What is Ladder Logic

Ladder Logic

Command systems integrate into a production process that is often driven and supervised from a central command and control point. The programmable logic controls must be able to properly interact with the peripheral terminals of the central system.

The first commissioning as well as possible changes due to the evolution of the production process requires a programming dialogue. The most common way to do this is to use a computer or a programming console. The connection between programming devices and APs is generally done using a serial communication line. Check it out here.

The printers are used to list the information in the AP memory or the information provided by the software used to program it, thus making it easier to view the program and AP data. The user can easily track the program whether it is in the form of a list of instructions or a diagram of the contact scheme. It is easier to verify both the correctness of the implemented scheme and its quality.

The main information that can be listed on the printer are:

– Diagram based on the schema of contacts;

– the program as a list of instructions;

– cross references;

– comment blocks;

– state of processor registers in the central unit;

– listing and condition of the conditions of fortification;

– values ​​for timers and counters.

  • The elements of the AP are:
  1. Central processing unit
  2. Programmer / Monitor
  3. Input / output modules
  4. Drawers and cases
  5. Printer (optional)
  6. Information storage units
  • The central processing unit contains:

– The processor, which consists of one or more microprocessors that make arithmetic and logical calculations as well as communication and interaction between the other components.

– Memory – ROM – used to store the AP operating system and RAM – divided into several blocks in which they keep images of AP outputs and inputs,preset and current values ​​of timers and counters, processor work area,program storage area,other memory areas with special destination.

The RAM loses its contents when the power supply is interrupted, so the AP must have an auxiliary power supply (battery) to maintain the program and the data stored in it.The power supply makes adaptation and conversion of alternating voltage into continuous voltage to feed various AP elements, such as elements connected to inputs and outputs (sensors, execution elements, etc.).

The programmer / monitor is a device that communicates with AP elements and which allows the application program to be transferred to the AP memory, monitoring and visualizing AP operation.

The programmer can be:a manual programmer,an industrial terminal,a personal computer.

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Input / output modules are devices through which the AP communicates with the operating system or the installation that it has to command and with the outside environment.Drawers and cases allow mounting of the programmable machine elements in a unitary assembly.

The printer is used to list the application program after which the AP works and other messages and information related to the system are ordered.

Information storage units are used as secondary storage devices for:

– application program in AP memory

– other data and useful information to save or load them on / off the magnetic support (magnetic tape or disc).

Application programs can be made and recorded on magnetic media in AP application base development laboratories and then loaded into AP memory in command systems placed in production halls.Old versions of the AP were equipped with tape recorders. Recent versions use flexible drives or hard disk drives.

  • Input / output modules all or nothing (digital)

Signals are logical. Typical values ​​are:

– “0” logic = 0 V

– “1” logic = 5 VDC, 12 VDC, 24 VDC, 48 VDC, 12 VAC, 24 VAC, 120 VAC, 240 VAC.

  • Digital input modules

Allows the central unit of the AP to read a logical state of the transducers or sensors associated with it.

Modules generally have 4, 8, 16 or 32 entries. Each input corresponds to a path that processes the electrical signal to produce binary information, the input bit that is stored.

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