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Floppy drive D2/D3 jumpers

M

Malvineous

Experienced Member
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Jun 18, 2010
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Brisbane, Australia
Hi all,

I have a 5.25" floppy that has four jumpers on it - D0, D1, D2 and D3. I am aware the normal set up is two drives with D1, and a twist in the cable. Two questions: (all in the context of standard PCs)

Does setting a drive to D0 allow you to use a straight-through 34-pin cable? In other words, does this change the drive-select and motor-on signals the drive responds to (like the cable twist does), or does it only change one of these signals (e.g. drive-select and not motor-on) so it's not enough to let you use two drives with a straight-through cable?

Secondly, as far as I understand it, the 34-pin PC floppy cable only carries select/motor signals for two drives. In this case, what does the drive respond to when it is set to D2 or D3? It seems to me there is no signal present on the cable for drives three or four, so I am not sure why the D2 and D3 jumper positions are present.

Thanks for any info!
 
Yes you can use it without the twist.

In the context of standard PCs, there is no purpose to DS2 and DS3.. But in the context of other machines (including PCs with standard "non-standard" adapters) you can have four drives per controller.
 
I've got a DTC RLL hard+floppy controller that has a series of jumpers. You can use the standard 2-drive setup or you can opt for the "flat" setup and use 4 drives on a cable with all drive selects. The downside is that if one drive motor is enabled, they're all enabled.

There is also the "3 drive" cable where drives a and b are wired as normal, but the third drive makes use of unused pins (4 and 6 IIRC) to handle the motor and select options. The third drive is set to DS0. Some Ultrastor and DTC hard/floppy controllers can handle this and I've even modified a Future Domain SCSI controller to do likewise. If you've got a National 8473 FDC on your board, it's a 4-drive drive controller and you need only bring a couple of jumpers out.

Another approach to add floppy drives to a 2-drive system is to use a "switch" card. Basically, you take the host FDC cable and run it though the card and plug the extra floppies into a third connector. The principle is pretty very simple--the card contains an address decoder for port 3F2 and processes the 2 high-order and low order bits of a register write to add the extra support. No problem, since 3F2 is a write-only register. These came out as an accessory to the 5170, mostly.

The FDC core itself (D765 model) isn't used to generate drive selects, so no matter.
 
That's very informative, thanks for the info!

I was looking at the manuals for my drives (Teac FD-55 series) and I now realise that the unused pins 4 and 6 are actually assigned a use at the drive end, and the motor on signals are also used as drive select lines. It seems the PC floppy controller assigns a different use to those pins, but it's arranged in such a way that properly configured drives will ignore those signals.

@Chuck(G): With your comment about all the drive motors switching on at the same time, I noticed my Teac manual has a jumper you can set so the motor only switches on when the drive select line is also set, specifically to avoid this problem of multiple drives activating when they are not being accessed. I just thought that was a clever solution to the issue.

That's also very interesting about some DTC controllers allowing four drives on the same cable. Until now I had only heard of four-floppy controllers that had two connectors, with two drives on each connector. I suspect however that a four-drive 34-pin ribbon cable with edge connectors and IDC connectors is so rare that you will probably have to make your own!

I don't think any of my boards can do this unfortunately. They are all generic Winbond/UMC/GoldStar multi-IO cards. At least it would be easier to stick a four-drive ROM into the machine, as from my other thread it looks like most of them support four drives on one controller, but I have yet to find one that can use the secondary controller's address.
 
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