![background image](http://html2.mh-extra.com/html/national-instruments/gpib-120b/gpib-120b_user-manual_3397767014.webp)
2-2
|
ni.com
Chapter 2
Hardware Overview
If a GPIB device on Bus B asserts ATN, the Bus B Active Controller state becomes true and the
Bus A Active Controller state becomes false.
Source Handshake Detection (SH)
A device is considered a source handshaker if it is an active Controller sourcing command bytes
or if it is a Talker sourcing data bytes.
After you power on, Bus A and Bus B Source Handshake states are false.
If a GPIB device on Bus A asserts DAV, the Bus A Source Handshake state becomes true and
the Bus B Source Handshake state becomes false.
If a GPIB device on Bus B asserts DAV, the Bus B Source Handshake state becomes true and the
Bus A Source Handshake state becomes false.
Data Transfer Modes
The GPIB-120B isolator/expander has two data transfer modes—unbuffered mode and buffered
mode. The data transfer mode determines how data is transmitted across the expansion. The
switch on the back of the GPIB-120B sets the GPIB isolator/expander operation mode. The
default switch setting is for unbuffered transfer mode.
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