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Osborne 1(b)

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ChrisCwmbran

Experienced Member
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Oct 11, 2011
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Cwmbran, Wales, United Kingdom
Hi all,

Hoping one of you Osborne 1 gurus can confirm that the following is correct:

I have an Osborne 1 (B version) which has a version 1.43 BIOS. The machine appears to be fitted with two TM100-1 single sided 48tpi floppy disk drives. If I fit the machine with a double density option board the machine will then be able to read double density disks? I don't need to do anything else?

The reason I ask is as it stands my tweener will not write single density disks. Given I have two spare double density disk boards for the Osborne 1 it would seem this is the easiest and best way to solve the problem.

Thanks in advance,

Chris.
 
ChrisCwmbran said:
Hi all,

Hoping one of you Osborne 1 gurus can confirm that the following is correct:

I have an Osborne 1 (B version) which has a version 1.43 BIOS. The machine appears to be fitted with two TM100-1 single sided 48tpi floppy disk drives. If I fit the machine with a double density option board the machine will then be able to read double density disks? I don't need to do anything else?

The reason I ask is as it stands my tweener will not write single density disks. Given I have two spare double density disk boards for the Osborne 1 it would seem this is the easiest and best way to solve the problem.

Thanks in advance,

Chris.
Click to expand...

Try http://bitsavers.informatik.uni-stuttgart.de/pdf/osborne/osborne1/ for the installation pdf and tech manual. Is there a double density version of the boot disk?
 
ChrisCwmbran said:
...an Osborne 1 (B version)...fitted with two TM100-1 single sided 48tpi floppy disk drives. If I fit the machine with a double density option board the machine will then be able to read double density disks?...
Click to expand...

The tech manual I read yesterday (may not apply to the 1B) said that the O1 had its own modified FDD electronic cards and the FDD interface shown in that book had taken departures in the normal 34pin floppy cable signal definitions. Therefore it may be that the TM100s could never work without modification. Hopefully the 1B isn't like the O1 documented.

If that system has never run before you acquired it, perhaps the previous owner just tried to stick in some drives and couldn't get it to work, and decided to sell it. "As Is" covers that unfortunately.

If this is confirmed, the real question becomes, "What would it take to make a O1B compatible with normal drives?"

If you have a tech manual explicitly for the 1B, post it. I realize that 1983 tech manual you have migth be for the 1B... So far I only have to 1982 tech manual.
 
Extending the thought about the TM-100 drives in the Osborne-1B, the document said they made special boards for the MPI and Siemens drives. Its possible someone took the Osborne modified FDD electronics cards and mounted them on TM-100 and installed them. That might be sensible if the drives are similar. Its probably worth the time to pull one TM-100 and inspect the board to see if its got the Osborne card on it.

Its also possible a previous owner didn't know what Osborne did and just hooked up TM-100 drives to the Osborne-1B's 34 pin floppy cable, not know about departures from the normal 34 pin standard lines. Its worth tabulating the two 34 pin assignments and see if something there may have directly caused the PIA to fail.

The second case is perhaps more likely to occur because floppies are thought to be interchangeable on most systems. Looking at the cable signal tabulations might reveal a direct cause (I'll do that at the office tomorrow before local noon).

The first case could be more insidious because if the Osborne card moved over to the TM-100 FDDs, the questions of whether its compatible to MPI/Siemens and whether the reconnections were done well... makes the job of diagnosing it more complex.

Should be interesting to document the FDD cable departures.
 
The "native" drives on the O1 have power supplied via the ribbon cable nestled right in the middle of the ground lines. Hooking up a non O1 floppy will result in voltage being shot right into the drive and will definitely kill it!

