If the first 4 arguments are for specifying ways to output 2 strings, then you can get simpler than that!

int
tdSensor( protocol, protocolLen, model, modelLen, id, dataTypes )
        SV *    protocol
        int     protocolLen
        SV *    model
        int     modelLen
        int &   id
        int &   dataTypes
CODE:
    {
        STRLEN n_a;
        (void) SvPV_force( protocol, n_a );
        (void) SvPV_force( model, n_a );
        SvGROW( protocol, protocolLen );
        SvGROW( model, modelLen );
    }
    RETVAL = tdSensor(
        SvPV(protocol), protocolLen, SvPV(model), modelLen, &id, &data
+Types );
    SvCUR_set( protocol, strlen(SvPV(protocol)) );
    SvCUR_set( model, strlen(SvPV(model)) );
OUTPUT:
    RETVAL
    id
    dataTypes
[download]

But much simpler (and less likely to be buggy) than even that is:

int
_tdSensor( protocol, protocolLen, model, modelLen, id, dataTypes )
        char *  protocol
        int     protocolLen
        char *  model
        int     modelLen
        int &   id
        int &   dataTypes
CODE:
    RETVAL = tdSensor( protocol, protocolLen, model, modelLen, &id, &d
+ataTypes );
OUTPUT:
    RETVAL
    id
    dataTypes
[download]

Plus

sub tdSensor {
    my( $protocolLen, $modelLen ) = @_;
    $protcolLen ||= 1024;
    $modelLen ||= 1024;
    my $protocol = '\0' x $protcolLen;
    my $model = '\0' x $modelLen;
    my $id = 0;
    my $dataTypes = 0;
    my $ret = _tdSensor(
        $protocol, $protocolLen, $model, $modelLen, $id, $dataTypes );
    s/\0.*//
        for $protocol, $model;
    return( $ret, $protocol, $model, $id, $dataTypes );
}
[download]

Which has the significant advantage of making it much easier to improve the interface without having to futz with the super finicky XS part of the code.