HP 1630A Operating And Programming Manual page 142

An HP-113 Overview Model 1630A/D/G
Information is transmitted on the data lines undersequential control of the three handshake lines (DAV, NRFD,
and NDAC) . No step in the sequence can be initiated until the previous step is completed . Information transfer
can proceed as fast as devices can respond, but no faster than allowed by the slowest device presently
addressed as active . This permits several devices to receive the same message byte concurrently .
The ATN line is one of the five bus management lines. When ATN is true, addresses and universal commands
are transmitted on only seven of the data lines using the ASCII code . When ATN is false, any code of 8 bits or
less understood by both talker and listener(s) may be used .
The IFC (interface clear) line places the interface system in a known quiescent state.
The REN (remote enable) line is used with the Remote, Local, and Clear Lockout/Set Local Messages to select
either local or remote control of each device .
Any active device can set the SRQ (service request) line true via the Require Service Message. This indicates to
the controller that some device on the bus wants attention, such as a counter that hasjust completed a time-
interval measurement and wants to transmit the reading to a printer.
The EOI (end or identify) line is used by a device to indicate the end of a multiple-byte transfer sequence . When
a controller sets both the ATN and EOI lines true, each device capable of a parallel poll indicates its current
status on the DIO line assigned to it .
In the interest of cost-effectiveness, it is not necessary for every device to be capable of responding to all the
lines. Each can be designed to respond only to those lines that are pertinent to its function on the bus.
The operation of the interface is generally controlled by one device equipped to act as controller . The interface
transmits a group of commands to direct the other instruments on the bus in carrying out their functions of
talking and listening.
The controller has two ways of sending interface messages . Multi-line messages, which cannot exist
concurrently with other multi-line messages, are sent over the eight data lines and the three handshake lines.
Uni-line messages are transferred over the five individual lines of the management bus .
The commands serve several different purposes :
B-4
Addresses, or talk and listen commands select the instruments that will transmit and accept data . They
are all multi-line messages .
" Universal commands cause every instrument equipped to do so to perform a specific interface operation .
They include multi-line messages and three uni-line commands : interface clear (IFC), remote enable
(REN), and attention (ATN) .
" 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
instrument responds to an addressed command, however, only after an address has already told it to be
talker or listener .
" Secondary commands are multi-line messages that are always used in series with an address, universal
command, or addressed command 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 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).