OIS-IDE Interface
OK, so you have a large (140-series) OIS system, but no hard disk (and
no room for a washing-machine size SMD drive!) - what to do?
Because the OIS system is based on the Z80 processor, it is easy to identify
the system bus interface to create your own hardware for it. And if you have
any background in Z80 assembly language, you can patch the operating system
and the PROMS to enable the use of a simple interface built around an 8255
parallel interface chip and several standard TTL components.
These pages describe the construction of a prototype interface, and its
connection to an OIS-125A system. It is still an 'alpha' project, as you
can see by the photos below. It only supports one IDE device.
OIS-IDE In Action!
Inside the OIS-125A
You can see the Maxtor 850MB IDE drive near the CPU board.
This is the rats-nest prototype board of the interface.
The wires that connect the interface board to the CPU card have been soldered
directly to the pins that protrude through the board, for the connectors that
the memory expansion uses. I had to solder on to the control and reset pins
near one of the edge connectors. Eventually, a board will be constructed that
either plugs in place of the memory expansion, or plugs into the open slots
left for the old disk controller.
Limitations
It's slow. I do not know how it compares to some of the drives that one
would normally use on the OIS, but I know it could be faster. It works well
enough to run the Z80 assembler/linker and some other utilities.
The OIS only supports certain disk configurations by default. When you patch
the operating system for the drive you intend to use, it will still show up
in the System Generation list as the 5MB(?) drive (corresponds to drive address
14) - but if you define the total sectors to the actual for the size of drive
you are using, it will format to that capacity. I have not tested it above
170MB, though, it takes way too long to format. I have no idea what other
limitations you might run in to if you specify a larger configuration.
You have to edit two tables in the file SYSGEN.MASTER.ROOT (or OS image
on system disk) to specify the drive parameters. One table specifies the
cylinders,heads,sectors and the other specifies the sectors per cylinder
and total sectors per drive. You can set it to use less than 100% of the
drive by reducing the number of total sectors.
The interface uses only 8 bits of the 16-bit IDE data interface. Thus,
you get only 50% of the disk's total space. This works out good for the
sector size, as the IDE disk has 512-byte sectors, and the OIS expects 256-byte
sectors.
Construction
Schematics, instructions, and the source code for modification of the operating
system will be available here soon.
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