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Analog Inputs and Outputs
PLCs must also work with continuous or analog signals
...
Analog signals are
used to represent changing values such as speed, temperature,
weight, and level
...
The PLC must convert the analog signal
into a digital representation
...
The S7-200
analog modules convert standard voltage and current analog
values into a 12-bit digital representation
...
In addition, analog modules are available for use with
thermocouple and RTD type sensors used in to achieve a high
level of accuracy in temperature measurement
...
In the following example a scale
is connected to a load cell
...
In this example the load cell is converting a value of
weight into a 0 - 10 VDC output
...
This
load cell outputs 0 - 10 VDC for a 0 - 500 Lbs input
...
The example application can be expanded to include a conveyor
system with a gate to direct packages of varying weight
...
A
package that weighs at or greater than a specified value is
routed along one conveyor path
...
62
Analog Outputs
Analog outputs are used in applications requiring control
capability of field devices which respond to continuous voltage
or current levels
...
Like analog
inputs, analog outputs are generally connected to a controlling
device through a transducer
...
In
the following example a 0 - 10 VDC signal controls a 0 - 500 Lbs
...
63
Timers
Timers are devices that count increments of time
...
In this example timers
are used to control the length of time between signal changes
...
When a timer
receives an enable, the timer starts to time
...
The output of the timer
is a logic 0 as long as the current time is less than the preset
time
...
S7-200 uses three types of timers: OnDelay (TON), Retentive On-Delay (TONR), and Off-Delay (TOF)
...
The maximum value of
these timers is 32
...
7
...
By adding program elements, logic can
be programmed for much greater time intervals
...
In the following example, a
normally open (NO) switch (S1) is used with a timer (TR1)
...
When S1
is closed, TR1 begins timing
...
When S1 is open, deenergizing TR1,
the TR1 contacts open, immediately extinguishing PL1
...
ON delay indicates that once
a timer receives an enable signal, a predetermined amount of
time (set by the timer) must pass before the timer’s contacts
change state
...
The T-bit is a logic
function internal to the timer and is not shown on the symbol
...
65
In the following simple timer example, a switch is connected to
input I0
...
1
...
100
...
The preset time (PT) value has been set to 150
...
100 x 150 )
...
If the switch were
opened before 15 seconds had passed, then reclosed, the timer
would again begin timing at 0
...
3
T37
IN
150
T37
66
TON
PT
Q0
...
By reprogramming the T37 contact as
a normally closed contact, the function of the circuit is changed
to cause the indicator light to turn off only when the timer times
out
...
I0
...
1
The Retentive On-Delay timer (TONR) functions in a similar
manner to the On-Delay timer (TON)
...
The Retentive On-Delay timer times as long as the enabling
input is on, but does not reset when the input goes off
...
67
The same example used with the On-Delay timer will be
used with the Retentive On-Delay timer
...
3, timer T5 (Retentive timer) begins timing
...
3 is opened the timer
stops
...
3 is closed the timer will begin timing at
10 seconds
...
3
has been closed the second time
...
Here a pushbutton is connected to input I0
...
If after 10 seconds input I0
...
2
...
3 is closed again
...
2
R
I0
...
When the enabling bit
turns on the timer bit turns on immediately and the value is set
to 0
...
TXXX
IN
PT
68
PT
Q0
...
The specific T number chosen
for the timer determines its time base and whether it is TON,
TONR, or TOF
...
When the Start Button is
pressed at input I0
...
0
...
The program then starts pump
2, controlled by output Q0
...
Pump 2 runs for 3 seconds filling
the tank with the second chemical
...
The program starts the mixer motor, connected to
output Q0
...
The
program then opens the drain valve controlled by output Q0
...
4
...
A manual Stop switch is
also provided at input I0
...
69
Review 5
1
...
Three types of timers available in the S7-200 are OnDelay, ____________ On-Delay, and ____________Delay
...
The maximum time available on a 100 millisecond time
base timer is ____________ seconds
...
A count of 25 on a 10 millisecond time base timer
represents a time of __________ milliseconds
...
70
Analog signals are converted into a ____________
format by the PLC
...
Counters
Counters used in PLCs serve the same function as mechanical
counters
...
