Force a 1.4MB 3.5" to only act as a 720K drive? (CoCo)

[ColorComputerStore]

I want to utilize a 3.5" drive on a TRS-80 Color Computer. Setting the Radio Shack drives aside, that using a 5-1/4", the floppy controller is limited to 360K drives. And for 3.5", the floppy controller is limited to 720K disks.

360K disk drives are hard to find, but 3.5" 1.4M drives are plentiful. On the actual media, we just needed to cover up the HD hole with tape and the drive would treat the HD disks like SD disks.

So if the CoCo could talk to a 3.5" drive and utilize 720K disks, why can't I just buy a 1.4M 3.5 and cover up the HD holes so the drive treats the disks as 720k? I was told that the drive has to only be able to read/write 720k disks. If it does 1.4M and 720K, it won't work.

BTW, I already have SDC and GoTek. The above is for a different need.

Thanks,
Carlos

 

[Eudimorphodon]

So if the CoCo could talk to a 3.5" drive and utilize 720K disks, why can't I just buy a 1.4M 3.5 and cover up the HD holes so the drive treats the disks as 720k? I was told that the drive has to only be able to read/write 720k disks. If it does 1.4M and 720K, it won't work.

Whoever told you that is misinformed. On a “standard” 1.44mb drive the density selection jumper directly controls the write current, if you insert a 720k disk (or a 1.44mb disk with the hole covered up) the drive appears identical to the computer to a 720k-only drive.

(There are edge cases where some drives might have a jumper to allow the computer to control density select, that’s how IBM did it on the PS/2, but you have to go out of your way to find a drive like that.)

Bigger problem really is simply most newer floppy drives lack a full set of drive select jumpers so they can be awkward to wire up.

 

[Chuck(G)]

READY/ is also an issue for many HD drives, which usually use pin 34 for DISK CHANGED, which is not the same thing. Some older systems need READY. Some drives allow you to re-jumper the PCB, but it varies from drive model and maker. As far as drive select goes, that's mostly a matter for the interface side. That is, the drive selects after the jumpers all terminate at the same spot; it's mostly a wiring issue.

That being said, I find that most modern-ish HD drives outperform legacy DD drives in DD mode. Probably better head technology and read/write channel design.

However, that doesn't mean that all HD drives perform the same. More on this later...

 

[SomeGuy]

Basically, if you plop a 720k disk (or 1.44mb with notch covered) in to a 1.44mb drive, the drive will look and behave just like a real 720k drive, which also happens to look like a 360k drive if you don't look for extra tracks.

There are just a couple of possible wiring issue. I don't know what a CoCo expects, but it almost certainly uses a "flat cable", which means it uses a different drive select. IBM PC twisted cables only use DS1, but a flat cable would need DS0 on a primary and DS1 on a secondary. As such, a lot of 3.5" drives are hard-wired for DS1 only.

If the controller uses the Drive Ready/Disk Changed line, it may expect different behavior.

Also, as mentioned, if it outputs anything to the density line, that could cause problems for a few 3.5" drives. But I'd sort of expect these lines to not even be used.

A few of the better 3.5" drives have jumpers for this stuff.

 

[Ec1564]

Many many years ago a friend hooked a 1.4M 3.5 floppy drive to his CoCo and he said after formatting the disk worked like normal. Just don't try to use that disk on any other operating system it would come up a a unformatted disk.

CoCo is only a 168K single sided drive but I think he modded the 1.44M floppy drive to be a double sided setup knowing him.... I may have to try this but the Gotek works really good soo that's on back burner.