Data is read-from or written-to instruments and interfaces by executing WRDIO, BLKIO, BUFIO, or STRIO commands.  External commands (components) may also be used to preform IO to hardware, but this is rare.

The WRDIO command executes single reads, writes or command (dataless) operations.
The syntax for the WRDIO command is:
WRDIO < #DeviceName#,  #Address#, #SubAdd#, #FCode# > #Data#

The BLKIO command executes block transfers to or from a Kmax histogram widget.
The syntax for the BLKIO command is:
BLKIO < #DeviceName#,  #Address#, #SubAdd#, #FCode# >  #HistogramName# #CountVal#

The BUFIO command executes block transfers into the Kmax event data buffer.  The BUFIO command cannot be used to write data.
The syntax for the BUFIO is:
BUFIO < #DeviceName#,  #Address#, #SubAdd#, #FCode# > #CountVal#  #SizeVal#