Control applications that
commonly use counters include the following:
•
Count to a preset value and cause an event to occur
•
Cause an event to occur until the count reaches a preset
value
A bottling machine, for example, may use a counter to count
bottles into groups of six for packaging
...
Counters
increment/decrement one count each time the input transitions
from off (logic 0) to on (logic 1)
...
S7-200 uses three types of
counters: up counter (CTU), down counter (CTD), and up/down
counter (CTUD)
...
The same number cannot be assigned to more than
one counter
...
The
maximum count value of a counter is ±32,767
...
Input CU is the count input
...
Input R is the reset
...
If the current count is equal to or greater than the preset value
stored in PV, the output bit (Q) turns on (not shown)
...
When the current
value is equal to zero the counter output bit (Q) turns on (not
shown)
...
XXX
CTD
CD
LD
PV
Up/Down Counter
The up/down counter counts up or down from the preset value
each time either CD or CU transitions from a logic 0 to a logic 1
...
When the current value (CV) is equal to zero, the
output QD turns on
...
Similarly, the counter resets and loads the current value (CV)
with zero when the reset (R) is enabled
...
XXX
CTUD
CD
CU
R
LD
PV
73
Counter Example
A counter might be used to keep track of the number of
vehicles in a parking lot
...
As vehicles exit the lot
through an exit gate, the counter counts down
...
Up/down counter C48 is used in this example
...
0
...
1
...
2
...
This
value has been stored in the preset value (PV)
...
1
...
As cars enter the lot the entrance
gate opens
...
0 transitions from a logic 0 to a logic 1,
incrementing the count by one
...
Input I0
...
When the count has reached 150
output Q0
...
The “Parking
Lot Full” sign illuminates
...
74
High-Speed Instructions
As discussed earlier, PLCs have a scan time
...
Events may occur in
an application that require a response from the PLC before
the scan cycle is complete
...
High-Speed Counters
High-speed counters are represented by boxes in ladder logic
...
The CPU 224 and CPU 226
supports six high-speed counters (HSC0, HSC1, HSC2, HSC3,
HSC4, HSC5)
...
High-speed counters can be defined
by the definition box to operate in any of the twelve available
modes
...
Refer to the S7-Programmable
Controller System Manual for definitions available for each
counter
...
The maximum clock input frequency is 20 kHz
...
In
quadrature mode, 1x or 4x counting rates can be selected
...
At 4x rate the
maximum counting frequency is 80 kHz
...
In the following illustration a motor is
connected through a starter to a PLC output
...
The
encoder emits a series of pulses as the motor turns
...
Assume the encoder generates 600 pulses per
revolution, and it takes 1000 motor revolutions to move the
object from one position to another
...
The counter would count up 30,000 counts (5000
revolutions x 600 pulses per revolution) and stop the motor
...
Interrupts in the S7-200 are prioritized in the following order:
1
...
I/O Interrupts
3
...
The PTO
provides a square wave output for a specified number of pulses
and a specified cycle time
...
PTOs have a 50% duty cycle
...
The
number of pulses and the cycle time can be changed with an
interrupt
...
After four pulses an interrupt occurs which
changes the cycle time to 1000 ms
...
0
4 Pulses
1000 milliseconds Each
4 Pulses
500 milliseconds Each
Interrupt
Occurs
PWM
The Pulse Width Modulation (PWM) function provides a fixed
cycle time with a variable duty cycle time
...
In the following example
the output has a 10% duty cycle (on 10% off 90%)
...
On
Off
On
Off
Q0
...
This allows machine
operation to be varied to compensate for product variations or
mechanical wear
...
See the
section titled “Connecting External Devices” for examples
...
EM 241
In any complex system rapid communication is essential
...
The EM 241 is an expansion module
that supports communication between an S7-200 PLC and
STEP 7 Micro/WIN via a modem
...
This is useful for remote diagnostics and maintenance, machine
control, alarm systems, and general communication functions
...
Protocols
are rules that identify how devices should communicate with
each other
...
ModBus RTU has been widely used by other companies
...
LANs
are used in office, manufacturing, and industrial areas
...
Siemens has
been a leader in pushing the trend to open systems based
upon international standards developed through industry
associations
...
