PLC Hardware Components

Discrete I/O Modules Discrete type I/O interface modules connects field devices of the ON/OFF nature. Discrete output modules are used with field load devices such as lights, small motors, solenoids, and motor starters that require simple ON/OFF switching. Discrete input modules are used with field control devices such as selector switches, pushbuttons, and limit switches.

Discrete I/O Modules The classification of discrete I/O covers bit-orientated inputs and outputs. In this type of input or output, each bit represents a complete information element in itself. Each discrete I/O module is powered by some field-supplied voltage source of a specified value. Common voltage ratings are: Input Interfaces Output Interfaces 12 V ac/dc 12-48 V ac 24 V ac/dc 120 V ac 48 V ac/dc 230 V ac 120 V ac/dc 120 V ac 230 V ac/dc 230 V dc 5 V dc (TTL level) 5 V dc (TTL level) 24 V dc

Discrete I/O Modules The I/O modules receive their module voltage and current from the power supply module through the backplane of the rack enclosure into which they are inserted. Power from this supply is used to power the electronics, that reside on the I/O board. The relatively higher currents required by the loads of an output module are supplied by user-supplied power.

AC Discrete Input Module The input circuit is composed of two basic sections: the power section and the logic section. The power and logic sections are normally coupled together with a circuit, which electrically separates the two.

AC Discrete Input Module Operation When the pushbutton is closed, 120 VAC is applied to the bridge rectifier through R1 and R2. This produces low level DC voltage across the LED. The resulting light switches the phototransistor into conduction and the closed status of the pushbutton is communicated to the processor.

A Zener is not like a normal diode A Zener is not like a normal diode. A normal diode lets current flow in only one direction and needs to be installed in the correct direction. A Zener diode is placed in the opposite direction, against the flow of current. A Zener diode will prevent current from flowing until it reaches a certain voltage, depending on the diode rating. Once this critical value is reached, current is allowed to flow. The diode will also try to keep the outgoing voltage at its rated level, say 5 volts. If you feed 10 volts through the diode, the voltage might climb to 5.5-6 volts, but it will try to stay at the same voltage. If you feed too high a voltage through the diode, it will break down and fail.

Input Module Tasks Sense when a signal is received from a sensor Convert the input signal to the correct voltage level for the particular PLC Isolate the PLC from fluctuations in the input signal’s voltage or current Send a signal to the processor indicating which sensor originated the signal

AC Discrete Output Module The output circuit is composed of two basic sections: the power section and the logic section, coupled by an isolation circuit. The output interface can be though of as a simple electronic switch to which power is applied to control the output device.

AC Discrete Output Module Operation The processor sets the output status according to the logic program. This in turn switches the Triac into conduction which, in turn, turns on the lamp. When the processor calls for an output, a voltage is applied across the LED of the isolator, which switches the phototransistor into conduction.

AC Discrete Output Module Provided with LEDs that indicate the status of each output. Fuses are generally required for each circuit. Some modules also provide visual indicators for fuse status. Individual AC outputs are usually limited to 1 or 2 amps. The maximum current load for any one module is also specified.

Interposing Relay Connection Used for controlling larger loads such as motors. The low current output from the module operates the relay coil, the contacts of which operate the motor starter.

Types Of Discrete Output Modules Are used to turn two-state devices either ON or OFF Triac outputs can only be used only for control of AC devices. Transistor outputs can be used only for control of DC devices. Relay outputs can be used with AC or DC devices. However they have a much slower switching time compared to solid-state outputs.

Sinking And Sourcing References Sinking and sourcing references are terms used to describe a current signal flow relationship between DC field input and output devices in a control system and their power supply. Sourcing device with sinking output module circuit

Sinking and Sourcing DC References The design of DC field devices typically require that they be used in a specific sinking or sourcing circuit, depending on the internal circuitry of the device. Sourcing device with sinking input module circuit

Sinking And Sourcing References Sinking device with sourcing input module circuit Field devices connected to the negative side (DC common) of the field power supply are sinking field devices Sinking device with sourcing output module circuit

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