View Full Version : Curious 5.25 Drive Behavior (PC/XT clone system)

August 8th, 2017, 01:31 PM
Hi all,

I have a situation that is very strange. I have an IBM PC/XT clone (Computer Express brand, Phoenix Technologies BIOS ROM v2.52) that came with a unidentifiable 1.2M floppy controller card with 2 async ports. It originally came with two drives (Newtronics Mitsumi D503V). Upon my initial boot attempt with 3.2 PC-DOS in drive A:, I received a boot disk failure. After switching the 34 pin connectors around and using what was previously drive B: as the boot drive, I successfully booted the OS.

After closer inspection, it seems that the stepper motor within the defective drive is only capable of moving to track 00 when connected. Any boot attempt, or attempt to use the drive to format another 5.25 diskette, results in a general drive error as the read head only retracts to track 0 and does not step inward.

Now, here is the kicker. I can manually step the read head inward by pulling DIR, MOTOR ON, and DS1 low, and pulsing the STEP line by hand. I even attempted to replace the drive with another (Chinon FZ-506) I had around and it experienced the same behavior (FDC doesn't move the read head, but I can manually step it inward). Both drives were set to use DS1 drive select (As per twisted PC floppy cable behavior) and I've tried multiple pin combinations on both drives. I've also successfully moved the functioning drive to B: after the OS is booted and successfully read diskettes in the working drive without i/o errors.

Would this be written off as a controller error? A simple case of drive gone bad? Some problem with one of the drives affecting the control lines? Both original drives came with the termination jumpers lacking a pin header and thus disconnected. The other Chinon FZ-506 has a termination jumper but setting it while it was the final drive on the cable did nothing.

Any pictures or ROM images can be provided if needed and I appreciate any help or advice any of you have. Thanks :).