Command ReadLongString :

For most gpib device the read command is enough to talk with the device. In certain case,
the gpibDevice returns a very big string, which is larger than the read buffer. For concret
example, on :CALC:DATA? command, the hp3588 returns 400 strings representing a
spectrum, for a total size> 4Kbytes. This method reads these sort of long string.
For efficiency, its better to use the read command instead of readLongString. This method
is provided for exceptionnal case. Throws DevFailed on error.

ReadLongString Definition
Input Argument	Tango::DEV_LONG	Max expected string length.
Output Argument	Tango::DEV_STRING	The readed string.
DisplayLevel	EXPERT	..
Inherited	false	..
Abstract	false	..
Polling Period	Not polled	..
Command NOT allowed for	MOVING FAULT	..