Keithley 6220, Reference Manual

Page: 419 / 454

C-6 IEEE-488 Bus Overview Model 6220/6221 Reference Manual
Return to Appendix topics
The complete handshake sequence for one data byte is shown in Figure C-2 .
Once data is placed on the data lines, the source checks to see that NRFD is
high, indicating that all active devices are ready. At the same time, NDAC should
be low from the previous byte transfer. If these conditions are not met, the source
must wait until NDAC and NRFD have the correct status. If the source is a control-
ler, NRFD and NDAC must be stable for at least 100ns after ATN is set true.
Because of the possibility of a bus hang up, many controllers have time-out rou-
tines that display messages in case the transfer sequence stops for any reason.
Once all NDAC and NRFD are properly set, the source sets DAV low, indicating to
accepting devices that the byte on the data lines is now valid. NRFD will then go
low, and NDAC will go high once all devices have accepted the data. Each device
will release NDAC at its own rate, but NDAC will not be released to go high until all
devices have accepted the data byte.
The previous sequence is used to transfer both data, talk, and listen addresses,
as well as multiline commands. The state of the ATN line determines whether the
data bus contains data, addresses, or commands as described in the following
paragraphs.
Figure C-2
IEEE-488 handshake sequence
Bus commands
The instrument may be given a number of special bus commands through the
IEEE-488 interface. The following paragraphs briefly describe the purpose of the
bus commands which are grouped into the following three categories.
1. Uniline commands — Sent by setting the associated bus lines true. For
example, to assert REN (Remote Enable), the REN line would be set low
(true).
DATA
DAV
SOURCE
SOURCE
VALID
NRFD
NDAC
ACCEPTOR
ACCEPTOR
ALL READY
ALL ACCEPTED
Test Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176 - TestEquipmentDepot.com