Packet Format - HP 26067A Manual

HP-IB INTERFACE/26067B
packet originator needs performed. A Return
Command Status byte to indicate Status byte is
used to indicate to the packet originator the
status of the command operation. Also, if any
data, such as data to be printed, is associated with
the command, the packet Buffer Address byte
identifies the beginning I/O RAM memory
address where this data is stored and the Data
Length byte identifies the number of data bytes
stored.
BYTE 0-7
RESERVED
FOR INTERNAl
OPERAnONS
COMMAND
COMMAND MODIFIER
RETURN COMMAND STATUS
USED FOR
INTERNAL OPERATIONS
(low byte)
-
BUFFER ADDRESS
-
(high byte)
RESERVED
-
FOR INTERNAl
-
OPERAnONS
(low byte)
'---
DATA LENGTH
-
(high byte)
locations of the I/O RAM memory (0000 Hex
and 0001 Hex; referred to as the slave's mailbox)
and the slave places its requests for the master
into the second two memory locations (0002 Hex
and 0003 Hex; referred to as the master's
mailbox). These two bytes identify the beginning
I/O RAM memory address location where where
the originator has placed the packet. Thus, if the
master has a command for the slave, the master
will place the two packet address bytes into the
first two locations of memory (0000 and 0001
hexidecimal) and then interrupt the slave. The
master interrupts the slave by means of the Slave
Interrupt (INTSLV) signal line. This interrupt
prompts the slave to read its I/O RAM mailbox
memory locations to obtain the address of the
packet.
8
9
10
11
12
13
14
15
16
17
ORHPIB3J
Figure 3-2. Packet Format
The first four I/O RAM memory locations (0000
Hex -0003 Hex; see figure 3-3) are reserved for
originator (the master or slave) of a packet to
indicate to the other (slave or master) that an
operation needs to be performed. The master
places its requests for the slave into the first two
3-2
When either the master or the slave has a
command and/or data to transfer for the other,
the originator (master or slave initiating the
command) must create a packet for the command
information and transfers the packet to the I/O
RAM memory. Also, any data associated with
the command is transferred into memory. Then
the originator places the memory address of the
packet into the receivers (slave or master
performing the command). The receiver
retrieves the address of the packet from its
mailbox. Once the receiver has the packet
address, the receiver retrieves the packet and
performs the operation. When the receiver of
the packet has completed the packet operation
(or is unable to complete the command or gets
unexpected results), the receiver writes the status
into the packet Returned Command Status byte
(byte 10), returns the packet into the shared
memory, and interrupts (hardware interrupt) the
originator. The interrupt prompts the originator
to look for a packet address in its mailbox,
retrieve the packet, and read the status. This
sequence allows the master and slave a means of
transferring information to each other, through
the shared memory.