should be DS1 closed for middle of cable before the twist for "B" drive, same as teac's fd-55 series, not sure if some boards will work with a B but not an A drive (depends on the BIOS??)
Think it is a DOS thing: Re: Controlling drive letters in DOS

OK so I moved the jumper from DS0 to DS1 but it does not work as a B: drive with or without the TERM jumper.

I assume I also need to attach an A: device to the chain for that to work, presumably without the TERM jumper? I'll try that tonight..

I don't know what the TERM jumper does but that floppy drive is old enough to have removable resistor pack, check if it is installed. Shouldn't really work at all without one but it's best to make sure.
The drive should work as A: while jumpered as DS1 and sitting at the end of the cable, after the twist. So if it works as DS0 in the middle but not as DS1 after the twist, perhaps the flat cable is damaged?

All drives (whether A: or B:) must have the second drive select jumper set. If your DS jumpers count from 0, then you set DS1. If DS jumpers count from 1, then you set DS2.

After doing that, if the drive still works only as A: but not as B: then the cable or the plug before the twist is damaged, or your BIOS settings might be messed up.

appiah4 wrote on 2023-09-24, 18:27:

I have Chinon FD-502 that is content with running as A in the middle of a floppy cable but does not work as B on either positio […]
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I have Chinon FD-502 that is content with running as A in the middle of a floppy cable but does not work as B on either position.

Jumper settings are

TERM Closed
DS0 Closed
DS1 Open
DS2 Open
DS3 Open

That's actually a quite interesting scenario: The PC floppy drive systems uses a "twist" on the drive select and motor on signals that foreign to the standard Shugart bus system which allowed 4 drives on one cable. The shugart system uses a shared motor on line for all drives, but the IBM system uses dedicated motor on lines for both drives. The traditional "DS0" line is the motor on line for drive A in the IBM PC system. The traditional "DS2" line is the drive select line for drive A in the PC system. The traditional motor on line is the motor on line for drive B in the PC system. So you got your drive to listen to the "motor on" signal of drive A as drive select signal. Using motor on as substitute drive select signal can work in single-drive systems. But where the heck is your drive getting "motor on" from? If it would use the standard Shugart motor on pin, its motor would not be running when you access drive A: only, as that line is only asserted while drive B: is accessed. Possibly that drive is configured to not use a dedicated motor on signal, but use the chosen DS input as both motor on and drive select? Or possibly someone hacked the motor-on logic in some special custom way?

Setting DS1 instead of DS0 should make the drive respond to the "drive B select" signal if you connect it at the middle of the floppy cable, but having the drive to respond to DS1 won't help if the motor doesn't turn. I guess you need to focus on the motor-on-signal instead of the DS signal to solve your issue.

Thanks for all suggestions, serting it at DS1 and opening TERM makes it run flawlessly as B: in the middle of the cable but only when there is an A: drive after the twist at the end! Very curious!

Apparently this drive is from a Commodore PC10 if it is of any significance.

appiah4 wrote on 2023-09-25, 19:05:

Thanks for all suggestions, serting it at DS1 and opening TERM makes it run flawlessly as B: in the middle of the cablw but onky when there is an A: drive after the twist at the end! Very curious!

"only whene there is an A: drive" is not that surprising: You need a drive to terminate the bus. So if you open TERM on the B drive, you need the A: drive to terminate the bus.

Well, yeah 😀 And with TERM closed it works as A: on the end of the chain (aftrr the twist) when set as DS1, and in the middle position when set as DS0.