Changed setReferenceMode (write-only) to referenceMode, read-writable.
This allows us to see what we're doing when setting the reference mode.

For clarity of reading, use the 'STP' command for emergency stop motion
rather than u0018

The PIE873 is sending binary data as response, which cannot be processed with the string interpretation engine right now.
We use string as data type as this does not do any post processing, just to
avoid runtime errors.