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Chapter 5 Using GPIB Interface
164
to wait for all preceding commands to be completed before the commands that follow
*WAI are processed. For example, sending the following command line causes a red
rectangle to be drawn and then overwritten by the ColorBar image.
IMGL
COLORBAR; IMGU; IMGE;
Note that the IMGU command merely requests that the current image be rendered
eventually, not necessarily right away nor in a single attempt. By inserting a *WAI
command after the IMGU, the generator is instructed to render the ColorBar image first,
before rendering the red rectangle. This command ensures that the generator draws the
red rectangle on top of the ColorBar image.
IMGL
COLORBAR; IMGU; *WAI; IMGE;
The generator will wait until the ColorBar image has been completely rendered before it
reads and executes the RECT command.
The
command is required by the IEEE-488.2-1992 standard, but also works with
*WAI
GPIB and RS-232 protocols as well.
Completion handshake
The generator returns a > prompt immediately after an FMTU, IMGU, ALLU, BOOT, INIT,
or SCAL command is received, even if these commands have not finished executing. If
the system controlling the generator must know when the process started by one of these
commands has been completed, then append *OPC? to the command string.
For example, the following command causes the generator to wait until all processes have
been completed before responding with the number 1 and sending the > prompt.
FMTL
VGA_M3;
IMGL
The *OPC? command is required by the IEEE-488.2-1992 standard, but also works with
GPIB and RS-232 protocols as well.
Input buffer
Since some commands may take longer to execute, the generator has an input buffer.
This input buffer is 255 characters long and can be written to by the host controller while
the generator is busy executing or parsing previous messages. If the input buffer becomes
full, the generator will hold off the controller until there is room in the buffer. For this
reason, a program message cannot be longer than 255 characters including terminator.
RECT
RED 200 200 0 0 GRAYPAT100
RECT
FLAT; ALLU; *OPC?
RED 200 200 0 0 GRAYPAT100
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