Chapter 3
Configuration and Operation
© National Instruments Corp.
3-3
GPIB-140 User Manual
Setting the HS488 Mode
The two GPIB extenders in the extension system do not need to be set to the same HS488
mode. However, the maximum data transfer rate is achieved when both sides in the
extension system are doing transfers using HS488.
Use switch position 2 on the DIP switch of each GPIB extender to set the HS488 mode.
Slide the switch down to the ON position to enable the HS488 mode; slide the switch up
to disable the HS488 mode. See Figure 3-2.
OFF
ON
PARALLEL POLL IMMEDIATE
HS488 ENABLE
BUFFERED TRANSFER
= Not used to set HS488 mode
Figure 3-2. Switch Setting for HS488 Mode (Enabled)
Parallel Poll Response (PPR) Modes
According to ANSI/IEEE Standard 488.1-1987, devices must respond to a parallel poll
within 200 ns after the Identify (IDY) message—Attention (ATN) and End or Identify
(EOI)—is asserted by the Controller-In-Charge (CIC). The Controller waits at least 2
µ
s
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. You can solve this problem using one of the following two
approaches in your application program:
•
Specify in your program that the Controller must allow enough time to receive the
response. If you are able to do this, it is the easiest solution. For more information
about this option, see the Immediate PPR Mode section, later in this chapter. If you
are using National Instruments IEEE 488.2 driver software, you can use your
software configuration utility to set the amount of time the Controller waits.
•
Con duct two consecutive parallel polls and use the second response. For more
information about this option, see the Latched PPR Mode section, later in this
chapter.