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DB37 Pinout for External IBM Floppy

Holmes

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So a little while ago my IBM PS/2 4869 External 5.25 Floppy Drive "died". I was never sure as to why (either the MCA adapter card, the drive, or somewhere in-between).

I had some time recently, so I did some digging with it on my IBM PC 5150. I opened up the case of the 4869, connected one of the 5150's internal card-edge connectors to it and it worked! I checked the 4869's power supply, and it's working fine.

This means it must be that big, thick, honking DB37 cable that has a loose connection somewhere in it. So I'm probably going to have to build a new cable for it.

Does anyone know the pinout for the DB37 external floppy so I can build a DB37-to-17pin card edge connector adapter?

Or does anyone have any other thoughts or tips about this?

I've tried googling it and can only get bits and pieces of info, but nothing definitive.

Thanks!
 

Chuck(G)

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Are you married to the idea of a round cable? Making an external drive cable from ribbon cable is a snap--basically a conventional internal drive cable with a DC37M crimped on and the ribbon cable (34 conductors) shifted toward the "high side" of the DC-37.

Take a look at the techref for the 5150 diskette adapter--there's a chart of the internal vs. external connectors--it's pretty easy to see how they line up. The confusing thing is the convention used for conductor numbering. On the edge connector, it's odd-even (i.e. the first conductor in a ribbon cable is 1, then 2, etc.). On a DC37, it goes row-wise; i.e. 1-19 top row, 20-37, bottom row.

http://www.pinouts.ru should have the information.

Note that the PS/2 version for the Diskette Adapter/A does swap a couple of wires in the cable--see my blog in this forum.
 

Holmes

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Chuck,

Thanks for the link. I didn't know about the swapped wires.

Hmm. Now you've got me thinking. I took a look at your blog, and you talk about swapping some of the wires in the cable. I was testing this 4869 drive externally by simply connecting it to my 5150 by the external floppy DB37 port. Based on the swapped wires thing, though, this is wrong. You would NOT expect a vanilla 4869 external drive when connected to a 5150 to work this way (since the floppy is only wired to work in a PS/2 with an IBM external floppy adapter).

So it seems that I need to decide if I want this floppy drive to work on an 5150 or a PS/2, and wire it accordingly, since no single wiring method will work with both machines. Is that right?
 

MikeS

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So a little while ago my IBM PS/2 4869 External 5.25 Floppy Drive "died". I was never sure as to why (either the MCA adapter card, the drive, or somewhere in-between).
Sounds like you ruled out the drive and the power supply, but what about the adapter?
 

IBMMuseum

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...You would NOT expect a vanilla 4869 external drive when connected to a 5150 to work this way (since the floppy is only wired to work in a PS/2 with an IBM external floppy adapter).

So it seems that I need to decide if I want this floppy drive to work on an 5150 or a PS/2, and wire it accordingly, since no single wiring method will work with both machines. Is that right?

Actually the 4869 cable goes straight to a standard 5-1/4" drive. Do you have the 360Kb (an asterisk on the faceplate of the drive) or 1.2Mb version? Which microchannel adapter do you have, the "pass-through" with a few chips, or fully populated? Use this as a reference.
 

Holmes

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OK, I'm getting a bit confused. Quick answer is I have NOT excluded the MCA adapter from being the problem.

I just checked the model of my MCA adapter and it is a late-model FRU 15F7996 based on the link IBMMuseum provided. Also, my drive is the 360K model.

Originally I had an IBM PS/2 Model 70 (8570) with the MCA Floppy Adapter (FRU 15F7996) connected with a 4869 external 5.25 floppy. Working fine 6 months ago and packed for a move. It was placed on the back seat of my SUV (to absorb vibration) and moved that way (along with most of my other vintage computer equipment).

The move was completed, and I broke out the IBM 8570 with the 15F7996 MCA floppy adapter still in it. Connected the 4869 floppy to it, and it did NOT WORK. It would just blink the drive light and spin for about half a second on boot and then would not do anything else. I then shelved it (after a bit of cursing).

I tested just the 4869 drive on my IBM PC 5150 by connecting it to the DB37 external drive port on my floppy card in the 5150. It did NOT WORK. I used this to conclude that there was something wrong with my 4869 drive. Based on the info provided by ChuckG, however, I am thinking this may be an incorrect conclusion, given the 4869's "flipped pins". Anyway . . .

I opened up the 4869 drive, disconnected the floppy card-edge connector in the drive box. I then plugged into the exposed drive directly by using the internal card-edge connector from my 5150 (which was pulled out of the 5.25 floppy drive in the 5150's case). Note that the 4869 drive in this scenario is still being powered by the 4869's power supply. With this setup, it WORKED.

So based on this I have concluded the physical floppy drive INSIDE the 4869 works. The power supply in the 4869 works. Since I am unable to get the 4869 working EXTERNALLY with either the PS/2 8570 or the 5150, I concluded that the DB37 connector/cable is faulty (since it is the common denominator).

