<xs:attribute name="trigger_source" use="optional">
    <xs:annotation>
        <xs:appinfo>order:0</xs:appinfo>
    </xs:annotation>
    <xs:simpleType>
        <xs:restriction base="xs:string">
            <xs:enumeration value="TrigSel" />
            <xs:enumeration value="EncASel" />
            <xs:enumeration value="EncBSel" />
            <xs:enumeration value="EncoderQuadrature" />
            <xs:enumeration value="GatedTrigger" />
            <xs:enumeration value="EncoderDivider" />
            <xs:enumeration value="StartOfFrame" />
            <xs:enumeration value="StartOfLine" />
            <xs:enumeration value="EndOfFrame" />
            <xs:enumeration value="EndOfLine" />
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>