ControlLogix Input Modules

Introduction to ControlLogix Input modules

DiscreteInputControlLogix input modules read the status of equipment.  This could be any type of switch, such as a limit switch when a cylinder extends or retracts to a certain position.   Inputs are also used on valves:   If a valve is open or shut, a switch is made that will put a voltage on a terminal of the input module.  The project in the processor can then take another action, such as starting a motor, or activating a dump table.

Discrete (digital) inputs are either on or off, while analog modules read a “range” of values.   This could be a pressure transducer that measures the pressure on a pipe.   Another analog input could be the speed on  a motor.  Flow rates could be another good example of an analog input.

Let’s take a look at the status lights on the digital input module.  Here there are two rows of status lights (barely visible in this image).   The status lights indicate if a voltage on a terminal.    This is similar to the voltage testers you will buy for your automobile.   On the voltage testers, you just hook up a ground, and touch the probe to a voltage, and the indicator lights.  This is the same with the discrete input module.  Common is connected from an external power supply.   The + side of the power supply will usually go to the switches in a system.

Status Indicators

When the switch is shut, a voltage returns back to a terminal on the discrete input module.   The status light will then energize.  When the status light energizes, a logical 1 is placed in the memory location for that terminal.  The programmer knows what switch the terminal was connected to.    The programmer will then use thDiscreteInputStatusis input in logic to know if an even has taken place on the system, such as a valve being open or shut.  You will also find an OK status light.   When this is solid green, the modules is active and communicating.   A solid red OK light indicates a possible hardware failure in the module.   On the right side of the module, you will see a locking tab.    The purpose of the locking tab is to prevent the RTB (Removable Terminal Block) from coming off of the module.

Removable Terminal Block (RTB)

Input RTBThe terminal block is removable.    To remove the terminal block for wiring the module, be sure the locking tab is in the unlocked position.   The locking tab is at the top of the module on the right side.  There is nothing electronic about this tab.  It just locks the terminal block into place, so it cannot be inadvertently removed.   Once the RTB (removable terminal block) has been removed from the module, a sleeve will slide off the RTB as shown.   This will allow for easy access to the terminals for the purpose of wiring the module.

Please visit the ControlLogix page for more posts about this system!

—  Ricky Bryce

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