Redirect unit A: to unit B: on MS-DOS via software?

Posted by darthuna@reddit | vintagecomputing | View on Reddit | 25 comments

TLDR: Is there a way to redirect unit A to unit B on MS-DOS 6.22 via software? Maybe using the DEVICE command on CONFIG.SYS? Long story: This is an industrial 386 board inside a very small case with limited space, it has a 3.5" floppy drive connected to unit B: because it has a very short ribbon cable without the twist. There are only two connectors on the ribbon cable, so no chance to make it unit A:, and the other ribbon cables that I have are too long for the small space in the case. I'm used to the 3.5" FDD being A: so I always do A:, wait for MS-DOS to tell me it failed, then remember it's unit B: and do B:, until I repeat the error again and again... It's mildly annoying. So that's why I want to redirect A: to B: via software (maybe keeping B: as-is so both A: and B: access unit B:).

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Vanbursta1@reddit

subst a: b:

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dlarge6510@reddit

It's already done. By default if you only have one drive A: and B: use that drive. However perhaps that's not working as your drive is in the B connection (before the twist) in which case you need to get a cable that puts it after the twist. But when I use my DOS laptop, that has only one floppy, A: and B: both use that drive. It's supposed to assist with using one drive but I haven't figured out how yet!

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StreetSweet5357@reddit

Some 3.5" floppy drives still had jumpers to select which drive it is.

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AppropriateCap8891@reddit

Most of them have such a jumper somewhere.

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denislemire@reddit

No, IDE drives generally used jumpers but floppy drive ribbon cables has a twist in them and which drive was on which side of that twist determine which was drive A or B

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AppropriateCap8891@reddit

And the twist literally changes the position of the jumper without changing the jumper. The procedure if you have a cable with a twist is that you set the jumpers on the floppy so that both drives are Drive 1 (second drive in the chain), and the twist will change that to in the chain the drive at the end is Drive 0 (A) and the second is Drive 1 (B). You can tell who here has never had to work with floppy drives before the cables were commonly twisted. Oh, and it was the exact same way with MFM-RLL hard drives. You could set them both as the same device and the twist in the cable changed the drive assignment for you. Or you could use a straight cable and assign them manually. And as this is specifically a 386 system. it is doubtful that even when new that had an IDE drive. MFM dominated the industry at that time.

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Gadgetman_1@reddit

Sorry, but that's... Even my 386sx had an IDE(PATA) drive. Bough that one in 1990. The only machines still using older tech was crappy designs pushed to enterprises, such as the Olivetti M380. (That had a 20MHz 386DX, and my 16MHz sx ran circles around it) Also, MFM-RLL is a data-encoding technology, even used in floppy drives. What you're thinking of is ST-412 and ST506 interfaces. And the ST506 supported up to 4 drives. With SELECT on pins 26, 28, 30, and 32 (the in-betweens are GND)

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AppropriateCap8891@reddit

On the board, or added? Other than some name brands, on-board hard drive controllers were pretty rare in the 386 era.

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2raysdiver@reddit

IBM's first PS/Valuepoint line had 386 model that used IDE as well.

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Baselet@reddit

Old drives use to have some jumpers, newer ones have to be modded on the PCB.

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AppropriateCap8891@reddit

I would say 95% of those I worked with in that era had jumpers. And the few that did not have jumpers generally did the same thing via either traces you cut or dip switches. Because it has to be remembered, this is not just a "PC Thing". The same floppy in a Compaq or clone desktop could also be found in a Brother word processor or in a synthesizer. It cost a lot of money for some company to demand a floppy manufacturer make something special only for them, and most manufacturers were not willing to pay that money. So they used COTS floppies for damned near everything. Even the iconic "Teac Dual" at the end of the era had jumpers.

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StreetSweet5357@reddit

If your cable is too long, use a sharp pair of scissors and cut the cable just before the second drive connector leaving the section with the twist if it that is long enough. You can also move the connector the is on the computer end.

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Pyrofer@reddit

Subst A: B: I've been using Subst A: C:/installpath to install stuff that hard codes looking at floppy drives for the files. You can prob just do that.

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nricotorres@reddit

I had completely forgotten my DOS tricks, including this!

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dgeurkov@reddit

another option is to use grub4dos and reassign drives

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dgaxiola@reddit

You can put the twist in yourself. Pop off the end plastic connector. Count over the appropriate number of lines. Separate the group of lines (10-16) with a utility or x-acto style knife. Twist. Re-apply the end connector. https://retrocmp.de/fdd/general/floppy-cable.htm

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aspie_electrician@reddit

What machine is it if you don’t mind my asking?

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Scoth42@reddit

Some BIOSes do have a swal floppy option, though I mostly remember it coming on later BIOSes than 386s. It's also not too hard to modify the cable if you're careful with it. The connector probably has a little clip that comes off the back, then you can swap the bit of ribbon and put the clip back on. Floppy cables are also still pretty readily available in lots of lengths, you can probably find a replacement with the twist that will fit. Alternatively you might be able to take a standard three connector cable and cut off the "long" bit just leaving the two connectors with the twist, which could be pretty short.

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Redirect unit A: to unit B: on MS-DOS via software?

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25 Comments

Vanbursta1@reddit

dlarge6510@reddit

StreetSweet5357@reddit

AppropriateCap8891@reddit

denislemire@reddit

AppropriateCap8891@reddit

Gadgetman_1@reddit

AppropriateCap8891@reddit

2raysdiver@reddit

Baselet@reddit

AppropriateCap8891@reddit

StreetSweet5357@reddit

Pyrofer@reddit

nricotorres@reddit

dgeurkov@reddit

dgaxiola@reddit

aspie_electrician@reddit

Scoth42@reddit

gen_angry@reddit

CoolBlackSmith75@reddit

my-names-gavin@reddit

AdrianValles@reddit

TheMemo@reddit

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CrasVox@reddit