Measurement Computing Extender488 - Manual (Page 19)

Section 2 Getting Started
2. 9
2.5 PARALLEL POLL RESPONSE TIMES
The Parallel Poll function is the only extended bus transaction which is not completely transparent
to your system. This is because the s peed at w hich data is transferred serially is slower than the IEEE
specification for Parallel Poll response. Consequently, it is n ecessary for the controller to perform two
Parallel Polls.
When the local extender detects the parallel poll, it outp uts the mos t recent Parallel Poll
information to the controller on the local bus. This data should be discarded by the controller because it is
not current. After receiving a Parallel Poll, the local exten der commands the remote extender to perform a
Parallel Poll, and returns the remote Parallel Poll data byte to the local extender. The bus controller should
now perform another Parallel Poll, and use this data as th e valid Parallel Poll byte. The following table
describes the minimum times the controller should wait b etween consecutive Parallel Polls in order to
assure valid data on the second poll.
Extender488 Parallel Poll Response Times
1 msec
3 msec
8 msec
13 msec
24 msec
48 msec
92 msec
200 msec
115200
19200
9600
4800
2400
1200
600
300
Baud Rate Delay (typ)
The minimum time the controller should wait with the Extender488/F between consecutive
Parallel Polls is 1 millisecond.