HP 7470A Interfacing And Programming Manual page 194

messages, are sent over the eight data lines and the three handshake
lines. Uni—line m essages are transferred over the five individual lines of
the management bus.
The commands serve several different purposes:
0 Addresses or talk and listen commands select the instruments that
will transmit and accept data. They are all multi—line m essages.
0 Unviersal commands cause every instrument equipped to do so to
perform a specific interface operation. They include multi—linemes­
sages and three uni-line commands: interface clear (IFC), remote
enable (REN), and attention (ATN).
0 Addressed commands (also referred to as primary commands) are
similar to universal commands, except that they affect only those
devices that are addressed and are all multi—line commands. An in­
strument responds to an addressed command, however, only after an
address has already told it to be talker or listener.
0 Secondary commands are multi—line messages that are always used
in series with an address, universal command, or addressed com­
mand to form a longer version of each. Thus they extend the code
space when necessary.
To address an instrument, the controller uses seven of the eight data­
bus lines. This allows instruments using the ASCII 7-bit code to act as
controllers. As shown in the following table, five bits are available for
addresses, and a total of 31 allowable addresses are available in one
byte. If all secondary commands are used to extend this into a two—byte
addressing capability, 961 addresses become available (31 allowable
addresses in the second byte for each of the 31 allowable in the first
byte.)
Command and Address Codes
Code Form Meaning
X 0 0 A5 A4 A3 A2 A1 Universal Commands
X 0 1 A5 A4 A3 A2 A1 Listen Addresses
except
X 0 1 1 1 1 1 1 Unlisten Command
X 1 0 A5 A4 A3 A2 A1 Talk Address
except
X 1 0 1 1 1 1 1 Untalk Command
X 1 1 A5 A4 A3 A2 A1 Secondary Commands
except
X 1 1 1 1 1 1 1 Ignored
Code used when attention (ATN) is true (low).
X = don't care.
A-6 AN HP-IB OVERVIEW