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Osborne Floppy question

T

The Lightning Stalker

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Hello. I took apart my Osborne 01 and noticed there is a resistor network (RN3) on the A drive and a corresponding socket on the B drive with no resistor network. Would this be how the A&B drives are distinguished?

Thanks,
The Lightning Stalker
 
No. In those days, bus termination was done by placing a terminator on the device at the end of the bus - the floppy drive cable in this case.

A:/B: drive differentiation is most likely done by the 'drive select' jumpers on the floppy drives. I say 'most likely' because some other systems, such as the IBM PC, had the drive select jumpers on the drives set the same, and a crossover in the cable meant that it was the drive's position on the cable that defined A:/B:
 
The Lightning Stalker said:
No jumpers on the drives. The resistor network is the only difference.
Click to expand...

You are correct. I also took apart my osbourne because one of the drives wasn't working. The resistor network is both a termination and a drive select jumper. It has to be at the end of the cable or the drive doesn't work reliably. I also found that if everything is not grounded properly, the CRT emits enough EMI to interfere with the floppy interface.

-steve
 
It's been a long time, and I don't recall the exact details, but Osborne did something 'weird' to accomplish drive detection/selection. This was done to allow either drive to be used as a boot device (anyone remember the majikal key combination?), and whichever drive is booted from becomes A: which was customary in CP/M.

--T
 
Terry Yager said:
It's been a long time, and I don't recall the exact details, but Osborne did something 'weird' to accomplish drive detection/selection. This was done to allow either drive to be used as a boot device (anyone remember the majikal key combination?), and whichever drive is booted from becomes A: which was customary in CP/M.

--T
Click to expand...

The Floppy drives have Osbourne custom controller boards in them. They did things different. I definitely found that whichever drive had the resistor network installed was the A drive. That was the drive the system tried to boot from. If the A drive didn't work, it didn't boot. They also powered the drives through the ribbon cable.

Is there a way to select the B drive to boot on the Osbourne?

-steve
 
sjv said:
Is there a way to select the B drive to boot on the Osbourne?

-steve
Click to expand...

If I remember correctly, it's [shift] P when you put the boot disk in the B: drive. That switches the A and B drives to the Ozzie.

I know it's shift something up in that part of the keyboard! :confused:

Curtis
 
An operating system can do what it likes. For example, I can write my own OS that on boot checks physical diskette drive 0 for a boot disk, and if not found, then checks physical diskette drive 1 for a boot disk. Whichever drive was first found to have a boot disk is allocated drive letter A: with the other drive being allocated drive letter B:

But because these machines use existing operating systems, etc. such a swap has do be done at a lower level - the BIOS. There's a key combination on the Osbourne 01 that can do that swap. Can't remember what it is right now.

Anyhow, to help answer the original question, I pulled apart my Osborne 01. My Osborne 01 is the version that uses half height Chinon F-502 drives.
The diskette cable is a straight-though type, and the drives have jumpers.
On the left-hand drive (usually A: ), the jumper for DRIVE SELECT 0 and the jumper for TERM are the only ones on.
On the right-hand drive, the jumper for DRIVE SELECT 1 is the only one on.

That is exactly what I expected, and so I'm puzzled by SJV's, "I definitely found that whichever drive had the resistor network installed was the A drive."

Is it possible that type of drives used in some Osborne 01's do a 'self - drive select', that is, if the drive has a termination resitor pack, the drive self-selects itself as drive 0, and if the termination resitor pack is missing, the drive self-selects itself as drive 1.
I've never heard of such a drive, but that of course doesn't mean that they don't exist.
 
An operating system can do what it likes. For example, I can write my own OS that on boot checks physical diskette drive 0 for a boot disk, and if not found, then checks physical diskette drive 1 for a boot disk. Whichever drive was first found to have a boot disk is allocated drive letter A: with the other drive being allocated drive letter B:
Click to expand...

That's the way the Victor 9000 and the TI Professional behave (& probably some others I'm not thinking of). They may be booted from either floppy or a hard drive. The boot drive is assigned A: by the OS at boot.

--T
 
modem7 said:
I pulled apart my Osborne 01. My Osborne 01 is the version that uses half height Chinon F-502 drives.
The diskette cable is a straight-though type, and the drives have jumpers.
On the left-hand drive (usually A: ), the jumper for DRIVE SELECT 0 and the jumper for TERM are the only ones on.
On the right-hand drive, the jumper for DRIVE SELECT 1 is the only one on.

That is exactly what I expected, and so I'm puzzled by SJV's, "I definitely found that whichever drive had the resistor network installed was the A drive."
Click to expand...

My O1 uses full height 5.25 drives. I forget the manufacturer, but the controller board bolted to the drive is definitely a custom Osbourne assembly and not from the drive manufacturer. It sounds like the original poster has the same drives since he doesn't have a jumper. I didn't know they made O1's with half height drives!

-steve
 
I found this in the 1982 technical manual (page 33): "Two types of disk drives are used in the Osborne 1. The type supplied .. depends on when your computer was made. Both Siemens and MPI disk drives are used, with the manufacturer-supplied electronics replaced with a Osborne-designed electronics board."

And in the service manual, there's no reference to drive select jumpers/switches, just that RN3 gets fitted to drive A: with no RN3 on B:

So it appears that 'Osborne-designed electronics board' decodes the two 'drive select' lines from the motherboard (page 37 of technical manual) according to the presence/absence of RN3.

So what about my half-height Chinon drives? The installation of the half height drives is my machine is done to a standard that is consistant with the remainder of the machine. The drives have well machined metal sheilds. The installation is definately not a 'home job'.
The technical manual is dated 1982, and my machine is dated September 1983. According to a web site, Osborne filed for bankruptcy in September '83, the same month that my machine was built. Sounds like Osborne switched to half-height drives (unmodified) late in the production run.
 
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