GPIB-PC Functions — Introduction Section Three
GPIB-PC User Manual 3-14 ©National Instruments Corp.
More About Device and Board Functions
Before proceeding to the next group of functions, you will find it helpful
to compare how a high-level device function can be replaced by several
low-level board functions. Conducting a serial poll is a good example.
In the discussion of the IBRSP function of Group II, this BASIC
example of the device function was used:
CALL IBRSP (PLTR%,STATUS%)
This is equivalent to the following sequence using the board functions
just described:
CMD$ = "?" + CHR$(&H18) + "G!"
CALL IBCMD (GPIB0%,CMD$)
STATUS$ = SPACE$(1)
CALL IBRD (GPIB0%,STATUS$)
CMD$ = "_?" + CHR$(&H19)
CALL IBCMD (GPIB0%,CMD$)
The first IBCMD function is used to send the string of ASCII commands
assigned in the first program line. These are Unlisten (?), Serial Poll
Enable (CHR$(&H18)), talk address of the plotter (G), and listen
address of the board (!). Now that the plotter is enabled to send its
status byte and the board is addressed to receive it, the IBRD function is
called to read the byte and store it in the variable STATUS. The final
IBCMD function completes the poll by sending the command string
consisting of three messages: Untalk (_), Unlisten (?), and Serial Poll
Disable (CHR$(&H19)).
You can see that a high-level device function is easier to use. However,
when an application requires a more complex serial poll routine than the
one just described, such as a serial poll routine which polls several
devices in succession and provides other servicing operations at the
same time, low-level board functions can be used to create such a
routine.