Measurement Computing Isolator488 - Manual (Page 21)

Section 2 Getting Started
2.12
When ATN is unasserted, the Isolator488 asserts NDAC on both ports,
for a minimum of 200 nanoseconds, while looking for an active talker. If
the talker is detected on the CONTROLLER port, the NDAC lines are
released and the data and handshake line directions remain unchanged. If
the active talker is detected on the INSTRUMENT port, the data line
direction is force to the CONTROLLER port and the handshake line
directions are reversed. The Isolator488 delays a minimum of 1200
nanoseconds to allow the data lines to settle before releasing the NDAC
lines. Once changed, data transfers to 1 megabyte per second are possible.
2.6.4 Parallel Polling
During data transfer operations, the data line transceivers are operated
in tri-state. When a parallel poll is detected, the CONTROLLER port
transceiver is changed to open-collector and the data line direction is forced
from the Instrument Port to the Controller Port. Parallel polling
propagation delay through the Isolator488 is typically 85 nanoseconds.
2.6.5 Passing Control
If another device is included in the system which can receive control,
it must be placed on the CONTROLLER port. IEEE 488 control can not be
passed through the Isolator488.