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HP-IB INTERFACE/26067B
SECTION III.
THEORY OF OPERA TION
3-1.
INTRODUCTION
This section provides the theory of operation for
the HP-IB Interface.
A brief description of the
prin ter / in terf ace
comm unica tion
protocol
is
provided, followed by a description the HP-IB
interface.
The HP-IB
interface
circuits
are
described on a functional level (see figure 3-1) to
provide a general understanding of the operation
of the interface.
3-2.
MASTER/SLAVE
SHARED MEMORY
COMMUNICA TION
PROTOCOL
The printer control electronics and the interface
electronics
communicate
with
each
other
through the "shared" I/O RAM memory located
on the HP-IB interface (see figure 3-1). When
either the printer or the interface electronics has
information to transfer to the other (interface or
printer), the information
is stored into the I/O
RAM, by either
the master or the interface
initiating the packet, and removed by the other
(slave or printer).
That is, if the interface has
information for the printer, the interface stores
the information into the I/O RAM memory and
the printer reads the information
from the I/O
RAM memory. Thus, both printer and interface
share this memory for information
exchange.
This shared
memory
communication
protocol
identifies
the printer
as the master and the
interface as the slave.
MASTER
SHARED
MEMORY
FROM
SLAVE
FOR
SLAVE
MASTER/SLAVE
MAILBOXES
MASTER
(ADDRESS
0000-0003
HEX)
PRIN1ER
PACKET/
PACKET/
INTERFACE
CONTROL
DA.TA
I/O
RAM MEMORY
DATA
TRANSFER
(4K)
TRANSFER
-- FROM
FOR
SLAVE
MASTER
SLAVE
INTERRUPT
MASTER
INTERRUPT
Figure
3-1.
Master/Slave
Shared Memory
Communication Protocol
The master/slave shared memory communication
protocol uses a format referred to as a packet to
implement
the shared memory protocol.
The
packet
(see figure
3- 2) is created
by the
originator
and
contains
all the
information
required to inform the master or slave as to what
operation it is being requested to perform. These
operations
include the transfer
of print
data
from
the slave to the master
for
printing,
self -test
information
transfer
between
the
master and the slave, printer configuration
data
from
the
master
to
the
slave,
or
status
information
transfer
from
the slave to the
master.
The packet, created by either the master or slave,
consists of a block of 18 bytes of memory. Each
byte of the packet contains information that the
originator (master or slave) of the packet needs to
pass to the receiver (master or slave) of the
packet.
These commands might be print data
command,
self -test
data
exchange,
or
configuration
information.
The packet
bytes
include
a Command
byte
and
a Command
Modifier byte which identifies the command the
3-1
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