5.25" 1.2Mb floppy attached to an Intel D875PBZ

[Giuseppe Vitillaro]

I'm trying to connect an "old" (1990) 5.25" 1.2Mb floppy drive unit
to a D875PBZ original Intel motherboard.

It is the only floppy connected to the floppy onboard controller
as "A:" unit: it is connected through a standard five sockets
floppy 34 pin controller cable, after the "twist" obviously.

The BIOS motherboard seems to support 5.25 1.2M floppy units.

The problem is that the floppy motor do not start at system boot
and the floppy do not work at all.

I already tried to reconnect the 5.25" floppy unit to another
system and it works like a charm (through an old ISA floppy controller).
A 3.5" 1.44Mb floppy unit works without problem when connected
with the same cable to the D875PBZ onboard floppy controller.

Anybody knows if the I/O D875PBZ controller (SMSC LPC47M172) actually
supports 5.25" 1.2M floppy drives?

Thank in advance for any help you may provide to me, Regards
G. Vitillaro.

P.S. Please direct e-mail messages to "(e-mail address removed)"
after removing the "no-spam-" protection string.

 

[Arno Wagner]

I'm trying to connect an "old" (1990) 5.25" 1.2Mb floppy drive unit
to a D875PBZ original Intel motherboard.

It is the only floppy connected to the floppy onboard controller
as "A:" unit: it is connected through a standard five sockets
floppy 34 pin controller cable, after the "twist" obviously.

The BIOS motherboard seems to support 5.25 1.2M floppy units.

The problem is that the floppy motor do not start at system boot
and the floppy do not work at all.

I already tried to reconnect the 5.25" floppy unit to another
system and it works like a charm (through an old ISA floppy controller).
A 3.5" 1.44Mb floppy unit works without problem when connected
with the same cable to the D875PBZ onboard floppy controller.

Anybody knows if the I/O D875PBZ controller (SMSC LPC47M172) actually
supports 5.25" 1.2M floppy drives?

I don't know this. But there might be another problem: Originally
some signals of the floppy were used with a 330R pullup resistor
in the floppy and required a special 20mA driver on the PC side.
All the other signals are just standard 1.6mA TTL load. But today
most (all?) 3.5" floppies use standard TTL load on this line also.

It is possible that the chip used cannot physically drive the
lines with the higher load. That would also explain
why the old controller does not have a problem.

Arno

 

[Giuseppe Vitillaro]

Thank for all the very kindly answers to my question.

I resorted to connect another 5.25" 1.2Mb (YE DATA 381) to the
D875PBZ and it works pefectly. At the same time I received
a message from Intel (they are very kindly on user support)
that confirms (I actually do not need this confirm anymore) that
the SMSC LPC47M172 supports legacy 5.25" 1.2Mb floppies.

Unfornately on the SMS Web (www.smsc.com) there is not any data sheet
for the LPC47M172 I/O controller. That is very strange: on the D875PBZ
Intel User Manual they make an "explicit" reference to the SMS site.

For who answered by an e-mail message, the other unit is surely
configured as "B:" (DS1/Unit1) and was installed "after" the twist on
a working five sockets floppy cable (that perfectly works as A: on
an older floppy controller).

So the problem is probably related to the terminator. I have
to check. If I remember correcly (I don't have the older unit
in my hands by now) I saw an unpluggable resistor terminator
on the faulting unit.

Do you think that "unplugging" the terminator and attaching the 3.5"
floppy (powered on) on the same cable would solve the problem?

Thanks, G. Vitillaro.

 

[Arno Wagner]

Thank for all the very kindly answers to my question.

I resorted to connect another 5.25" 1.2Mb (YE DATA 381) to the
D875PBZ and it works pefectly. At the same time I received
a message from Intel (they are very kindly on user support)
that confirms (I actually do not need this confirm anymore) that
the SMSC LPC47M172 supports legacy 5.25" 1.2Mb floppies.

All you need to know for the logical part of the question.

Unfornately on the SMS Web (www.smsc.com) there is not any data sheet
for the LPC47M172 I/O controller. That is very strange: on the D875PBZ
Intel User Manual they make an "explicit" reference to the SMS site.

O.k., leaves no real possibility to find out whether the chip has
strong drivers or not except doing measurements with a scope.
Possibly needs a digital scope as well.

[...]

So the problem is probably related to the terminator. I have
to check. If I remember correcly (I don't have the older unit
in my hands by now) I saw an unpluggable resistor terminator
on the faulting unit.

That would likely be the 330R or 270R resistors.

Do you think that "unplugging" the terminator and attaching the 3.5"
floppy (powered on) on the same cable would solve the problem?

The critical lines are shared, as far as I remember. So the
pull-up in the other floppy should be enough. Otherwise you can
replace the terminator with one in the 3300R range.

Try it and format a floppy. If that goes through, you should be
good for reading at least. Writing maight be a little more
critical and lacks the error detection of reading.

One other test you can do: If the 3.5" floppy stops working
(warning, test may damage data on floppy!), when the 5.25"
one is attached, then it is likely to be a terminator problem.

Arno