It is possible to switch the lines around, but trust me, you don't want to...
 
curtis said:
The "native" drives on the O1 have power supplied via the ribbon cable nestled right in the middle of the ground lines. Hooking up a non O1 floppy will result in voltage being shot right into the drive and will definitely kill it!...
Click to expand...
Thanks. I looked at the schematics a few days ago and thought I saw a power connection as you describe, wanted to check it when I got into the office. I'm going to see if I can figure out *WHY* Osborne departed from standard interface.

I think they used a 10 sector 256 byte single density track native format. This is sounding similar to a Zorba format issue a few months ago on another blog about their 10 sector 512 byte double density format being packed on the track requiring different gaps to be reliable... my first impression was that Osborne did their own FDD electronics to address it another way.

My quick scan of the O1 FDC schematics looked like they added pre-compensation pins to the interface. Guess it would be nice to replace the Osborne FDC board with a more compatible one (in an unrealistic, ideal situation).

I'm curious to see if that cable plugged into a standard drive would affect the PIA too; it has an open-collector buffer between it and the FDD, so that cound have protected it. It might make more sense that the PIA started to fail and that meant it couldn't enable either drive, so the owner simply replaced them with the TM-100s and unfortunately just added to the problem. :S

As you said, its possible to wire-wrap an converter to connect a normal drive, removing all the incompatible signals and putting compatible signal on their standard pin assignments.

Do you know if that has been tried *AND* the Osborne could reliably access floppy media on stardard FDDs?

I'll look at the Osborne-1 (not 1B) documents during lunch today and post what I discover.
 
Last edited:
I tabulated the Osborne-1 (not 1B) 34 pin connector along side the TM100's 34 pin connector (I had a printed copy from 1980):
Code: 
Pin  Osborne-1      |   | TM100
===  ===============|   |==================   Legend:
01:  GND            | = | GND                 =  same assignment (may have other issues like STEP)
02:  GND            | ~ | Connector clamp     ~  probably useable
03:  GND            | = | GND                 x  not the same
04:  GND            | ~ | Spare               !  major problem
05:  GND            | = | GND
06:  GND            | x | Drive Select 3
07:  GND            | = | GND
08:  INDEX          | = | Index/Sector
09:  GND            | = | GND
10:  Drive Select 1 | ~ | Drive Select 2
11:  +12 Volts      | ! | GND
12:  Drive Select 2 | ~ | Drive Select 1
13:  +12 Volts      | ! | GND
14:  nc             | ~ | Drive Select 0
15:  +12 Volts      | ! | GND
16:  4Mhz Clock     | x | Motor On
17:  +12 Volts      | ! | GND
18:  DIR            | = | Direction Select
19:  GND            | = | GND
20:  STEP           | = | Step
21:  +5Volts        | ! | GND
22:  Write Data     | = | Composite Write Data
23:  +5Volts        | ! | GND
24:  Write Gate     | = | Write Enable
25:  +5Volts        | = | GND
26:  Track 00       | = | Track 0
27:  GND            | = | GND
28:  Write Protect  | = | Write Protected
29:  GND            | = | GND
30:  Read Data      | = | Composite Read Data
31:  GND            | = | GND
32:  Side Select    | = | Side Select
33:  GND            | = | GND
34:  Late           | ~ | Connector clamp
Note that every +12Vdc and +5Vdc signal Osborne puts on the cable is normally a GROUND signal.
Also some of the TM100 pins can be jumpered... but the Osborne-1 floppy controller is in no way compatible with normal floppy disk drives. I'm sure the Osborne owners/users have known this for a long long time.
Newbies should be careful purchasing Osbornes or restoring them based on standard assumptions.

One should pull the floppy drives and assure that the Osborne floppy electronics are on the drive.

Be careful on e-Bay not to get a system slapped together just so it looks right enough to sell as is. I've only seen MPI and Siemens listed as drives Osborne used in their documentation. I don't know if other drives have been swapped with the Osborne card and worked successfully.
 
Last edited:
Chris confirmed that his Osborne-1B does have the Osborne electronics installed on its TM100 FDDs.
 
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