© National Instruments
|
3-3
GPIB-140A User Manual
To enable HS488, set DIP switch 2 to the ON position, as shown in Figure 3-2. To disable
HS488, set DIP switch 2 to the OFF position.
Figure 3-2.
DIP Switch Setting for Enabled HS488
Parallel Poll Response Modes
According to IEEE 488, devices must respond to a parallel poll within 200 ns after the
Controller-In-Charge (CIC) asserts the Identify (IDY) message—Attention (ATN) and End or
Identify (EOI). The CIC waits at least 2 µs 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. To solve this problem, use one of the following two
solutions in your application:
•
If possible, specify in your application that the CIC must allow enough time to receive the
response. For more information, refer to the following section,
Immediate PPR Mode
. If
you are using the NI-488.2 software, you can use the NI-488.2 Configuration utility to set
the amount of time that the CIC waits.
•
Execute two consecutive parallel polls and use the second response. For more information,
refer to the
Latched PPR Mode
section later in this chapter.
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.
= Not
us
ed to
s
et H
S
4
88
mode
PARALLEL POLL IMMEDIATE
H
S
4
88
ENABLE
BUFFERED TRAN
S
FER
OFF
ON
