National Instruments GPIB-140A User Manual - page 19
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Chapter 3
Configuring and Using Your Hardware
The software driver library of most Controllers contains an easy-to-use parallel poll function.
For example, if the function is called
ibrpp
and your application is written in BASIC, the
sequence to execute a poll in latched PPR mode might be similar to the following sequence:
CALL ibrpp (brd0%, ppr%)
CALL ibrpp (brd0%, ppr%)
IF ppr > 0 GOTO 300
Selecting a PPR Mode
To select a PPR mode, consider the type of Controller present in your GPIB system and the
length of cable between the GPIB-140A 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. In some
Controllers, the IDY signal is toggled on and off, and you can change the duration of the signal
to accommodate delayed responses over extenders. If you are using these types of Controllers,
you should set the GPIB extender to immediate PPR mode.
Most other Controllers pulse the IDY signal for approximately 2 µs and expect a response within
that time. If you are using this type of Controller and if the cable between the extenders is longer
than 60 m, you should set the GPIB extender to latched PPR mode. For shorter cable distances,
use immediate PPR mode.
The two GPIB extenders in your extension system do not need to use the same PPR mode. Select
the PPR mode of the local GPIB extender based on the Controllers on the local GPIB system.
Likewise, select the PPR mode of the remote GPIB extender based on the Controllers on the
remote GPIB system. If no Controllers are physically connected to one of the GPIB extenders,
the PPR mode of that GPIB extender has no effect on your system.
Setting the PPR Mode
To use immediate PPR mode, set DIP switch 3 to the ON position, as shown in Figure 3-3. To
use latched PPR mode, set DIP switch 3 to the OFF position.
Figure 3-3.
DIP Switch Setting for Immediate PPR Mode
= Not used to set Parallel Poll Response (PPR) mode
PARALLEL POLL IMMEDIATE
HS488 ENABLE
BUFFERED TRANSFER
OFF
ON