A system diagram is a visual model of a system, its components, and their interactions. With supporting documentation, it can capture all the essential information of a system's design. There are many variations of diagramming style that all fall under this rubric. The style presented here is intended to be optimally consistent with the rest of this courseware.
A system diagram is a visualization of a system as a flow-chart-like diagram.
A system is marked by a box. The box marks the boundary of the system and completely contains it. The boundary need not be physically distinct. We place a label in the box identifying the system.
Since systems transform their inputs into their outputs, we use labelled arrows to represent specific interactions between systems:
Inputs are arrows coming from outside the system, and either:
figure 1 is an example of a very simple system diagram. It shows the very beginning of a system design for an elevator. We have marked the elevator system as a labelled box, and indicated the principal inputs and outputs as different types of arrows.
Some notes about figure 1 are in order.
Since systems are composed of elements that may be other systems, a box representing one system may include smaller boxes for its subsystems.
Figures 1 and 2 show an initial system diagram, one in which the inputs and outputs have been established (in accordance with the requirements), but the design of the product itself has not.
Once a system has been designed, one can expect to generate a system diagram more like figure 3.
The nodes (boxes) inside the main system represent the functional subsystems of the elevator.
The arrows representing flows of mass, energy, and information have been extended from the boundary of the elevator to specific subsystems. Some points of interest about this type of diagram:
electricityflows. Others require more explanation (e.g.
feedback. It is important to be as specific as possible, without making assumptions about design decisions that have not yet been made.
Exercise for the reader: Take some time to study Figure 3, and try to identify as many shortcomings or ambiguities as possible. Discuss them in class.
figure 5 shows an entirely different example of a system diagram. In this case, the product is a toaster (and was created with a different software package than the preceding figures, which accounts for the stylistic differences). In this case, there are far more annotations on the arrows and as such this constitutes a better diagram. Still, this diagram is not perfect.
Exercise for the reader: Take some time to study Figure 5, and try to identify as many shortcomings or ambiguities as possible. Discuss them in class.
As the saying goes, a picture is worth a thousand words. A good diagram can capture a huge amount of information in a very small space. This makes diagrams a dense way to represent information.
Diagrams can also represent non-linear information, such as the multi-dimensional relationships between systems. Text is linear, which prevents it from representing non-linear information efficiently.
Therefore, diagrams are a richer form of information capture than simple text.
One very important reason for creating a specification of a system is to record the decisions made during its design. Besides the legal requirements - being able to account for your actions as an engineer - documentation provides one with the means to remember what one did. It is not likely that one will remember everything one does, and why one did it. A record of decisions taken can work as a memory aid, so that one can work in the future knowing sufficiently well what one did in the past and why.
Diagrams, if properly constructed, are excellent mechanisms to communicate complex information to others, much like a well-made technical drawing. Designing is, among other things, an act of communication, so making sure one has an accurate and relatively easy to read account of design decisions is important for communicating that information to others.
A system diagram captures the results of design activities. While it is possible to use other means to record one's work during designing and then generate the system diagram at the end of the systems design task, it is not recommended.
System diagrams can be used as thinking tools. By studying a system diagram, one can discover problems and shortcomings of the design it represents, and at the same time construct a final document that will capture the entire design.
Thus, system diagrams should be constructed during systems design, and used as the basis for refining and specifying various aspects of the system.
One may use pencil and paper, or software, to build a system diagram. Software-based diagrams are easier to maintain and to produce in a neat and concise form. The final decision is, however, up to each design team.
Salustri's recommendation is draw.io. It's free, works online, and has standalone versions (also free) for most platforms. Also, it works very well.
The development of a system diagram is often done at the same time as the identification of subsystems because the two tools complement each other. The PAS forces the designers to think in terms of the operation of the product, which can often bring to light shortcomings in the system identification chart.
Here are the rules for PAS diagrams in a nutshell.
Boxes for systems: Each box, or node, marks a subsystem. Generally, all boxes look the same, and contain the name of the subsystem.
Inputs and outputs: Every system has inputs and outputs by definition.
Interactions and links: An interaction is shown as a transfer of mass, energy, or information between system elements.
Three arrow types: By convention:
Connectivity matters: Every subsystem must:
Interface specifications: Each arrow (interaction) represents an interface between system elements. Be as specific as possible on the nature of that interface, without deciding how the interface will be implemented – unless you can justify your design decisions.
Here is a bad example of a PAS for a medical stretcher.
Here is a better version of the same thing.
Exercise for the reader Explain why the bad PAS is bad, and how the good one is better. How could you improve the good PAS even more?