© National Instruments Corp.
3-1
GPIB-140 User Manual
Chapter 3
Configuration and Operation
This chapter describes how to configure and operate a GPIB-140 or GPIB-140/2 system.
Data Transfer Modes
The GPIB extender has two data transfer modes: unbuffered mode and buffered mode.
The data transfer mode determines how data is transmitted across the extension.
Unbuffered Mode
In unbuffered mode, each data byte is transmitted using the GPIB double-interlocked
handshaking protocol. For long data streams, transfers using unbuffered mode are slower
than transfers using buffered mode. However, the GPIB extension is transparent in
unbuffered mode.
Buffered Mode
In buffered mode, the GPIB extenders use FIFO (first-in-first-out) buffers to buffer data
between the remote and local units. For long data streams, you can obtain a much higher
data throughput with buffered mode than with unbuffered mode.
Consider the following situation: a GPIB device on the local side of the extension is
addressed to talk, another device on the remote side is addressed to listen. When the
talking device sources data bytes, the GPIB extenders accept the data bytes and store
them in a FIFO buffer. At the same time, the GPIB extenders read data from the FIFO
buffer and source data bytes to the Listener. Whenever the FIFO buffer contains data, the
number of bytes sourced by the Talker differs from the number of bytes accepted by the
Listener. Because of this behavior, a few applications may not operate properly in
buffered mode.
GPIB command bytes are not stored in the FIFO buffers; they are transmitted using the
GPIB double-interlocked handshaking protocol.
Setting the Data Transfer Mode
The two GPIB extenders in the extension system do not need to be set to the same data
transfer mode. Use switch position 1 on the DIP switch of each GPIB extender to set the
data transfer mode. Slide the switch down to the ON position to set buffered mode; slide
the switch up to set unbuffered mode. See Figure 3-1.