HP 7942 Service Manual page 30

Channel In terf ace
7942 and 7946
Table 2- 3. HP-IB Definitions
HP-IB TERM
TALKER
LISTENER
CONTROLLER
SYSTEM
CONTROLLER
DEFINITION
Any device which sends informa-
tion over the HP-IB.
Any device which receives in-
formation over the HP-IB. Some
devices can function as
LISTENERS or TALKERS.
Any device that has been pro-
grammed to manage data flow
between the TALKER and the
LISTENER(s) in addition to
being a TALKER and a LISTENER.
Any device that functions as a
CONTROLLER and is able to gain
absolute control of the HP-IB
with the Interface Clear (IFC)
signal.
CONSIDERATIONS
There can be only one TALKER send-
ing information over the HP-IB at
a time.
In a parallel poll system, there
can be up to eight LISTENERS re-
ceiving information over the HP-
at the same time.
The CONTROLLER manages data flow
by addressing one device as a
TALKER and one or more devices as
LISTENERS. There can be only one
active CONTROLLER on the HP-IB at
at any time. The active CONTROLLER
is called the CONTROLLER-IN-CHARGE
(CIC) .
There can be only one SYSTEM
CONTROLLER connected to the HP-IB.
2-10. UP-IB COMMUNICATIONS
This section describes the formats and sequences
for the HP - IB commands, messages, and transac-
tions that occur between the Controller-In -Charge
(CIC) and the disc drive. The following list explains
the terms used in this section.
talk and the address of a particular device. The
primary II command terminates the message with
an unlisten or un talk command.
COMMAND -- A parcel of information transmit-
ted over the channel (HP- IB) relating to a specific
operation. Channel commands (usually a single
byte) are used to manage operations on the inter-
face channel. Device commands (usually more than
one byte) are used to control the operation and are
contained within the text of a command message.
UNIVERSAL COMMAND -- A channel command
that causes all devices on the bus to perform a
predetermined interface function. Refer to table
2-4.
PRIMARY COMMAND -- The pnmary I com-
mand is a channel command that begins the mes-
sage sequence. It contains the command to listen or
2-12
Table 2-4. Universal Command Formats
UNIVERSAL UNIVERSAL
COMMAND
DEVICE CLEAR
ATN ATN
[POO1CCCC]
[POO101OO]
P=Parity Bit P=Parity Bit
CCCC=Command Code
SECONDARY COMMAND -- The secondary
command sets up the action required of the disc
drive in the text of the message.
TEXT -- The text of the message can be 1 to n
bytes depending on the required action. The
required action can be to receive further
qualifying information or instructions {such as a