PLC learning series 16: PLC Documentation

Documentation is the process of orderly collection and record of data about machines or process and about both hardware and software components of its control system. Such documents are a helpful guide in the design, implementation, deployment, testing, and maintenance of applications.

Benefits of using PLC documentations:

PLC documentation is used by system designers during the design phase. Following are the main benefits:

  • This will provide an easy way for all those concerned with the program to get accurate information through.
  • They’ll act as a project guide during and after the design phase.
  • These will act as instructional material for both the operators interfacing with the network and the maintenance staff who will operate it.
  • This would require replication or modification of the device to meet certain purposes.
  • They can assist the author, answer questions, identify possible problems and adjust the software if the needs change.

Documentation components:

  • System abstract
  • System configuration
  • I/O wiring connection diagram
  • I/O address assignments
  • Internal storage address assignments
  • Storage register assignments
  • Variable declaration
  • Control program printout
  • Stored control program

System abstract:

System abstract provides the following:

  • A clear statement of the issue or mission with the regulation
  • a statement of the objectives to be achieved
  • A summary of the design approach or methodology used to introduce a solution to the problem, which describes the roles of the system’s main hardware and software components and why they have been chosen.

The task specifies that the PLC should control the conveyors so that the parts are sorted correctly. The design philosophy would suggest that two commodity conveyor lines be operated by a single CPU, situated at the central area of the warehouse. It would also indicate that for those areas remote subsystems located in rooms 4 and 5 control the sorters.

At last, the quantitative declaration (i.e. 90 percent sorting accuracy) will allow the user to measure the success of the operation of the system. This descriptive method can communicate general information on functionality to the end user or anyone else who needs to understand the initial control function.

System configuration:

System configuration is a system arrangement diagram. This is a pictorial illustration of the hardware elements laid out in the abstract method. This shows the location, simple interfaces, and basic information of the major hardware components of the device (i.e. CPU, subsystems, peripherals, GUIs, etc.).

The device configuration not only specifies the subsystem’s physical location but also the I / O rack address assignment label.

I/O wiring configuration:

An I / O wire connection diagram shows the actual field input and output device connections to the PLC circuit. The drawing normally includes power supplies and interfaces to the CPU by module.

The below diagram shows the position of each field unit in the rack, community and module to demonstrate the termination address of each I / O point

If the instruments in the field are not specifically connected to the I / O board, the diagram will display terminal block numbers, as shown below:

I/O Address assignment:

An I / O address assignment paper defines each device in the field by address (rack, set, and terminal), type of input or output module (e.g. 115 VAC, 24 VAC), type of device in the field (e.g. limit switch, solenoid), and role of the device in the field.

The table below shows a typical I/O address assignment document:

Internal storage address assignment:

An internal storage address assignment record is an essential part of the overall package of documents. Since internals are used for programming clocks, counters and replacement control relays and are not connected with any field tools, programmers prefer to use them freely without taking into account their use.

The table below illustrates a typical I/O address assignment document for internals:

Storage register assignment:

Many systems use registers to store or retain timer, counter and/or compare the information. It is very critical to keep an accurate record of the use of and modifications to these registers. Just like with I / O assignment records, the storage registry assignment table will indicate if a register is being used or not.

Variable declaration:

Documentation of physical I / O addresses, internal storage addresses, and storage address allocations in a programming environment IEC 1131-3 allows the devices connected to the PLC to be specified or identified as variables via its I/O.

A proper declaration of variable should be included in each assignment document (e.g. I / O assignment, storage register assignment) and contains the name of the input, output, or internal.

Control program printout:

The control program printout is a hard copy of the control logic scheme which is stored in the memory of the controller. The hard copy should be an exact replica of the controller’s memories, whether stored in ladder form or some other language. A typical ladder program document is shown below:

A basic printout of a hard copy displays each coded instruction with the address associated with each input and output. Moreover, this printout does not provide readily knowledge about the role of each instruction or which field tool is being assessed or managed.

For this purpose, software coding alone is not sufficient of understanding of the control system without the previously mentioned documentation. Many vendors provide a set of documents that allows the programming system to enter labels or mnemonic nomenclature for the control program components, usually a PC (personal computer).


Instrumentation Engineer

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