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.
Selecting a PPR Mode
Refer to the following descriptions when selecting the PPR mode.
Immediate PPR Mode
In immediate PPR mode, the GPIB extenders do not use the internal PPR data register. When a
Controller on the local system asserts IDY, the local extender sends the IDY message to the
remote bus and the response is returned as fast as propagation delays permit. Your application
must allow enough 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 system asserts IDY, the local extender sends the contents of the PPR data register
to the local data lines. At the same time, a parallel poll message is sent to the remote bus.
When the local system unasserts IDY, 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 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.
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
Setting the PPR Mode
To enable immediate PPR mode, set the
PARALLEL POLL IMMEDIATE
DIP switch to
the ON position, as shown in the following figure. To enable latched PPR mode, set this
switch to the OFF position.
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GPIB-140B User Manual