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Translating Logical System Design Into a PCB Layout

Key Takeaways

  • Why logical system design supersedes physical design for PCBs.

  • Different diagram types support logical system design.

  • How hierarchical diagrams keep expansive schematics organized and easy to reference during layout.

Traces on a blue PCB

Your design software will help you translate your logical system design into a physical design

Whether aware of it or not, people tend to collapse everyday decision-making into logical black boxes: If I put in this, what do I get out? The exact details of the journey are unimportant. It behooves the average user to not have to concern themself with the minutiae of why a system works, just how it operates in the grand scheme.

When it does come time to design, the black box still holds its use as a learning framework. Getting from input to output is simply the greater nexus of the process, and designers need to determine the waypoints in between as smaller, manageable I/O problems become readily approachable with the proper background. In effect, designers need to take the collapsed black box function that most products begin as and expand it until it has been sufficiently described in detail. 

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 several factors. Complex systems that work with multiple data structures and process flows will likely use all three of these, so systems designers need to understand the information they can communicate.

Logical System Design Is the Basis of Electronics

It’s what’s on the inside that counts for electronic devices. While enclosures and cable assemblies provide critical functionality, they can be seen as supporting the board-level operation. In other words, the logic system comes first as the I/O arbiter and all else falls into place afterward. Because of this, logical system design is more abstract than physical system design; the former generally precedes the latter, as it requires visualization of 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 of 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.

The tasks involved in physical system design are relatively straightforward; PCB layout and mechanical packaging are the two primary portions. With newer PCBs becoming ever more complicated, designers are being asked to do more than 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, inspiring new design techniques. Some examples include rigid-flex or flex PCBs, which require mechanical analysis as part of the design process.

Start With Diagrams and Flesh Them Out Into Systems

Multiple diagrams are used in the construction of logical system design. Arguably, data flow diagrams make the most sense from the perspective of PCB design, as they show how data is 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 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 electronic schematics. Although the term “data flow” implies that this diagram is only applicable to digital systems, this is not entirely 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 the connectivity of nets. 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 help with your new layout.

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