Chapter 6 - Wiring Techniques
unless: (1) one of the input devices requires power that is different from the PLC input or
(2) if the current required by the device is more than the PLC can deliver.
Figure 6-9 illustrates the wiring diagram for a system with two normally open pushbutton switches and one photoelectric sensor connected to a PLC with 24 VDC inputs and an internal 24 VDC power supply. The power supply in this case is able to supply enough current to operate all three inputs and power the photoelectric sensor. Notice that the negative output of the internal power supply is connected directly to the INPUT COM of the input unit. The positive terminal of the internal power supply is connected to the two pushbutton switches and the power and collector of the photoelectric sensor. Also, only normally open switches are used to avoid any problems with loss of 24VDC causing an input to be wrongly detected. INPUT 3 is connected to the emitter of the photoelectric sensor to allow the sensor to pull INPUT 3 up when active. This also prevents any problem with loss of power since the collector of the sensor would be open on power loss resulting in the input being OFF. When possible, all inputs should be connected to input devices in such a manner as to cause the inputs to be normally OFF.
Figure 6-9 - Typical PLC Wiring Diagram
6-6. Output Wiring
PLC outputs are of two general types: (1) relay (2) solid state. Relay outputs are mechanical contacts and solid state outputs may take the form of transistor or TTL logic
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(DC) and triac (AC). Relay outputs are usually used to control up to 2 amps or when a very low resistance is required. Transistor outputs are open collector common emitter or emitter follower. This type of output can control lamps and low power DC circuitry such as small DC relays. TTL logic outputs are available to drive logic circuitry. Triac outputs are used to control low power AC loads such as lighting, motor starters and contactors. As with input units, output units are available with a common terminal and isolated from each other. The type of output unit selected will depend upon the outputs being controlled and the power available for controlling those devices. Typically, power for driving output devices must be separately provided since there can be a wide range of requirements depending upon the device.
6-7. Relay Outputs
As stated before, relay outputs are normally used to control moderate loads (up to about 2 amps) or when a very low on resistance is required. Refer to Chapter 1 for a description of a relay. Relay contacts are described as three main arrangements or forms.
The three arrangements are FORM A, FORM B and FORM C. A FORM A relay contact is a single pole normally open contact. This is analogous to a single contact normally open switch. The FORM B relay contact is a single pole normally closed contact which is similar to a single normally closed switch. The FORM C relay contact is a single pole double throw contact. The schematic symbols for the three arrangement types are shown in Figure 6-10.
Figure 6-10 - Relay Contact Arrangements
PLC output units are available with all three contact arrangements but typically FORM A and FORM C are used. By specifying a FORM C contact, both FORM A and FORM B can be obtained by using either the normally open portion of the FORM C contact as a FORM A contact or by using the normally closed portion of the FORM C contact as a FORM B contact. Relay outputs are also available with a common terminal and as isolated contacts.
An output unit with three FORM C contacts having a common terminal is shown in Figure 6-11. Note in this figure that the common terminal of each of the three relays is connected to one common terminal of the output unit labeled OUTPUT COM. Since all relays have one common terminal, all power supplies (there can be one or several) associated with the outputs to be driven must have one common connection. Note that each output has two labeled outputs, NC (normally closed) and NO (normally open). The NC and NO have a number following which is the number of the output associated with the terminal. When an output is turned OFF, the OUTPUT COM terminal is connected to the
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NC terminal associated with that output. When the output is turned ON, the OUTPUT COM terminal is connected to the associated NO terminal.
Figure 6-11 - Common Relay Output
A typical connection diagram for a relay output unit with three FORM C contacts having common output is shown in Figure 6-12. In this drawing, the power source shown is an AC supply which could be the 120VAC building power. Notice that the wiring of this figure shows lamp LT1 as only lighting when OUTPUT 1 is turned ON. This is because the lamp is connected to the NO terminal for OUTPUT 1. Lamp LT2, however, is connected to the NC terminal for OUTPUT 3. This lamp will be ON whenever OUTPUT 3 is turned OFF. This means that if the PLC were to lose power, lamp LT2 would light since there
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would be no power to energize the output relays in the PLC. In the ladder diagram, OUTPUT 3 could be programmed as an always ON coil. The result would be that while the
PLC was powered and running, lamp LT2 would not be lit. If the PLC lost power lamp LT2 would light and provide the operator with an indication that the PLC had a problem. This method could also be provided as a maintenance tool to allow maintenance to troubleshoot and repair the system faster. Also in the drawing of Figure 6-12, a coil K1 is shown connected to OUTPUT 2. This could be a solenoid which drives a plunger into a slide to lock it in place or it could be the coil of a motor starter used to control power to a motor which requires more current than the relay in the output unit can safely carry. Note that to be used in this situation, the coil K1 would have to be rated for AC use at the voltage available from the AC power source. The wiring of these outputs may each be thought of in terms of a switch controlling a lightbulb. A normally closed switch or a normally open switch may be used. The switch is placed in series with the lightbulb and the power source to control current to the light.
Figure 6-12 - Common Relay Wiring
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A PLC relay output unit with three isolated FORM C contacts is shown in Figure 6-13. In this type output unit, the relay contacts have no connection between them.
These output contacts may be used for any purpose to drive any three output devices with no concern for connection between power sources. Each output has three terminals. The
C terminal is the common terminal of the relay. The NO terminal is the normally open contact and the NC terminal is the normally closed contact of the relay. The NC and NO terminals have a number following them that is the associated output number and the same number is indicated for each C terminal as well as indicating that it is associated with a particular output. As with the common relay output unit of Figure 6-11, the NO contact only closes when the output is ON and the NC contact only opens when the output is ON.
Figure 6-13 - Isolated Relay Output
Figure 6-14 shows a typical system output wiring diagram using an output unit having three FORM C isolated outputs. In Figure 6-14, the three outputs are controlling devices with three different power requirements. OUTPUT 1 is controlling a DC lamp, LT1,
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