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HP-IB INTERFACE/26067B
MASTER/SLAVE
(Service Request)
Mailboxes
MEMORY
ADDRESSES
,------------------,
1
Moster Outgoing
(Slave Incoming)
Slave Outgoing
(Master
Incoming)
information
for
the
master.
It
provides
control signals to the I/O control circuitry and
to the HP-IB processor.
The I/O processor
provides control through the operation of the
I/O operating program contained in the I/O
ROM.
b. I/O Control Circuitry
The
I/O
control
circuitry
receives
input
control signals from the I/O processor and
from the printer
control
processor control
lines and address buses.
The I/O control
circuitry
controls access to the I/O
RAM
memory by the master or I/O slave circuits.
Since the I/O RAM memory is shared by both
the master and I/O slave and only one may
access the memory at a time, the control
circuitry regulates which processor (master or
slave) is allowed to access the memory. This
includes control of the address multiplexer for
selection of either the master or slave address
bus used for addressing the I/O RAM memory
and control of the bi-directional
bus gates to
allow the master data bus or the I/O slave
data bus access to the I/O RAM. The control
circuitry controls the address multiplexer by
means of the address select
(AS)
signal.
Whichever
processor is selected to read or
write into or from the I/O RAM memory, its
address bus is selected for addressing. The I/O
RAM memory
access is provided
to
the
processors on a first-come-first-serve
basis.
OOOOH Moster to slave low byte
0001 H
Moster to slave high byte
0002H
Slave to master
low byte
0003H
Slave to master
high byte
Figure 3-3. Master/Slave Request Format
3-3.
FUNCTIONAL CIRCUIT
DESCRIPTION
The HP-IB interface
(see figure 4-,4) transfers
data to the printer from the computer and status
information
from the printer to the computer.
This information exchange is accomplished across
the Hewlett-Packard
Interface Bus (HP-IB) using
HP-IB
protocol.
The
interface
functional
circuits
are described
below, followed
by a
functional circuit operation description.
3-4. FUNCTIONAL CIRCUITS
The
interface
circuitry
consists
of
control
electronics and memory circuits.
The memory
includes 8K of read only memory (ROM) and 4K
of I/O random
access memory (RAM).
The
control electronics consist of the I/O processor,
the
HP-IB
processor
and
the
I/O
control
circuitry.
a. I/O Processor
The I/O
processor (a Z80
microprocessor)
provides
the
main
control
for
interface
operations
which includes interpreting
and
executing commands received from the printer
processor and from across the bus, writing
data
into
the
I/O
RAM
memory,
and
generation
and
regulation
of
packet
The bi -directional
bus gates divide the data
bus into separate sections, to allow either the
master or slave processor data bus use of the
shared I/O RAM memory. Also, since the data
bus is common through the printer, these gates
separate the bus, allowing both processors to
transfer data across their respective sections of
the bus at the same time.
These gates are
normally disabled and must be enabled by the
control circuitry to allow data to pass.
3-3
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