page 210
Rack, shoebox or micro
# of inputs/outputs (digital)
Memory - often 1K and up. Need is dictated by size of ladder logic program. A ladder element will take only a few bytes and will be specified in manufacturers documentation.
# of I/O modules - When doing some exotic applications, a large number of special add-on cards may be required.
Scan Time - The time to execute ladder logic elements. Big programs or faster processes will require shorter scan times. The shorter the scan time, the higher the cost. Typical values for this are 1 microsecond per simple ladder instruction.
Communications - Serial and networked connections allow the PLC to be programmed and talk to other PLCs. The needs are determined by the application.
7.23.1 SWITCHED INPUTS AND OUTPUTS
The Obvious:
A PLC is just a computer. We must get information in so that it may make decisions and have outputs so that it can make things happen.
Inputs:
Switches - Contact, deadman, etc. all allow a voltage to be applied or removed from an input.
Relays - Used to isolate high voltages from the PLC inputs, these act as switches.
Encoder - Can keep track of positions. etc.
page 211
e.g.
normally open push-button
24 V AC
Power
Supply
Normally open temperature switch (See appendix in textbook for more symbols)
I:0.3
Push Button
01
I:0.3
Temperature Sensor
03
PLC Input Card
24V AC
00
01
02
03
04
05
06
07
GND
it is in rack 0 I/O Group 3
7.23.1.1 - Input Modules
•Input modules typically accept various inputs depending upon specified values.
•Typical input voltages are:
12-24 VDC
100-120 VAC
200-240 VAC
12-24 VAC/DC
24VAC
•DC voltages are usually lower and, therefore, safer (i.e., 12-24V)
page 212
•DC inputs are very fast. AC inputs require a longer time (e.g., a 60Hz wave would require 1/60sec for reasonable recognition).
•DC voltages are flexible being able to connect to greater varieties of electrical systems.
•DC input cards typically have more inputs.
•AC signals are more immune to noise than DC, so they are suited to long distances and noisy (magnetic) environments.
•AC signals are very common in many existing automation devices.
7.23.1.2 - Actuators
• Inductive loads - Inductance is caused by a coil building up a magnetic field. When a voltage is removed from the coil, the field starts to collapse. As it does this, the magnetic field is changed back to current/voltage. If this change is too sudden, a large voltage spike is created. One way to overcome this is by adding a surge suppressor. One type of design was suggested by Steel McCreery of Omron Canada Ltd.
page 213
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inductive load |
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output |
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VDC+/VAC |
Uncompensated |
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common |
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VDC-/COM. |
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(PLC) |
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Control Relay |
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Power supply |
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inductive load |
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output |
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L |
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VAC |
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+ |
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for AC loads |
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R |
Vs |
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C |
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- |
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COM. |
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common |
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Relay or Triac |
R = Vs(.5 to 1) ohms |
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C = (.5 to 1)/Adc (microfarads) |
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Vcapacitor = 2(Vswitching) + (200 to 300) V |
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Adc is the rated amperage of the load |
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Vs is the voltage of the load/power supply |
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Vswitching may be up to 10 time Vs |
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inductive load |
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output |
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+ |
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common |
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- |
for DC loads |
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Relay or Transistor |
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Power supply |
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7.23.1.3 - Output Modules
• Typical Outputs
Motors - Motors often have their own controllers, or relays because of the high current they require.
Lights - Lights can often be powered directly from PLC output boards, etc.
• WARNING - ALWAYS CHECK RATED VOLTAGES AND CURRENTS FOR PLC’s AND NEVER EXCEED!
page 214
24 V DC |
120 V AC |
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Output Card |
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Power |
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00 |
Supply |
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COM. |
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01 |
Relay |
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02 |
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03 |
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04 |
Motor |
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05 |
24 V lamp |
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06 |
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07 |
+24 V DC |
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Power |
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COM |
Supply |
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in rack 01 |
GND |
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I/O group 2 |
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O:1.2 |
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Motor |
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03 |
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O:1.2 |
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Lamp |
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07 |
page 215
e.g. output example with dry (relay) contacts |
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120 V AC/DC |
24 V DC |
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Output Card |
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Power |
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Supply |
120 V AC |
00 |
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Power |
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Supply |
01 |
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02 |
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03 |
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04 |
Relay |
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05 |
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06 |
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Motor |
07 |
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in rack 01 |
24 V lamp |
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I/O group 2 |
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O:0.2
Motor
03
O:0.2
Lamp
07
• Typical outputs operate in one of two ways:
Dry contacts - A separate relay is dedicated to each output. This allows mixed voltages (AC or DC and voltage levels up to the maximum) as well as isolated outputs to protect other outputs and the PLC. Response times are often greater than 10ms. This method is the least sensitive to