National Instruments GPIB-100A - Manual (Page 25)

Theory of Operation Chapter 4
GPIB-100A User Manual 4-4 © National Instruments Corporation
the remote poll response into register U16. Clearing U45B unasserts XEOI and XATN, and after
they propagate to the remote side, BUS PP is also unasserted. This causes FF U36D to be cleared
as well, terminating the parallel poll process and removing the NRFD condition of the local
extender.
To recap, FF U45B is set from the start of the local poll until the remote response is available and
the local poll is over. FF U36D is set from the time U45B is cleared until the remote poll
handshake is over. While either is set, the local unit remains in an NRFD holdoff.
Data Direction Control
The unit drives the GPIB data lines DI01 to DI08 through transceivers U3 and U4 if there is a
local parallel poll in progress (U47B and U26A) or if the RS FF is set and a remote parallel poll is
not in progress (U47A and U27B). Otherwise, these lines are not driven.
The source for these data lines when they are driven is the remote unit through receivers U15 and
U17 when Unbuffered PPR mode is selected (Switch S1, position 3 open) or when the RS FF is
set and a remote parallel poll is not in progress (U47A and U27B). Otherwise, the source is the
Buffered PPR register U16.
The unit drives the transmission data lines BUS DIO1-8 through drivers U5 and U7 if there is a
remote parallel poll in progress (U37D and U27B) or if the LS FF is set and a local parallel poll is
not in progress (U47C and U26A). Otherwise, these lines are not driven.
EOI
The local unit transmits EOI to the remote side as XEOI if the LS FF is set (transceiver U1A,
U25B, U46B,and driver U30C). Furthermore, XEOI is asserted from the start of a local parallel
poll until the poll handshake signal BUS PP is received from the remote unit and the local poll
stops.
XEOI is received as REOI at the remote unit through receiver U19B. It propagates to the remote
GPIB if the local unit is conducting a parallel poll (U27A/B, U37A and transceiver U1A) or if the
RS remote response (RR) is set and the local unit is not conducting a poll (U47A).