Configuration and Operation
Chapter 3
GPIB-130 User Manual
3-2
© National Instruments Corporation
Setting the Operating Mode
Both GPIB-130s in the extension system must be set to the same operating mode. Use switch
position 1 on the side of each GPIB-130 to set the operating mode. Slide the switch to the up
position to set Buffered mode; slide the switch down to set Unbuffered mode. See Figure 3-1.
On Of
f
. . . . . . . . . . . .
A. Buffered Mode
B. Unbuffered Mode
Key
= Buffer position
= PPoll position
On Of
f
. . . . . . . . . . . .
PPoll Im
PPoll St
Buffered
Unbuffered
PPoll Im
PPoll St
Buffered
Unbuffered
Figure 3-1. Switch Setting for Operating Mode
Parallel Poll Response (PPR) Modes
According to ANSI/IEEE Standard 488.1-1987, devices must respond to a parallel poll within
200 nsec after the Identify (IDY) message (Attention (ATN) and End Or Identify (EOI)) is
asserted by the Controller-In-Charge (CIC). The Controller then waits at least 2
µ
sec 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. The
GPIB-130s can solve this problem using one of the following two approaches:
Approach 1:
Respond to IDY within 200 nsec with the results of the previous poll of the
remote bus.
Approach 2:
Ignore the 200 nsec rule and assume that the Controller will wait long enough to
capture the response.
