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108 lines
4.1 KiB
Markdown
108 lines
4.1 KiB
Markdown
---
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title: Program Structure
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---
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Program Structure
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=================
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Programs in XOD are quite similar to electronic circuits. Whereas to build an
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electronic circuit you use various electronic components and connect them
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with wires, in a XOD program you use *nodes* and connect them with *links*.
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Nodes
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-----
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What a node does depends on its type. Like in reality there are ICs to control
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motors, amplify audio signals, store data, in XOD there are many types of
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nodes available. It is easy to create your own as well.
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Some nodes represent physical devices like LED or digital thermometer, other
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are used to transform and filter data. Here are few examples:
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* [`thermometer-tmp36`](/libs/xod/common-hardware/thermometer-tmp36/)
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* [`console-log`](/libs/xod/core/console-log/)
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* [`add`](/libs/xod/core/add/)
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* [`to-percent`](/libs/xod/core/to-percent/)
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You place nodes you’ve chosen for your program into slots to be later connected
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with links.
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Pins, inputs, and outputs
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-------------------------
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Nodes alone are black boxes. To interact with them they expose *pins*. Think of
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pins as of sockets, ports, IC legs, jack-connectors.
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A pin can be either *input* or *output*. Once you feed an input with a new value
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the node is evaluated. As a reaction it can update values of its output pins or
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perform some interaction with real world, e.g. change a motor speed.
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Some nodes send an output on their own as a reaction to some external event. For
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example the [clock](/libs/xod/core/clock/) node sends output with regular time
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intervals.
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Pins are depicted as holes with short labels. Inputs are placed on a darker
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background and outputs are placed on a lighter background.
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Links and values
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----------------
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Nodes talk to each other by transmitting values over *links*. A link is a
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kind of wire that you use to connect one node output to another node input.
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Values in XOD are quite similar to electric signals. However unlike their
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electric counterparts they could carry not only a primitive voltage value, but
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more sensible data like arbitrary numbers and text strings. Learn more about
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values in [Data Types](../data-types/) article.
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In digital electronics voltage values are switched discretely and their change
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usually accompanied by some kind of clock signal. The clock signal is seen as
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a sequence of “moments” which are defined by observing falling or rising signal
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edges on the clock line. Interactions and changes actually happen at that
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moments. I.e. a digital circuit is static until a new clock signal would
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appear.
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Behavior of values in XOD is very much similar. Values change and propogate
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instantly. These cascade updates of values are called *transactions*. And things
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that play a role of clock signals are called *pulses* in XOD. [Execution
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Model](../execution-model/) article describes all principles in detail.
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There are few rules that define which pins are allowed to be linked and which
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are not. They are intuitive enough, although for a formal description you can
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see [Linking Rules](../linking-rules/).
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Patches
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-------
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Nodes linked together form a *patch*. Patches are like modules, documents, files
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in other systems.
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You would have a single patch in a simple project and for complex projects you’d
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likely to have many.
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What makes a patch pretty interesting is that once you’ve created it you can
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use it as a new type of node on other patches! That’s the main idea behind XOD
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extensibility.
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You use special *terminal nodes* to denote input and output pins when the patch
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is used as a node.
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<div class="ui segment">
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<span class="ui bottom attached label">
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Photo by <a href="https://www.flickr.com/photos/26735065@N00/">cutwithflourish</a>.
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</span>
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<p>
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<span class="ui blue ribbon label">Note</span>
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If you’ve heard of modular synthesizers they are very similar to XOD programs.
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The nodes are modules, the links are CV cables with banana connectors,
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the patches are rack chassis for modules.
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</p>
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<div class="ui fluid image">
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<img src="modular-synth.jpg" alt="Modular synth" />
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</div>
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</div>
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