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DB37 Pinout for External IBM Floppy

IBMMuseum

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...Since I've tested this drive in my PS/2, and it did not work, it could be either the cable or my MCA floppy controller. When I booted my PS/2 last, I believe it did recognize the controller because when it started up it said it was mounting the floppy. Not 100% sure on this...

There is a DOS driver required for your microchannel adapter. Do you have the earlier version which has a switchblock, or the adapter without (both have the same 15F7996 FRU)? I haven't determined what the switches did (a physical switch on a microchannel adapter is always intriguing, because the architecture defines that configurations should be done in software).

I did a DOS utility years ago that checked for presence of the adapter (the purpose was actually to determine which main FDC chip was present for the different PS/2 systems), but need to get back into that research...
 

MikeS

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In my experience good quality cables are usually the least likely problem, which is why I asked about the adapter (and any related drivers etc. of course).

But it's been interesting; BTW miniature 4PDT switches are readily available for that interface switch (see KISS principle ;-) )
 

Chuck(G)

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But it's been interesting; BTW miniature 4PDT switches are readily available for that interface switch (see KISS principle ;-) )

Consider that alternate wires on the floppy interface are grounded--you need only a DPDT swtich (common as dirt) to change the signals.
 

IBMMuseum

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...But it's been interesting; BTW miniature 4PDT switches are readily available for that interface switch (see KISS principle ;-) )

Even through they aren't "double-throw", I've wondered what the four line switchblock was on the initial 15F7996 microchannel adapter (my notes said that factory settings seem to be all switches "ON"). IBM had the drive select signals mapped so that the internal drive (if present) is primary, making the single external drive secondary.

That adapter version was covered in SOIC chips compared to the later released 15F7996 (without the 4 position switchblock). Having the same IBM FRU is supposed to mean the same functionality, but I wonder if the switches changed the drive select mappings between the internal and external connections. The I/O port of the FDC would have been changed by other means (in the ADF), and I've not heard of any odd adaptations that IBM would have done to connect a PS/2 to an older floppy unit.
 

Holmes

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Have you tried a different power supply with the drive? The "spins slowly" behavior has me wondering...

No I haven't, but the drive did work when connected via the internal cable while using the external power supply, so it's definitely working.
 

Holmes

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Dec 13, 2010
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There is a DOS driver required for your microchannel adapter. Do you have the earlier version which has a switchblock, or the adapter without (both have the same 15F7996 FRU)? I haven't determined what the switches did (a physical switch on a microchannel adapter is always intriguing, because the architecture defines that configurations should be done in software).

I did a DOS utility years ago that checked for presence of the adapter (the purpose was actually to determine which main FDC chip was present for the different PS/2 systems), but need to get back into that research...

I believe I have the later version of the MCA adapter. The thing is, this floppy was previously working on this PS/2 and ceased to function after the move.
 

Holmes

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Dec 13, 2010
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313
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In my experience good quality cables are usually the least likely problem, which is why I asked about the adapter (and any related drivers etc. of course).

But it's been interesting; BTW miniature 4PDT switches are readily available for that interface switch (see KISS principle ;-) )

Cool. Again, something else I've never heard of!

I just looked up the "4PDT switch" and it looks like it would take care of it in one fell swoop. And it sounds like a DPDT would work, too. It's nowhere near as "fun", though! :)OK. Fine, I guess I can't really build my fancy circuit switch with a solution as simple as this out there.

So now I know how I'm going to build the switch, but the cable mod didn't work so there's no point building the switch until I figure that one out.

Does anyone have any thoughts about the "drive constantly on" phenomenon after ChuckG's mod? Again, the power supply is working. Also, the drive didn't respond to any commands or boot - it just spins if turned on and connected to the IBM 5150's floppy adapter via external cable. The PC doesn't even have to be on! If the drive is on and connected to the DC37 port, it spins slow and constantly.

Doesn't all this make one think it's a cable problem?
 
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Holmes

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Dec 13, 2010
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OK, I got my 4869 working on my IBM 5150!

First of all, the "slow spin" behavior is because the card-edge connector is installed UPSIDE-DOWN. IBM in their infinite wisdom did not use plugs with the "non-reversible tab". It's my fault, though. I should have checked this first.

Also, my config.sys line was wrong. Here's what it should have been:

device=driver.sys /d:3 /f:0

I had previously used "/d:2". I was about to throw the towel in when I just tried "/d:3" for fun, and it ended up working!

So ChuckG's 4869 cable modification worked great.

Thanks everyone!
 
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