HP 7942 Service Manual page 31

device command), to receive write data, to send
read or status data, or to perform a specific
operation such as a CLEAR.
MESSAGE -- A unique sequence of command and
text bytes transmitted over the channel during
which the communication link between the devices
(for example, CIC and the disc drive) remains un-
broken.
COMMAND 1vtESSAGE -- A single message con-
taining all the information required to address a
device and initiate an operation, set up a
programmable parameter, or set up an operation to
be executed by an execution message.
EXECUTION MESSAGE -- A single message con-
taining all the information required to carry out
an operation previously set up by a command mes-
sage.
TRANSACTION - - A complete process or opera-
tion carried out over the channel. Some transac-
tions are completed with only a command/report
message, and some require a command, execution,
and a reporting message.
2-11. CHANNEL MANAGEMENT
The following techniques are used by the CIC to
manage the HP-IB: Parallel Poll and Universal
Device Clear.
2-12. PARALLEL POLL. The CIC conducts a
parallel poll on the HP - IB by asserting A TN - Land
EOI -L simultaneously. Each device requiring ser-
vice can then respond by asserting the DIO line
corresponding to its address. The CIC then address-
es only the device requiring service. If more than
one device requires service, the CIC addresses the
device with the highest priority (lowest address)
first. Parallel Poll Enable (PPE) and Parallel Poll
Channel In terf ace
7942 and 7946
Disable (PPD) are internal states of the disc drive
controller. PPE occurs when the disc drive requires
service from the CIC. PPD is the opposite state
and occurs whenever the disc drive is active (for
example, busy executing a command) or idle. A
Parallel Poll Response (PPR) from the disc drive
will occur if the CIC asserts both A TN - Land
EOI-L and if the disc drive is in the PPE state.
2-13. UNIVERSAL DEVICE CLEAR. A univer-
sal command is a channel command that causes all
devices on the HP-IB to perform a pre-determined
interface function. Universal Device Clear erases
inf orma tion stored in the disc drive con troller and
places the disc drive in a known reset state. The
universal device clear format is shown in table
2-4.
2-14. MESSAGE STRUCTURE
Each message contains the following components
(reJ er to table 2 - 5).
- - Primary I Command
(unidirectional from CIC to device)
- - Secondary Command
(unidirectional from CIC to device)
- - Text (bidirectional)
- - Primary II Command
(unidirectional from CIC to device)
The CIC asserts A TN - L during primary and secon-
dary commands to distinguish them from text in-
formation. The disc drive decodes the information
contained in both the primary I and secondary
commands to prepare for action specified in the
text.
2-13