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Chapter 3
Configuration and Operation
© National Instruments Corporation
3-3
GPIB-130 User Manual
Stored PPR Mode (Approach 1)
In the stored PPR mode, the GPIB-130s use an internal PPR data register. When a Controller on
the local GPIB system asserts IDY, the local GPIB-130 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, the control program executes two parallel polls back-to-back and uses 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 the control program is written in BASIC, the
sequence to conduct a poll in Stored PPR mode might be similar to the following sequence:
.
.
.
CALL ibrpp (brd0%, ppr%)
CALL ibrpp (brd0%, ppr%)
IF ppr > 0 GOTO 300
.
.
.
Immediate PPR Mode (Approach 2)
In this mode, the GPIB-130s do not use the internal PPR data register. When a Controller on the
local GPIB system asserts IDY, the local GPIB-130 extender sends the IDY message to the
remote bus and the response is returned as fast as propagation delays permit. The Controller
must allow time to receive the response.
Choosing the PPR Mode
Choosing the right PPR depends on the type of Controllers present in the GPIB system and the
length of cable between the GPIB-130s. Of course, if the control program does not use parallel
polls, the PPR mode does not matter.
Many 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 control program. In other 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 these types of Controllers, the GPIB-130 should be set to Immediate PPR mode.
Most other Controllers pulse the IDY signal for about 2
µ
sec and expect a response within that
time. When used with this type of Controller, the GPIB-130 should use the Stored PPR mode if
the cable between the extenders is longer than about 60 m. For shorter cable distances, using the
Immediate PPR mode is recommended.