Chapter 2
Hardware Overview
©
National Instruments Corporation
2-5
GPIB-120B User Manual
Parallel Poll Operation
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 some cases,
a remote device on an expanded system cannot respond to parallel polls this
quickly because of propagation delays across the expander and the longer
cables.
When the GPIB-120B notices that a GPIB controller is conducting a
parallel poll, it sends a message to the secondary side to initiate a parallel
poll. The parallel poll on the secondary side is of the same duration as the
poll on the primary side, but delayed by the time it takes to get the message
to the secondary side. In the GPIB-120B this time is approximately 400 ns.
When the poll on the primary side finishes, a message is sent to finish it on
the secondary side, where the poll finishes about 400 ns later. Therefore, if
the secondary side of the GPIB-120B waited until the end of its parallel poll
to send the result of the poll to the primary side, it would be after the
primary side poll has ended. Thus the controller would miss the responses
of the devices on the secondary side.
To solve this problem, the secondary side of the GPIB-120B samples the
state of the bus every 600 ns during parallel polls and sends that data back
to the primary side. Therefore, taking the start of the poll on the primary
side as time 0, the state of the secondary bus is sent to the primary side at
times 600 ns, 1200 ns, 1800 ns and so on, and again when the poll actually
ends.
For slow devices or topologies in which a device on the secondary bus
responds to the parallel poll after the last data packet was sent to the
primary side, the controller would miss the response from this device.
If you encounter this situation, you must configure your controller to
conduct parallel polls longer than 2 µs.