Operation
Chapter 3
GPIB-232CV User Manual
3-2
© National Instruments Corp.
D Mode Operation
In D mode, the GPIB-232CV can act only as a GPIB Talker or Listener.
The GPIB-232CV should not be passed control. If it is, the GPIB circuitry
in the GPIB-232CV will accept control and immediately assert ATN*.
This is an error condition that can lock up your system and should therefore
be avoided.
Data Transfers and Buffering
When operating in D mode, the GPIB-232CV accepts data from the GPIB
port using the onboard DMA Controller. The DMA Controller transfers the
data directly to the memory buffer without processor intervention. As a
result, very high-speed GPIB transfer rates are possible—as high as 900
kbytes/sec. This means that the GPIB system performance will not be
degraded by connecting a "slow" printer or plotter to the interface. The
buffered data is then dumped to the device on the RS-232 port at its own
rate. These two asynchronous events take place concurrently and without
user interaction. All operations are completely transparent to the user.
Any serial data that is sent to the GPIB-232CV will be buffered and then
passed along to the GPIB port whenever the GPIB-232CV is addressed to
talk. Depending upon how much data, if any, is expected to be sent to the
GPIB-232CV, via the serial port, two different serial port input buffer sizes
can be used for maximum efficiency. The size of the serial input buffer can
be set to either 256 bytes (U22 switch 2 set to OFF), or 32 KB (U22 switch
2 set to ON). The serial input buffer size should be set to 256 bytes if only
small amounts of data, such as status information, are expected to be sent
from the serial device to the GPIB-232CV. The serial input buffer size
should probably be set to 32 KB if large amounts of data are expected to be
sent from the serial device to the GPIB-232CV, such as an RS-232
instrument sending thousands of bytes of measurements.
Choosing a Serial Port Buffer Size
Whether you use a 256-byte or 32 KB buffer size, the remainder of the
RAM space is used as the GPIB input buffer. Thus, if a large amount of
GPIB data is expected to be sent to the GPIB-232CV it would be
advantageous to use the small serial port buffer size if possible. Remember
that because the GPIB-232CV implements a hardware handshake scheme as
well as the XON/XOFF software handshake scheme if selected, serial data
Summary of Contents for GPIB-232CV
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