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HP 9895A Disc Memory Command Set
Command Compatibility
The 9895A belongs to a set of command compatible HP-IB interface discs. All of these discs
meet the "HP-300 Compatible HP-IB" standards. In addition, the same sequence of HP-IB
operations can be used to transfer data to and from any of these discs.
There are some subtle differences between HP-300 Compatible HP-IB and IEEE Standard
#488-1978.
1.
An identify code sequence by the host to determine what class of devices and which
device is connected, is not supported by IEEE #488-1978.
2. Disc read and write operations cannot be suspended and then resumed; i.e., an
Untalk or Unlisten command terminates command operation. This is not consistent
with IEEE #488-1978.
Sipce the capacity and organization of a flexible disc is different from other HP-IB compati-
ble discs, the allowable range of certain parameters is also different from the other discs.
Certain commands used in formatting a disc or for diagnostic purposes are unique to the
9895A. Similarly, certain commands supported by other discs are not supported by the
9895A. An unrecognized command causes an error to be set, but has no detrimental effect
on controller operation.
Command Sequences
Much of the 9895A command set shown in this section is made up from two basic types of
HP -IB sequences.
To send information (commands or data) to the 9895A, the bus controller addresses it to
listen, and then sends a secondary command byte followed by a series of information bytes.
,.,
The last information byte sent must be tagged with an EOI. Finally, the bus controller sends
an Unlisten command, and the sequence is complete.
To receive information (status or data) from the 9895A, the bus controller addresses it to
talk, and then sends a secondary command byte. At this point the device sends back a series
of information bytes. In some cases the last information byte will be tagged with an EOl. In
cases where the last information byte is not tagged with an EOI, an additional byte tagged
with an EOI is made available. The extra byte may be used to detect that a byte was dropped
on.the HP-IB, or it can be used to determine the end of a transfer without maintaining a byte
count. Finally, the bus controller issues an Untalk and the sequence is complete.
Sequences other than the ones shown may, in some cases, work; but there is no guarantee
that they will be compatible with other HP-IB discs or with
futur~
HP disc memories.
The controller only operates on a single command at any given time; i.e., overlapped
operations on multiple drives are not possible.
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