Building block diagrams

Back to Basics: The term “block diagram” refers to a type of diagram used by engineers to visualize system interactions at a high level. Block-diagram visualization allows system engineers to separate needs analysis from system design, and start system-level design before finishing component-level designs.

03/01/2008


The term “block diagram” refers to a type of diagram used by engineers to visualize system interactions at a high level. Block-diagram visualization allows system engineers to separate needs analysis from system design, and start system-level design before finishing component-level designs.

 

Many authors have attempted to lay down formal rules for building block diagrams. In general, however, these rules tend to have limited applicability. As Captain Jack Sparrow’s crew likes to say: “We think of them more as guidelines than as actual rules.”

 

Some useful guidelines include:

 

  • Represent subsystems by rectangle shapes.

  • Interconnect blocks with unidirectional directed (arrow) lines.

  • Identify each block with a subsystem name.

  • Write transfer function formulas for each output into the each block.

  • Label all inputs and outputs with an appropriate variable designation (such as Vi for input voltage and Vo for output voltage).

Block diagrams styles are almost infinitely variable.

Valid block diagram styles vary enormously. Sometimes block diagrams in Control Engineering use two dimensional rectangular shapes, sometimes they use three dimensional boxes, and sometimes they use graphical representations. What is always important is that the visual representation portrays the important relationships in a concise and easily understandable way.

 

Function block programming

 

Because block diagrams are a visual language for describing actions in a complex system, it is possible to formalize them into a specialized programmable logic controller (PLC) programming language. Function block diagrams are one of five programming languages defined in part 3 of the IEC 61131 standard. Since this is a real, bona fide computer programming language, it is highly formalized with strict rules for how diagrams are to be built.

 

Directed lines are used to connect input variables to function inputs, function outputs to output variables, and function outputs to inputs of other functions.

 

It is important to keep in mind that these blocks portray mathematical or logical operations that occur in time sequence. They do not represent the physical entities, such as processors or relays, that perform those operations.

 

The rules require logical sequence to go from left to right and top to bottom. This rule expresses the fact that the function block diagram is a computer language depicting actions that happen in time sequence.

 

Software packages are available to make building specialized block diagrams for most engineering disciplines. Microsoft’s Visio Technical includes templates for general purpose, as well as special purpose, block diagrams. Of course, engineers have been scratching out block diagrams by hand on paper, chalkboard, and backs of envelopes for centuries, and this method is often the quickest and easiest way to organize your thoughts.

 

 

 

 

Author Information

C.G. Masi is a senior editor at Control Engineering. Contact him at charlie.masi@reedbusiness.com .

 



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Robotic safety, collaboration, standards; DCS migration tips; IT/OT convergence; 2017 Control Engineering Salary and Career Survey
Integrated mobility; Artificial intelligence; Predictive motion control; Sensors and control system inputs; Asset Management; Cybersecurity
Big Data and IIoT value; Monitoring Big Data; Robotics safety standards and programming; Learning about PID
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This article collection contains several articles on how automation and controls are helping human-machine interface (HMI) hardware and software advance.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Big Data and bigger solutions; Tablet technologies; SCADA developments
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.

(copy 5)

click me