Troubleshooting an IBM 5155 - (portable personal computer)

From: Tom Jennings <>
Date: Mon Jan 17 02:59:02 2005

On Sun, 16 Jan 2005, Tony Duell wrote:

>> same diskette put back in the working drive formatted to 360K. So, how do
>> these drives tell if they're dealing with SS vs. DS meadia? Does it just
>> look for a signal from the second head? Is there another sensor that
>> indicates "yep - double sided". This is how it works with 8" drives. The
>> index hole is in a physically different spot.

Why we always sneered at the hateful 5.25's.

The FATID byte is used to know the sidedness on reading; the FAT,
like boot sector, is always on the first side. 160, 180, 320, 360,
make up your mind! :-)

> b) There is no useful signal from the side-1 head (this is the head on
> the top surface of the disk, of course),

For all the possibilities you state, likely cause.

> Well, although _now_ I have some very nice test gear, I debugged my first
> homebrew computer (This was when homebrewing meant soldering chips to
> stripboard and wiring everything up, not pluging a video card into a
> motherboard) with an anlaogue multimeter and an LED+resitor 'logic
> probe'. So it certainly _can_ be done...

Likewise! On my very first own computer, SWTPC 6800, I debugged
the fact I miswired the (20mA loop) tty using a Tek 555 scope to
ID the Mikbug code loop waiting for the start bit. I was too dumb
to realize I should have checked for Rx into the PIA :-)

Soldered every damn chip and all those molex "card edge"
connectors one at a time. Ugh. Plus a whole tube of 2102's (and
not 21L02's) a year later.

Now I buy only laptops, nearly impossible to even open them, and
quite happy with that. :-) Even having to stick in a PCMCIA card
these days seems invasive. Why isn't that built in?
Received on Mon Jan 17 2005 - 02:59:02 GMT

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