HS488 Enabled
After the Talker indicates that it wants to issue HS488 transfers, HS488 is enabled and the
GPIB extender accepts data using the HS488 protocol. Also, when talking, the GPIB extender
always tries to use the HS488 mode. In HS488 mode, FIFO buffers buffer data during HS488
transfers, even if the data transfer mode is set to unbuffered. When you use the HS488
protocol with the GPIB extender, you should set the GPIB cable length to 5 m for both the
local and the remote system. To do so, use your IEEE 488.2 software configuration utility.
Setting the HS488 Mode
The two GPIB extenders in your extension system do not need to use the same HS488 mode,
however, the system uses the maximum data transfer rate when both sides in your extension
system use HS488.
To enable HS488, set the
HS488 ENABLED
DIP switch to the ON position, as shown in the
following figure. To disable HS488, set this switch to the OFF position.
Figure 10. DIP Switch Setting for Enabled HS488
OFF
= Not used to set HS488 mode
PARALLEL POLL IMMEDIATE
HS488 ENABLED
BUFFERED TRANSFER
Parallel Poll Response Modes
According to IEEE 488, devices must respond to a parallel poll within 200 ns after the
Controller-In-Charge (CIC) asserts the Identify (IDY) message—Attention (ATN) and End or
Identify (EOI). The CIC waits at least 2 µs before reading the Parallel Poll Response (PPR). In
many cases, a remote device on an extended system cannot respond to parallel polls this
quickly because of cable propagation delays. To solve this problem, use one of the following
two solutions in your application:
•
If possible, specify in your application that the CIC must allow enough time to receive the
response. For more information, refer to
If you are using NI-488.2 software, you can use the NI-488.2 Configuration utility to set
the amount of time that the CIC waits.
•
Execute two consecutive parallel polls and use the second response. For more
information, refer to
PPR Mode Considerations
When selecting a PPR mode, consider the type of Controller present in your GPIB system and
the length of cable between the GPIB-140B extenders. However, if your application does not
use parallel polls, you do not need to select a PPR mode.
Some Hewlett Packard GPIB Controllers remain in a parallel poll state with IDY asserted if
they are not performing another function. A change in the response interrupts the application.
GPIB-140B User Manual
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© National Instruments Corporation
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