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        <article data-uid="Technique-ChainCode">
          <h1 id="machine-chain-code">Machine Chain Code</h1>

<p>A machine tool file can carry its kinematic topology as a <strong>chain code</strong> — a short bracket notation
for the same connectivity the mechanism builder produces, meant for hand-editing the XML. It is read
and written by <a class="xref" href="../../api/Hi.Numerical.Xyzabc.CodeXyzabcChain.html">CodeXyzabcChain</a><small>(API)</small>, which builds the very
same mechanism, so a hand-written code and a builder-drawn structure are interchangeable — with two
exceptions on the way back: a mechanism with two components sharing one name, or with an unnamed
component that more than one segment would have to mention, cannot be written down as code.</p>
<p>The code carries <strong>connectivity only</strong>. Pivots, exact axis vectors, end-anchor offsets and shapes
are properties of the mechanism, and editing the code preserves them for every component whose name
is unchanged.</p>
<h2 id="the-notation">The Notation</h2>
<p>Each segment declares branches from left to right:</p>
<pre><code>[O][base][Z][C][w];[O][base][Y][X][B][S][t]
</code></pre>
<ul>
<li><p><code>[A][B][C]</code> is shorthand for the branches <code>[A][B];[B][C]</code>, so a segment is just a path.</p>
</li>
<li><p>Words are components. The <strong>same word in different segments is the same component</strong>, which is how
segments join into one structure.</p>
</li>
<li><p>The first word of the whole code names the ground anchor. Within a segment each branch runs from
the word on its left to the word on its right.</p>
</li>
<li><p>Reserved words are the topology keywords: <code>X</code>, <code>Y</code>, <code>Z</code>, <code>A</code>, <code>B</code>, <code>C</code> become motion axes, and <code>w</code>
and <code>t</code> become the end anchors. <strong>Any other word is a plain component</strong> carried along without
motion, such as <code>base</code> or <code>S</code> above. A code must name exactly one <code>[w]</code> and one <code>[t]</code>; the six
axes are optional, and each may be used at most once. Words merge by exact name, so no two
components can claim one role; what the check refuses is a code naming no table buckle at all —
<em>the chain code must contain the table buckle [w].</em> A code carrying only <code>[W]</code> is refused by
that same check, and the message names the near miss: <em>The code carries [W], which differs only
in letter case; the keyword is matched exactly, so rename it to [w].</em></p>
</li>
<li><p><strong>Spell the keywords exactly</strong> — upper case for the axes, lower case for <code>t</code> and <code>w</code>. The code
itself and everything downstream of it compare them as written: role resolution, axis discovery,
the default collision pairs, and the end-anchor check a machine file is loaded through. So a
mis-cased axis is a plain component contributing no motion, and a mis-cased end anchor stops the
code building, with a message naming the near miss.</p>
</li>
<li><p><code>[]</code> is an anonymous component. Each occurrence is a separate one, it cannot be referred to from
another segment, and it cannot hold a geometric shape — shapes are stored by component name.</p>
</li>
<li><p>Declaring the same branch twice is harmless; it states the same connection.</p>
</li>
<li><p>A segment with a single word just declares a component, which is how a ground anchor that only
receives branches gets named.</p>
</li>
<li><p>Any number of segments is allowed, so auxiliary structures can branch off any component:</p>
<pre><code>[O][base][Z][C][w];[O][base][Y][X][B][S][t];[base][loader][gripper]
</code></pre>
</li>
</ul>
<h2 id="direction-and-convergence">Direction and Convergence</h2>
<p><strong>Branch direction is free.</strong> A branch authored towards the ground anchor is simply written that way,
as in <code>[Z][O]</code>. A branch belongs to the component farther from the ground anchor, so <code>[O][Y]</code> and
<code>[Y][O]</code> both make <code>Y</code> the axis; only the direction differs. Pointing every branch away from the
ground anchor remains a readability convention — it makes the structure read outwards, ground → base
→ motion axes → the <code>t</code> and <code>w</code> end anchors — but the motion, the axis keywords and the default
collision pairs all read a branch the same way round regardless.</p>
<p><strong>Limbs may meet again.</strong> <code>[A][B][D];[A][C][D]</code> gives <code>D</code> two incoming branches. Only one route
through such a loop drives the motion, so use it for structure rather than for a second motion path.</p>
<p>Axis directions follow the machine-building convention: axes between the ground anchor and <code>w</code> are
seeded negative, the others positive.</p>
<h2 id="see-also">See Also</h2>
<ul>
<li><a href="topology.html">Kinematic Topology</a> — the anchors, branches and assemblies this notation encodes</li>
<li><a href="../../manual/setup/machine-tool.html">Machine Tool</a> — building the same structure in the mechanism builder, and the keywords both forms share</li>
</ul>

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