99 lines
4.9 KiB
INI
99 lines
4.9 KiB
INI
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This file was downloaded from ALIENS BBS ISRAEL, the BEST BBS in ISRAEL!
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Call now! 052-506539 24H 250Mb on-line!
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NOTES ON THE USE OF 1.4 MB DRIVES IN AT-COMPATIBLES:
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I wasn't long after 1.4 Mb drives became available on the
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aftermarket that users discovered there are significant problems
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is applying then to AT's. Many of Second Nature's users have
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asked for help or advice, as SecNat has a feature to help in 3.5"
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disk installation on older AT's with no BIOS 3.5" support. All
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sorts of strange stories are filtering back from the people who
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sell such drives, some of it contradictory, so I felt it was time
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to get all the facts in one place.
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The use of 720k 3.5" drives is non-controversial, and the drive
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manufacturers' instructions are generally adequate. 1.4 Mb
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drives, however, _will_not_ and _cannot_ work "out of the box",
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due to a _basic_hardware_incompatibility_:
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a) The Western Digital AT-type controllers (1002 & 1003 WA2)
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_hard-wire_ the state of the /Reduced Write Current output on Pin
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2 to the _data_rate_in_use_: Low for 300 kHz, High for
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_all_others_. 1.4 Mb drives use the 500 kHz data rate for 1.4 Mb
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disks, and the 250 kHz data rate for 720k media. THUS /RWC IS
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ALWAYS HIGH!
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b) The 1.4 Mb drive requires that /RWC be LOW for 720k media, and
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HIGH for 1.4 Mb media. Think of it being much like tape bias on
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a cassette deck -- if it's not matched properly to the media,
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full fidelity recording is _impossible_.
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Note that both of the above factors are _built_into_ the
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hardware, and CANNOT be changed by the BIOS, or software drivers!
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Thus 1.4 Mb drives, when simply plunked in an hooked up, CANNOT
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properly record on 720k media, and will normally TRASH as disk,
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if a write is attempted! Two solutions are known to work:
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1) A switch on the drive allowing /RWC for the 1.4 Mb drive to
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be independent of input Pin 2's state. This may be done by
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installing a small toggle switch in the extra front panel space
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of the 3.5" to 5" adapter used for AT-mounted 1.4 Mb drives. The
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connection to pin 2 of the cable should be broken (on the drive
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side of the connector), and the switch wired so that in the
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"720k" position the circuitry previously connected to Pin 2 is
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closed to Logic Ground. The switch is then open in the other
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"1.4 Mb" position, and produces Low and High levels to /RWC,
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respectively. If 1.4 Mb BIOS support is available (if it's an
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option in your SETUP program), operation will be exactly like a
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PS/2 1.4 Mb drive, EXCEPT you must set the switch appropriately
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to the MEDIA IN USE. If you do not have BIOS support, you may
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use DRIVER.SYS to define the drive, with an entry like:
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PCDOS 3.3: DEVICE=DRIVER.SYS /D:1 /F:7 /S:18 /C
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AST MS-DOS 3.2: DEVICE=DRIVER.SYS /D:1 /F:4 /S:18 /C
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^
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(See your DOS Manual for the
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correct /F: number for 1.4 Mb)
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Command syntax for formatting will be as described in your DOS
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Manual. REMEMBER TO USE THE DRIVE LETTER ASSIGNED BY DRIVER.SYS
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WHEN FORMATTING, if you do not have BIOS support!
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2) The drive can be internally modified to control the state of
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/RWC from the sensor for the extra hole in the body of a 1.4 Mb
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diskette. Surprisingly I know of no commercial drive that does
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this, but I verified it's practicality by today actually
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modifying an Astro brand Internal Drive Kit, supplied as type 1),
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above. Regrettably, modifying the Mitsubishi drive in the Astro
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required milling of the casting, and modification of numerous
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internal parts, but with some 4 hours work it was done, and does
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indeed perform automatically as desired. I suspect that the
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absence of Media Type Sensors in most 1.4 Mb drives is due to the
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proximity of the required sensor location to the Eject Button
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mechanism, usually carried over from the manufacturers' earlier
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720k drive. With this type of media-sensing drive operation is
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exactly as in a PS/2, in that the only operator decision to be
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made is matching the Formatting commands to the actual type of
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media in use. There is an added advantage in that you can't
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"wipe out" the media, by writing to, or formatting, a 720k
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diskette with /RWC High -- a mistake that usually requires a
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powerful bulk eraser to repair, and causes the loss of any data
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already on the disk. Anyone wishing to supply a drive of this
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type may contact me for full particulars of the Media Sensor to
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/RWC interconnection.
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I'll note in closing that the Astro unit (advertised in
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InfoWorld) is supplied with software said to work with any DOS
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2.0 and above, at $295, however if you have BIOS support you may
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use PCDOS 3.3 (or AST MS-DOS 3.2), instead of the supplied
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software. They will not unbundle the drive/software package.
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Clyde Washburn
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70305,1211
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