Configuration and Operation
Chapter 3
GPIB-140 User Manual
3-4
© National Instruments Corp.
Immediate PPR Mode
In immediate PPR mode, the GPIB extenders do not use the internal PPR data register.
When a Controller on the local GPIB system asserts IDY, the local GPIB extender sends
the IDY message to the remote bus and the response is returned as fast as propagation
delays permit. In your application program or driver configuration utility, the Controller
must allow time to receive the response.
Latched PPR Mode
In latched PPR mode, the GPIB extenders use an internal PPR data register. When a
Controller on the local GPIB system asserts IDY, the local GPIB extender responds by
outputting the contents of the PPR data register to the local GPIB data lines. At the same
time, a parallel poll message is sent to the remote bus. When the local IDY signal is
unasserted, the PPR from the remote system is loaded into the internal PPR data register.
Consequently, the register always contains the response of the previous complete poll.
To obtain the response of both local and remote GPIB systems, your application should
execute two consecutive parallel polls and use the second response.
The software driver library of most Controllers contains an easy-to-use parallel poll
function. If, for example, the function is called
ibrpp
and your application is written in
BASIC, the sequence to conduct 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
Choosing the PPR Mode
Choosing the right PPR mode depends on the type of Controller present in the GPIB
system and the length of cable between the GPIB-140 extenders. However, if your
application does not use parallel polls, the PPR mode does not matter.
Some Hewlett-Packard GPIB Controllers remain in a parallel poll state with IDY asserted
whenever they are not performing another function. A change in the response causes an
interrupt of the application program. In some Controllers, the IDY signal is toggled on
and off, and the duration of the signal can be varied to accommodate delayed responses
over extenders. When used with any of these types of Controllers, the GPIB extender
should be set to immediate PPR mode.
Most other Controllers pulse the IDY signal for about 2
µ
s and expect a response within
that time. When used with this type of Controller, the GPIB extender should use latched
PPR mode if the cable between the extenders is longer than 60 m. For shorter cable
distances, you should use immediate PPR mode.
