VG 93

The subject is unclear. In Fedin's book it is written like this:

Port #FF.

The system register is used to select disk drives and perform other
auxiliary actions. Its structure:

7 6 5 4 3 2 1 0
│ │ │ │ └─┴─Disk drive number (0 - 3).
│ │ │ └─────Reset VG-93, if 0.
│ │ └───────Head loading.
│ └─────────Disk side (0 - bottom).
└─────────────Recording method (0 - FM, 1 - MFM).

But in Solonov’s book “repairing disk drives” this is not at all the case.

bit 2 - not used
bit 3 - reset
bit 4 - side
bit 5 - FM/MFM

bits 6 and 7 of type are free :-/ Not a word about head loading.

Where is the truth? #2C and #3C rule, but still? And why in all the programs I have seen
the 5th bit is always one?

By the way, does MFM coding have practical applications? Is it being used by anyone anywhere? What is it all about?

WBR, Jason.

/*e-mail: jason2000(scary dog)yandex.ru ICQ: 62235830*/

/np:/ *silence*

11 Dec 02 10:20, Valerij Kozhevnikoff wrote to All:

VK> The system register is used to select drives and perform other
VK> auxiliary actions. Its structure:

VK> 7 6 5 4 3 2 1 0
VK> │ │ │ │ └─┴─Disk drive number (0 - 3).
VK> │ │ │ └─────Reset VG-93, if 0.
VK> │ │ └───────Head loading.
VK> │ └─────────Disk side (0 - bottom).
VK> └─────────────Recording method (0 - FM, 1 - MFM).

VK> But in Solonov’s book “repairing disk drives” this is not at all the case.

VK> bit 2 - not used
VK> bit 3 - reset
VK> bit 4 - side
VK> bit 5 - FM/MFM

VK> bits 6 and 7 of type are free :-/ Not a word about loading the head.
VK> Where is the truth? #2C and #3C rule, but nevertheless?

bit 2 (counting from zero) - reset (0=active)
bit 3 - index pulse for 1818VG93 (0=pulse present)

There is an error in Solonov's book.

VK> And why in all the programs I have seen the 5th bit is always one?

MFM coding method is used.

VK> By the way, does MFM coding have practical applications?
VK> Is it used by anyone anywhere?

Yes, it is used on 3.5 and 5.25-inch drives.

VK> What is it anyway?

FM (frequency modulation) coding with pulse eliminationsynchronization.
MFM encoding provides a twofold increase in recording density compared to
FM.
FM encoding is used on 8-inch drives.

VK> bits 6 and 7 of type are free :-/ Not a word about loading the head.

The Solonovites are buggy.

VK> Where is the truth? #2C and #3C rule, but still? And why in all the
VK> programs I have seen is the 5th bit always one?

There is an assumption that Larchenko and Rodionov screwed up their
bit 3 is actually bit 5, so everywhere you see a one in the
fifth bit.

Nikolaj

VK> │ └─────────Disk side (0 - bottom).
VK> └─────────────Recording method (0 - FM, 1 - MFM).

This option is more “correct”.
Bit 3 is used to stop the disk, the signal from it is sent through a diode to IP
(index marker). Through it, a marker is emulated in the absence of a disk. (VG93
after spin-up will not stop the drive until it receives several index
pulses or receives a reset signal).

VK> Where is the truth? #2C and #3C rule, but still? And why in all
VK> programs that I have seen is the 5th bit always one?

There is a suspicion that in some controllers bit 5 is supplied to switch the FM/MFM
through an inverter...

The protoss escaped. The zerg dug in. It's time for me to call it a day. Ivan.

- Divorce the scheme, big and small... [Sprinter] Sprinter-II [Forth-CPU]

... ivan...@mail.ru * http://winglion.spb.ru * http://www.petersplus.ru

The Protoss fled. The zerg dug in. It's time for me to call it a day. Ivan.

- Divorce the scheme, big and small... [Sprinter] Sprinter-II [Forth-CPU]

... ivan...@mail.ru * http://winglion.spb.ru * http://www.petersplus.ru

The Protoss fled. The zerg dug in. It's time for me to call it a day. Ivan.

- Divorce the scheme, big and small... [Sprinter] Sprinter-II [Forth-CPU]

... ivan...@mail.ru * http://winglion.spb.ru * http://www.petersplus.ru

KF>> bit 3 -- index pulse for 1818VG93 (0=pulse present)
KF>> There is an error in the Solonov book.

12 Dec 02 15:49, Ivan Mak wrote to Valerij Kozhevnikoff:

IM> There is a suspicion that in some controllers bit 5 is supplied to
IM> FM/MFM switching via an inverter...

Then they would have a standard They didn't work with TR-DOS. But there were controllers without
this bit (always MFM).

13 Dec 02 03:06, Valerij Kozhevnikoff wrote to Yuri Potapov:

YP>> 5 bits - FM/MFM
VK> Nothing like that. Here is a piece of the Pentagon scheme. In Scorpio, definitely
VK> also.

It is possible that some controllers use D6 directly (1=FM, 0=MFM),
and other controllers use inverse D5 (0=FM, 1=MFM). This may
explain why 0x3c and 0x2c are written.