View Full Version : Osbourne 1 heelp with siemens B drive

March 17th, 2018, 02:15 PM
I have a working Osbourne 1 with 2 Seimens FDD100-5 drives.
A drive works great but B dose not. Drive B is missing the socketed C30 (is this a cap?)
I removed that cap and put in drive B. I swapped the drives on the ribbon cable and the computer would not boot from the new A. I moved the cap to original A now connected to B part of ribbon cable, and the computer booted from B (which I think it thinks is A)
I cleaned the heads of B and made sure the the head drive moves easily. I unplugged and re-plugged all the connectors on the drives PCB.
Question is there a hidden jumper for A and B for these drives? I can not see one. The online manual for this drive has a very different PCB then what I have. My show Rev M. There is a row of open pins at TP1. The only other difference I see is the trimpot screw is in a different position. But I do to think I want to mess with that. The Dive's led lights when the power switch is turned on.
Suggestions, help, links??

March 17th, 2018, 02:19 PM
The C30 is the blue item lower right in pic.
It has 8 pins and shows 8X151K G8220. What is this?

March 17th, 2018, 05:05 PM
That looks to be a termination resistor which is marked as RN3. C50 is likely the green cap closer to the edge of the PCB but the silkscreen was poorly positioned. It's normal for only one terminator to be present in a two drive machine.
Does the floppy ribbon cable for both drives have a twist? The old fashioned way to set DS0 and DS1 back then without jumpers was to add a twist between the two connectors for the drives.

March 17th, 2018, 05:51 PM
http://www.vcfed.org/forum/archive/index.php/t-5630.html is an old discussion of the Osbourne 1. The terminator block also sets the Drive Select. Osbournes are weird; grab all the manuals available.

March 17th, 2018, 06:24 PM
You are right about RN3 & C50.
What is the terminator? RN3?
This unit has no twist on the ribbon cable.

March 18th, 2018, 04:41 AM
I don't want to derail this conversation, but I was looking at a FDD100-5 manual and the photo shows a completely different PCB, one that clearly has a jumper block for configuring the drive and device select.


Are you certain you have an FDD100-5 drive? The photo you posted shows a completely different PCB - and one that has no visible jumper block or configuration jumpers. I'm wondering if it is a later model drive, perhaps intended for a system with "cable select" (twist in the drive cable).

March 18th, 2018, 08:20 AM
The is a FDD 100-5, but the PCB shows Ver M, with M hand written. There is no other letters or numbers after the -5. I figured out the RN3 with the blue 8 pin thingy(?) is the drive select. With it in the drive is A, without it is Drive B. These drives do not have a separate power connection. Power comes in through the ribbon cable.

The drive I was working with is actually bad. I think the zero track sensor is bad. I do have another compatible drive I was able to put into the Osbourne.
Thanks everyone.

March 23rd, 2018, 11:52 PM
Got it figured out. These drive being from before the 'standard' was set makes things interesting.
Thanks, everyone!!! Alway much appreciated!!

March 24th, 2018, 02:44 AM
Just trying to make sense of this, for my own sanity (some would say that's a lost cause).

So, both drives are identical models? There is a "resister network" RN3 (previously called "C30") which is actually a *socketed* part? I'm surprised, but might vaguely recall that sort of thing. That RN3 part is a 6x 150/220 termination resister, and I am having trouble imagining a schematic that would turn that into a drive select. However, proper placement of that termination resister pack is (or at least can be) critical. It usually needs to be at the physical end of the cable (farthest away from the computer main board), which ever drive that is. And there must be only one. But I still wonder how drive select is done. I can only guess there are some other jumpers someplace that program the drive select, or else there must be a twist in the cable but it's too subtle to see.

I guess 6 terminations is enough. That would cover MOTOR, DIR, STEP, WG, SIDE, and WD. If I recall correctly, drive select lines are not terminated (selectively). So, that part makes sense as a termination pack.

March 24th, 2018, 03:23 AM
If you look at the following URL you will see the typical layout of SIP & DIP Resistors.


And this site shows the typical connections of RN3 for several Drive Signals:

WR Gate RN3 Pin 1 & Pin 5
WR Data RN3 Pin 1 & Pin 3
STEP RN3 Pin 1 & Pin 4

So, it is a Type B - SIP -- Single row, ONE Common pin with all resistors bussed to remaining pins. Total pin and resistor count
given for each part. For RN3 it is a 150 OHM Bussed SIP, used as a PULLUP Resistor for the Signals. Typically, the PULLUP resistor
is on the LAST connector (drive) on the Cable


March 24th, 2018, 03:56 AM
So, it is a Type B - SIP -- Single row, ONE Common pin with all resistors bussed to remaining pins. Total pin and resistor count
given for each part. For RN3 it is a 150 OHM Bussed SIP, used as a PULLUP Resistor for the Signals. Typically, the PULLUP resistor
is on the LAST connector (drive) on the Cable

Ah, ok. I was originally thinking pullup but then the photo of the PCB looked like RN3 also connected to the ground trace. That PDF states that RN3 should only be installed on drive A, which makes me think that drive A is at the end of the cable.

That schematic also solves the mystery. The signals "terminated" by RN3 include WG, WD, STEP, DIR, DS0, DS1, and XXX. The last one being significant to drive select. The "XXX" is an unnamed signal that internally enables either DS0 or DS1. So, absence of RN3 enables DS1 (a.k.a. drive B) and presence of RN3 enables DS0 (drive A). So, that forces the signal termination to be on "drive A" - which really should dictate that drive A be at the end of the cable.

Most drives (that I dealt with) did not use logic gates to do drive select. You'd simply jumper pins (using various methods) such that one of DS1, ... DS4 got routed to the drive select input. TTL logic must have been getting pretty cheap by that time, and also the use of jumper plugs, etc, getting more expensive for PCB fabricators.