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Translating Logical System Design into a PCB Layout

Traces on a blue PCB


It wasn’t until I started working on larger software projects that I realized the importance of different types of diagrams for logical system design. Whether it’s hardware or software, designers and engineers need a way to easily see how data flows through a system and is manipulated along the way.

There are three main types of diagrams that are used as part of logical system design, and the right choice for your system depends on a number of factors. Complex systems that work with multiple data structures and process flows will likely use all three of these, so it is important for systems designers to understand the information they can communicate.

Physical vs. Logical System Design

Electronics design, particularly digital electronics, requires breaking design tasks into two categories: physical and logical design. Each of these categories is generally undertaken by a different design team: mechanical engineers and PCB designers will oversee the physical design tasks for a new product, while electronics engineers tend to address the logical portion of a system.

The tasks involved in physical system design are somewhat obvious; your PCB layout and mechanical packaging are the two primary portions of physical system design. With newer PCBs becoming ever more complicated, designers need to do more than just connect the dots between components on a board. Newer devices, especially consumer electronics, have sleeker, more compact packaging that imposes tighter mechanical constraints on PCBs and has motivated new design techniques. Some examples include rigid-flex or flex PCBs, which require serious mechanical analysis as part of the design process.

Logical system design is more abstract. This task generally precedes physical system design and requires visualizing inputs, outputs, and data flows throughout the system. When most product designers first place their ideas for a new device on paper, they are likely creating a box diagram or flowchart that shows how data moves through the system without drawing a schematic.

The goal in logical system design is to create an initial idea that shows how data moves through a system and is manipulated by different portions of the system along the way. There are different ways to represent this, although the best way to do this is by creating one of many different types of block diagrams for your system. This allows you to focus on the flow of data through the system so that the system’s requirements and capabilities are implemented without creating an entire schematic.

It Starts with Diagrams

There are a number of different diagrams that are used in logical system design. Arguably, data flow diagrams make the most sense from the perspective of PCB design as they show how data are transformed and stored as it moves from the input to the output in the system. Each functional block in a data flow diagram refers to its own set of components, inputs, and outputs, although the components required for a functional block to perform its intended function are omitted from a data flow diagram.

This type of diagram provides a convenient visual representation of how different portions of the system are related and is easy to translate into electronics schematics. Although the term “data flow” implies that this diagram is only applicable to digital systems, this is not really the case. The same diagram could be used to show the intended design for a purely analog system, or a mixed-signal system.


Blue data flow diagram as part of logical system design

Your data flow diagram might look something like this...


An entity-relationship diagram provides another level of abstraction in that it shows the relationship between different sets of data within a system. This type of diagram is more useful when designing embedded software for a system as this requires defining some relationship between data structures. These relationships are then used to program logic devices for a system. They are less useful for generating a schematic as they do not show how data moves between different components.

The third type of common logical system design diagram is the entity life history diagram. This diagram shows how data within a system changes over time as various inputs are added to the system. This might also account for the relationship between two sets of data, thus it communicates some aspect of a data flow diagram and an entity relationship diagram simultaneously.

Logical System Design and Hierarchical Schematics

Whichever method you use to model your data flow and relationships in your system, you will eventually need to translate this information into schematics. With complicated systems that include multiple functional blocks, you’ll be able to enforce some level of organization and clarity when you use hierarchical schematics in your PCB design software.

This methodology allows you to create parent-child relationships between schematics and consolidate multiple components into a single schematic while maintaining net connectivity. This also allows you to place each schematic as a single block in a higher-level schematic. Once you’ve designed your schematics and their hierarchy, you can use a schematic capture tool to place models for your components in an initial layout.


Schematic in OrCAD

Schematic for a portion of a digital system


Logical system design might seem abstract, but the hierarchical schematic tools in your PCB layout and design software can help you take your next idea for a digital system and make it a reality. Allegro PCB Designer and Cadence’s full suite of design tools includes an editor for hierarchical schematics and a schematic capture tool to being your new layout.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.