The PROFIBUS-DP EM 277 module allows connection of
the S7-200 CPU to a PROFIBUS-DP network as a slave
...
79
PROFIBUS DP
PROFIBUS DP is an open bus standard for a wide range
of applications in various manufacturing and automation
processes
...
Through PROFIBUS DP the features of S7-200 PLCs
can be used to their full extent within a distributed system
...
This
provides uniform communication between all SIMATIC devices
on the PROFIBUS DP network as well as devices from other
manufacturers
...
Until recently,
extensive parallel control wiring was needed to connect sensors
to the controlling device
...
The cable is designed so that devices can
only be connected correctly
...
80
PLCs, for example, use I/O modules to receive inputs from
binary devices such as sensors
...
EM 253
Position control describes a range of applications that involve
movement with varying degrees of precision
...
The EM 253 is a positioning module that enables the user to
control the speed and position for either stepper motors or
servo motors
...
81
EM 253 Features
The EM 253 provides functionality for single-axis, open-loop
position control
...
siemens
...
Review 6
1
...
2
...
3
...
4
...
5
...
6
...
7
...
83
Review Answers
Review 1
1) a: input module, b: CPU, c: output module, d: programming
device, e: operator interface; 2) 2; 3) 16; 4) 1010, 0001 000, A
...
Review 3
1) 221, 222, 224, 226, 226XM; 2) b; 3) 2, 7; 4) 8, 6; 5) 14, 10;
6) Q0
...
Review 4
1) a: box, b: contact, c: coil; 2) AND Function - a: 0, b: 0, c: 0, d: 1,
Or Function - e: 0, f: 1, g: 1, h: 1; 3) I0
...
0, and I0
...
Review 5
1) digital; 2) retentive, off; 3) 3276
...
Review 6
1) 256; 2) CTU, CTD, CTUD; 3) ±32,767; 4) high-speed; 5) 12;
6) CP 243-2 Communication Processor; 7) EM 253
...
The book
may be used during the exam
...
After completing the test, mail the answer sheet in for
grading
...
Upon successful
completion of the test a certificate will be issued
...
The component of a PLC that makes decisions and
executes control instructions based on the input signals is
the ____________
...
c
...
b
...
8 bits
32 bits
11
101
b
...
100
111
analog
high-speed
b
...
discrete
normally open
A programming language that uses symbols resembling
elements used in hard-wired control line diagrams is
referred to as a ____________
...
c
...
2 bits
16 bits
An input that is either On or Off is a/an ____________ input
...
c
...
Input module
Operator interface
The binary equivalent of a decimal 5 is ____________
...
c
...
b
...
One byte is made up of ____________
...
c
...
CPU
Programming device
ladder logic diagram
network
b
...
statement list
PLC scan
A type of memory that can be read from but not written to
is ____________
...
c
...
d
...
Which type of interface cable is needed when a personal
computer is used as a programming device for an S7-200
PLC?
a
...
8
...
d
...
a
...
c
...
9
...
a
...
none
10
b
...
7
30
10
...
a
...
11
...
d
...
c
...
d
...
The address designation for output four of an S7-200 is
____________
...
c
...
4
Q0
...
d
...
3
Q0
...
The maximum clock input frequency for high-speed
counters is ____________
...
c
...
37 µs
20 kHz
b
...
10 kHz
1 ms
14
...
a
...
86
3
...
67
b
...
32
...
7
15
...
a
...
3
...
67
b
...
32
...
7
16
...
a
...
17
...
1
1
b
...
10
100
The maximum count of an S7-200 up counter is
____________
...
c
...
d
...
A/An ____________ is used to assign a mode to a highspeed counter
...
c
...
d
...
____________ instructions allows communication
with external devices, such as modems, printers, and
computers
...
c
...
d
...
____________ is used to temporarily override the input or
output status in order to test and debug the program
...
c
...
d
...
sea
...
com/step
...
Online courses
include reviews, a final exam, the ability to print a certificate
of completion, and the opportunity to register in the Sales
& Distributor training database to maintain a record of your
accomplishments
...
These files contain the most
recent changes and updates to the STEP 2000 courses
...
The pictorial glossary can be accessed from anywhere within
a quickSTEP course
...
88