<xs:attribute name="source" use="optional">
    <xs:simpleType>
        <xs:restriction base="xs:string">
            <xs:enumeration value="None" />
            <xs:enumeration value="Low" />
            <xs:enumeration value="High" />
            <xs:enumeration value="Diff" />
            <xs:enumeration value="TTL" />
            <xs:enumeration value="VFG0Trig" />
            <xs:enumeration value="TSCT0" />
            <xs:enumeration value="Button" />
            <xs:enumeration value="CXPTrig" />
            <xs:enumeration value="SoftwareTrig" />
            <xs:enumeration value="ScanStep" />
            <xs:enumeration value="VFG0TSCT0" />
            <xs:enumeration value="BoxInTTL0" />
            <xs:enumeration value="BoxInTTL1" />
            <xs:enumeration value="BoxInTTL2" />
            <xs:enumeration value="BoxInTTL3" />
            <xs:enumeration value="BoxInTTL4" />
            <xs:enumeration value="BoxInTTL5" />
            <xs:enumeration value="BoxInTTL6" />
            <xs:enumeration value="BoxInTTL7" />
            <xs:enumeration value="BoxInTTL8" />
            <xs:enumeration value="BoxInTTL9" />
            <xs:enumeration value="BoxInTTL10" />
            <xs:enumeration value="BoxInTTL11" />
            <xs:enumeration value="BoxInDiff0" />
            <xs:enumeration value="BoxInDiff1" />
            <xs:enumeration value="BoxInDiff2" />
            <xs:enumeration value="BoxInDiff3" />
            <xs:enumeration value="BoxInDiff4" />
            <xs:enumeration value="BoxInDiff5" />
            <xs:enumeration value="BoxInDiff6" />
            <xs:enumeration value="BoxInDiff7" />
            <xs:enumeration value="BoxInDiff8" />
            <xs:enumeration value="BoxInDiff9" />
            <xs:enumeration value="BoxInDiff10" />
            <xs:enumeration value="BoxInDiff11" />
            <xs:enumeration value="BoxInOpto0" />
            <xs:enumeration value="BoxInOpto1" />
            <xs:enumeration value="BoxInOpto2" />
            <xs:enumeration value="BoxInOpto3" />
            <xs:enumeration value="BoxInOpto4" />
            <xs:enumeration value="BoxInOpto5" />
            <xs:enumeration value="BoxInOpto6" />
            <xs:enumeration value="BoxInOpto7" />
            <xs:enumeration value="BoxIn24V0" />
            <xs:enumeration value="BoxIn24V1" />
            <xs:enumeration value="BoxIn24V2" />
            <xs:enumeration value="BoxIn24V3" />
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>