So I guess the question I still have is:
Will a 4869 External 5.25 Floppy Drive work externally connected to an IBM 5150 Floppy Drive Adapter via the DB37 port? Or, does the PS/2's MCA Floppy Drive Adapter (15F7996) external floppy DB37 port work differently from the same port on the IBM 5150's Floppy Drive Adapter?

Thanks!

(please forgive my liberal use of CAPITALIZATION - I'm doing it for emphasis because this is getting confusing)
 
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Chuck(G)

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Yes, if you modify it per my instructions. Way back when IBM surplused their stock of these boxes I picked up a few cartons of them. Some I resold to customers as special 3-mode external drive boxes (used for machine tool floppies); others I converted for my own use. The IBM part number is 4869-002 (on the bottom of the box).

My modification has not been a casual one-off affair. To do my blog, I pulled out a brand-new one and made the change.

If you're convinced that it's the cable that's bad, drop me a line--I still have a spare (I modified a few of these boxes to hold SCSI devices) un-modified cable or two and would be happy to provide one for the cost of shipping.
 

IBMMuseum

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...does the PS/2's MCA Floppy Drive Adapter (15F7996) external floppy DB37 port work differently from the same port on the IBM 5150's Floppy Drive Adapter?...

Yes, in that the microchannel adapter only has one drive select signal per connection (did you notice the internal 34-pin header on your adapter?). The drive should have the select jumpers set just like it were inside the case (for the internal connection too, if you use the microchannel adapter; it is designed for PS/2s other than a Model 50 or 70, where there was a power plug for the drive). With your microchannel card, you can run the 1.2Mb 4869-002, where you normally would not be to do that on the PC.

Chuck mentioned the 4869 submodel (the only difference is the capacity of the standard drive inside, and what is needed for a power source):

4869-001 = 360Kb drive, 120VAC power source
4869-002 = 1.2Mb drive, 120VAC power source
4869-501 = 360Kb drive, 240VAC power source
4869-502 = 1.2Mb drive, 240VAC power source
 

Holmes

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Thanks everyone for all the great info. Yes, I'm a "DB-everything" person (DB37, DB9, DB1653, ...) ;)

Chuck, sorry, I didn't mean to insinuate anything inaccurate at all about the information in your blog. I'm certainly not in a position to. If I did, I apologize. And thank you for the very generous offer about the cable. I might take you up on it once I've figured out a plan for this drive.

OK, so basically IBM hosed us with the DC37 wiring differences between the ISA Floppy Drive Adapter (in the 5150) and the MCA Floppy Drive Adapter (in the PS/2). Just great!

I had hoped to fix this drive and then use it in either computer. I wonder if there's a way I could put a switch on the case that would allow me to flip between these modes (essentially flipping the wiring at the flip of a switch), so it could do either. That would be cool.

Hey, I've been working a bit on this since it's been so long since I've done anything with switching theory. Could anyone who knows more about this check this out?

I figure that I could implement an external 4869 floppy drive which would support both a PS/2 or an IBM 5150 by building a switching circuit. This circuit would basically switch from a PS/2 wiring setup to a 5150 setup by flipping a switch.

I came up with the following logic diagram for a simple switching circuit. I'm sure it's been done by many others and more elegantly than me, but does it look right?

Switching Logic.jpg

If so, it looks like ChuckG's flipped wires includes 4 wires that will need to be switched. The above circuit uses 2 AND gates, 1 NOT gate, and 1 OR gate. This would mean needing a total of 8 AND gates, 4 NOT gates, and 4 OR gates to build the switching logic for all 4 wires.

Does that sound right to anyone? Again, I'm not really a hardware person, but I've got the bug. Please tell me if I've got something wrong or if it can be done better.

Thanks!
 
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IBMMuseum

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...I had hoped to fix this drive and then use it in either computer. I wonder if there's a way I could put a switch on the case that would allow me to flip between these modes (essentially flipping the wiring at the flip of a switch), so it could do either...

I have a third-party 5-1/4" drive enclosure with no PSU, that had a circuit board and switch. Ultimately the switch was there to adapt to the ISA Model 30 or Model 50/70. There was a ribbon cable (that ran to a MCA or ISA slot bracket for the external connection) with edge-connectors and a PCB that routed wiring and adapted to different PS/2 configurations, vampiring off the main FDC connection as a "B:" drive.

The typical gray folder for the Tawainese adapters of the time identifies it as an "FX-360". I got it as NIB surplus that was purging all the units as-is. Apparently, as I quickly discovered, and just as quickly corrected (even trying to document it back to the surplus house, with no contact back) there was a design flaw by one misplaced wire (for the vampired drive power) at the drive end.

Now I wish I had picked up more of those enclosures...

I am using as my prototype to adapt a standard 1.44Mb replacement drive to the different PS/2 connections, and I've had great success. When on an 8086-based Model 30, it runs any brand of standard 1.44Mb (Teac, Sony, Panasonic, etc.) I threw at it. By comparision, only a Sony brand PS/2 1.44Mb drive would work there as an upgrade from the stock 720Kb drive.

