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giobbi
August 19th, 2016, 05:39 AM
Hi,

I already told about a problem I have with a floppy drive in another thread, but I'm opening this fresh thread because I think some informations could be interesting for future reading.


I have a good FDD board (the electronic mounted on the drive itself) and a bad one. Below, I will call them FDE for short.

Note: the FDD is A: if the SIP resistor is inserted in the proper FDE socket, it is B: without the SIP resistor.


Behavior:

== Switching on the computer, both leds blink and spindle motors run for a little while.



-- If I plug both FDE and press Enter, nothing happens (drives "dead", not even the led).

-- If I plug the good FDE only (configured as A: drive) and press Enter, nothing happens (I'm sure it worked with just one drive plugged).

-- If I plug the good FDE only (configured as B: drive) and press Enter, nothing happens (of course).

-- If I plug the good FDE only (configured as B: drive) and press Shift + [ " ' ] (I mean the key above the Enter; this key sequence swaps A: and B: drives), the computer boots fine

-- If I plug both FDEs and press Enter, nothing happens.

-- If I plug both FDEs and press Shift + [ " ' ], both drives starts and the good FDE one becomes bouncing the heads.



>>> I don't understand why the computer doesn't boot from A: drive without the B: drive connected. I'm sure previously it worked with just one drive.

Randy McLaughlin
August 19th, 2016, 08:16 AM
The SIP should be on the last physical drive - it is a cable terminator. I would still expect another jumper for drive selection.


Randy

Randy McLaughlin
August 19th, 2016, 09:15 AM
The SIP should be on the last physical drive - it is a cable terminator. I would still expect another jumper for drive selection.


Randy

I am wrong the SIP does do drive select.

Osborne rather than buying an SA-400 compatible drive like apple they would buy the drive frame without the controller board (from either MPI or Siemens) and use their own controller board.

90% of the controller board is standard but they change a couple things it gets power and data on one cable and the terminator provides a bias to drive selection.

You need to first find out if the motherboard is OK.

This can be determined by swapping drives from a known good Osborne or at least using a known good drive.

Any SA400 compatible drive will do with a modified cable. If you have the money I would recommend an SDHxC - it is an electronic drive emulator. It emulated drives A and B. you can simply load Osborne disk images and work from there:


http://lotharek.pl/download.php


Randy

giobbi
August 19th, 2016, 01:40 PM
I will try to understand what's going on with these floppy boards and with the FDC too.

This computer has a strange behavior, sometimes boot, sometimes it seems the heads are trying to find zero track, bumping... I could use parts from my good Osborne, but it is SS-SD while this one has the double density card, so I don't know if FDDs are compatibles. Also, I don't want to use parts of a computer that's working...

I have the Lotarek emulator, but I should make a cable adapter, since the Osborne flat cable is different and it carries power too, not only data.

Randy McLaughlin
August 19th, 2016, 02:04 PM
I will try to understand what's going on with these floppy boards and with the FDC too.

This computer has a strange behavior, sometimes boot, sometimes it seems the heads are trying to find zero track, bumping... I could use parts from my good Osborne, but it is SS-SD while this one has the double density card, so I don't know if FDDs are compatibles. Also, I don't want to use parts of a computer that's working...

I have the Lotarek emulator, but I should make a cable adapter, since the Osborne flat cable is different and it carries power too, not only data.

The drives are completely compatible - the difference is in the motherboard.

Single density sends out both a clock bit and a data bit - it works by looking between the clock bits to see if the data in the middle is high or low, this is FM (frequency modulated). Double density drops the clocking signal and instead has a complicated bit pattern that codes the data bits - doubling the amount of data in same space MFM (modified frequency modulation). The drives still see a series of pulses coming at the same speed - the disk controller handles the hard part.


Randy