input voltage from +15 V until +30 V is recognized as a logical “1”.
◾
Input current for a logical “1”. This indicates the minimum consumed current from a digital
input that the input device should supply to the input circuit of the unit so that the latter
recognizes the input signal as a logical “1”. This current is usually at a level of about 10 mA.
◾
Current consumption of a digital input or output. This indicates the electrical current that is
being consumed by the input or output circuit from the power supply of the PLC. In this
case, there should be no confusion of this current with the current that is being supplied
to the PLC from external sources of power. The values of the consumption current vary
depending on the module type. As an example, a unit of 32 digital inputs can consume an
electrical current of 30 mA, a unit of 16 digital outputs an electrical current of 160 mA, and
a unit of 32 digital output an electrical current of 200 mA.
◾
Leakage current of a digital output. This indicates the maximum value of the electrical cur-
rent that flows in a digital output circuit even when the circuit is in OFF state, which means
a logical “0”. This value has significant importance when the output device contains solid
state components. In this case, it is possible for the flow current to be able to falsely energize
the device, even in the case that the output is in the OFF state. A solution to this problem
will be discussed in Section 6.10.6.
◾
Switching frequency of an output. This indicates the maximum frequency that the switching
component on the digital output circuit can change state, or equivalently it expresses the
frequency that the output can open and close. The switching frequency of an output can be
different for the case of a resistive or conductive load.
◾
Region of permitted rippling of the output voltage. This indicates the minimum and maximum
value of the operational voltage of a digital output module, where between these values the
output circuit could operate without a fault. At this point it should also be noted that the
operational voltage and the corresponding rippling refer to the external power source that is
going to be connected to the output module. For example, in a digital output module, with
a nominal operational voltage of 24 V DC, the allowed variation can be 20–30 V DC, while
for 120 V AC, this could be 92–138 V AC.
Apart from the previous technical specifications, PLC manufacturers may indicate additional
ones that concern very specific cases, which will not be further analyzed and presented at this
point. In general, modules of digital inputs or digital outputs, with different technical characteris-
tics, can be at the same PLC that comes in a modular form. This means that a PLC of this type is
able to control an industrial system with different types of I/O modules, depending on the allow-
able cost and the requirements of the specific industrial application.
216
◾
Introduction to Industrial Automation
Since the beginning of their history, PLCs contained only units of discrete inputs/outputs and thus
only devices of an ON-OFF type, which were able to be connected to PLCs and, correspondingly,
to be controlled. Based on this restriction, at that time, PLCs were able to control only specific parts
of the industrial process (the digital ones). Every analog control scheme requires the utilization of
analog inputs and outputs, and thus with the corresponding huge improvements in the area of elec-
tronics, specific analog modules equipped the PLCs to provide the ability to interact with analog
signals, a step that created a significant impact in the area of industrial control.
An analog input module contains all the necessary circuits for the connection of analog signals and
their corresponding alteration into digital ones. The basic component of an analog input module
is the analog to digital (A/D) converter that converts the analog input signals into a digital value,
indicated in the simplified diagram o
to the analog input’s storage memory where the central processing unit can access it, based on the
requirements stated in the execution program in the PLC. When the analog input module con-
tains a rather large number of inputs and thus has a corresponding high cost, it is not financially
profitable for the manufacturers to offer separate modules for every kind of analog signal that the
industrial application might include. For this reason, the analog input module is equipped with a
signal adapting block that is a simple electrical circuit, able to transform the input analog signal
to a necessary signal compatible with the utilized A/D converter. Thus, if in such a module, the
type of the analog input is altered (e.g., a change of the input sensor), the only thing required is
To CPU/
memory
Digital
value
Analog input module
Signal
conditioning
circuit and
dip switches
Analog
signal
A/D
converter
d
0
d
1
d
2
d
3
d
4
d
5
d
6
d
7
250
128
200
48
300
200
100
mV
1
2
3
4 Sample
converter
Sample Code (8-bit)
Equivalent
analog input (mV)
1
2
3
4
11111010
10000000
11001000
00110000
250
128
200
48
Analog signal input
to A/D converter
(a)
(b)
A/D
Figure 6.21 Simplified block diagram of an analog input module (a) and the basic principle of
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