Chapter 2
Hardware Overview
©
National Instruments Corporation
2-3
GPIB-120B User Manual
Selecting a Data Transfer Mode
To select a data transfer mode, refer to the following descriptions of
each mode.
Unbuffered Mode
In unbuffered mode, each data byte is transmitted using the GPIB
double-interlocked handshaking protocol. For long data streams, transfers
are slower than transfers using buffered mode. However, the GPIB
isolator/expander is transparent in unbuffered mode.
Buffered Mode
In buffered mode, the GPIB isolator/expander uses FIFO (first-in-first-out)
buffers to buffer data between the remote and local sides of the isolation
barrier. For long data streams, the data throughput is much higher than with
unbuffered mode.
However, a few applications may not operate properly in buffered mode.
For example, a GPIB device on the local side of the isolator/expander is
addressed to talk, another device on the remote side is addressed to listen.
When the Talker sources data bytes, the GPIB isolator/expander accepts the
data bytes and stores them in a FIFO buffer. At the same time, the GPIB
isolator/expander reads data from the FIFO buffer and sources data bytes to
the Listener. If the FIFO buffer contains data, the number of bytes sourced
by the Talker differs from the number of bytes accepted by the Listener.
Therefore, there could be situations in which the talker will assume the
listener has accepted data which the listener has not yet received because
it is still in the FIFO buffer. If this situation is unacceptable for your
application, you must use unbuffered mode, in which the 3-wire interlocked
behavior of GPIB is maintained.
Buffered mode applies only to data transfers. GPIB command bytes are not
stored in the FIFO buffers; they are transmitted using the GPIB
double-interlocked handshaking protocol.