As said, the internal connector of your microchannel adapter will run a drive as well (connect a [EDIT] straight 34-pin cable with no [END EDIT] twist). The adapter has DOS drivers (the floppy drive letter ends up above all hard drives) and I believe it also runs under OS/2 (the FDC itself is conventional, just at a strange port range, so something could be done for Windows 95 too). I looked at bridging all of the drive select signals to both connections, but gave up. It is easy to supply switch parameters under DOS to run a 1.44Mb 3-1/2" drive instead of the 5-1/4" 360Kb or 1.2Mb the DOS driver expects.

In your place, I would construct a dongle that would adapt the DB-37 to run from the PC, and leave the microchannel setup as designed...
 
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MikeS

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Hey, I've been working a bit on this since it's been so long since I've done anything with switching theory. Could anyone who knows more about this check this out?

I figure that I could implement an external 4869 floppy drive which would support both a PS/2 or an IBM 5150 by building a switching circuit. This circuit would basically switch from a PS/2 wiring setup to a 5150 setup by flipping a switch.

I came up with the following logic diagram for a simple switching circuit. I'm sure it's been done by many others and more elegantly than me, but does it look right?

View attachment 5400

If so, it looks like ChuckG's flipped wires includes 4 wires that will need to be switched. The above circuit uses 2 AND gates, 1 NOT gate, and 1 OR gate. This would mean needing a total of 8 AND gates, 4 NOT gates, and 4 OR gates to build the switching logic for all 4 wires.

Does that sound right to anyone? Again, I'm not really a hardware person, but I've got the bug. Please tell me if I've got something wrong or if it can be done better.

Thanks!
Sounds like what you want is something like a 74157 (if they're all 2 in/1 out) or something like a (74HC)4066...
 

Holmes

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Wow, 2 great suggestions.

IBMMuseum's idea of a dongle would definitely be an easier solution to mine, and seems like it would definitely work great.

MikeS's 74157 chip sounds perfect for a switching solution on the 4869. I looked up the schematic on Jameco and it looks like just what I need. Hey, I didn't know they make such things!

I admit, I'm partial to the switching solution because I've just constructed my first circuit board (XTIDE), and I really want to learn more about circuits and hardware. I'd love to build this as a switch solution because it would be the very first circuit I've DESIGNED, which would teach me so much. Building the XTIDE was really fun, but I found myself thinking "OK, I can follow someone else's design and directions. Now if I could design something like this, that would really be something". OK, so I'm a long, long way from that, but this is a start!

I have thoughts about building this switch on the 4869 and replacing the floppy drive in it with one of those dual 3.5/5.25 Teac FD-505 drives that ChuckG mentioned in his blog. That would give me an external drive with could be used in either a PS/2 or 5150/5160 that would provide either a 3.5 or 5.25 floppy. Wow! And it seems like something I could design and build myself (with a little help ;) ).

This is exciting!
 

IBMMuseum

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...I have thoughts about building this switch on the 4869 and replacing the floppy drive in it with one of those dual 3.5/5.25 Teac FD-505 drives that ChuckG mentioned in his blog. That would give me an external drive with could be used in either a PS/2 or 5150/5160 that would provide either a 3.5 or 5.25 floppy. Wow! And it seems like something I could design and build myself (with a little help ;) )...

As long as you have the drive select signals, the dual drives are two independent assemblies put together (there is also a Panasonic version)...
 

Holmes

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OK. So I'm planning on building the switch for my 4869. Before I do this, though, I've got to figure out why my drive will not work externally with anything. Based on my previous posts, I believe I've narrowed it down to either the physical cable in the 4869 or the floppy controller in the PC.

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.

So I tried ChuckG's cable modification to adapt the 4869 drive to standard cabling. His blog is very clear with excellent pictures, so I'm pretty sure I did it correctly. I plugged the drive into my IBM PC 5150 via the external floppy DC37 port on my floppy controller. All the drive does now is it spins at a constant, slow speed. Putting a disk in does not change anything. I tried booting the PC after adding the following line to my config.sys:

device=driver.sys /d:2 /f:0

which I believe should make it mount my 3rd floppy (/d:2) as a 360K drive (/f:0).

It booted, mounted the drive as D:, but when I tried to access it, it gave me the "failed, abort, retry, fail" type disk error. All through this, the drive just spins at a steady slow rate (drive light on constantly) with no changes.

I tried disconnecting the DC37 cable while the drive was on, and the drive stopped. Plugged it in again, and it spins constantly again.

This leads me to believe that the cable is bad. This is assuming I did the mod correctly. On top of that, it's highly unlikely that 2 floppy controllers are bad. Also, my internal floppies on my IBM PC 5150 work fine which seems to indicate that the floppy controller is good.

Anyone have any insights on this?
 
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