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IBM 5161 MFM Contrller questions

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Exceter

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What did the IBM 5161 originally ship with for drive controllers in a two drive setup? Can a single controller support multiple drives or would you need two controllers?
 
The 5161 shipped with an Xebec controller that could support 2 drives. Because of some design mistakes, the first Xebec controller as supplied by IBM only worked with the ST-412 thus the second drive also needed to be an ST-412. Later revisions could support a couple of 20 MB hard drives as well but the selection was still quite limited.

Getting two controllers to work in the same system is difficult.
 
How do you cable two drives on a Xebec controller? It looks like there's only 3 ribbon connectors. Wouldn't you need 4? I must be missing something.
 
Exceter said:
How do you cable two drives on a Xebec controller? It looks like there's only 3 ribbon connectors. Wouldn't you need 4? I must be missing something.
Click to expand...

The 34 pin "control" cable is shared by both drives. (IE, for a two drive setup you'd have a cable with three connectors, like a floppy cable.) Each gets its own 20 pin data cable.
 
Please note that you can not use a 34-pin floppy cable with a twist for two MFM hard drives. You will need to either find a straight through cable and set the drive select manually or find the special MFM twisted cable IBM used. The MFM twist is different from the floppy twist.
 
krebizfan said:
Please note that you can not use a 34-pin floppy cable with a twist for two MFM hard drives. You will need to either find a straight through cable and set the drive select manually or find the special MFM twisted cable IBM used. The MFM twist is different from the floppy twist.
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Yes, this. (Realized some time after posting it would probably be good to mention this.) Just like they did with floppies IBM went with twisted cables instead of jumpering on machines where they supported multiple MFM drives, but it is indeed a *different* twist.

Technically you may also need to pull the terminating resistor pack out of the drive that's not on the end of the cable as well when installing two.
 
krebizfan said:
Because of some design mistakes, the first Xebec controller as supplied by IBM only worked with the ST-412 thus the second drive also needed to be an ST-412.
Click to expand...
I would use the word "limitation" rather than "mistake".

And per [here], the first and second major variations of the subject controllers are (unmodified) capable of use with other 'MFM' drives that have 306 cylinders and 4 heads.
 
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