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Other Floppy Disk

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Looking for CoCo help? If you are trying to do something with your old Color Computer, read this quick reference. Want to contribute to this wiki? Be sure to read this first. This CoCo wiki project was started on October 29, 2004. --OS-9 Al

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Home / Hardware - Other Floppy Disk


Yes, drives other than those from Tandy may be used with the CoCo. In particular, the common 360K PC drives are CoCo-compatible. To put it into the proper perspective, IBM chose for the PC the same pre-existing floppy standard that Radio Shack chose for the CoCo. Actually, IBM deviated from the standard more than Radio Shack did! The standard specifies the possibility of up to four drives connected to one controller. All decent floppy drives have four drive selection jumpers, usually numbered 0 through 3 (occasionally 1 through 4). While they were used properly in some early PCs, the standard was still not utilized to its full potential (by allowing only two drives). The CoCo supports all four possible drives. All 'modern' PCs use a twisted floppy interface cable, with all drives permanently set to ID 1 (or 2; the second of the four possible choices). The same selection method may be used with the CoCo, but will result in the same limitation of only permitting two drives. The usual selection method Tandy used was different. They jumpered all drive select lines on all drives, and removed all but the desired 'teeth' from the edge connectors on the ribbon cable. This is commonly referred to as the 'dental extraction' method of drive selection. :) PC and CoCo floppy drives alike may be connected to their controllers in the proper way, with a straight-through ribbon cable feeding all edge connectors, and the proper use of drive select jumpers on the drives themselves.

Another area in which the PC 'world' deviated from the standard is the issue of drive cable termination. The standard specifies that a terminating resistor array shall be installed at the end of the interface cable farthest from the controller, and nowhere else. As with SCSI interface cables, 10base2 networking, and all other RF applications, proper termination prevents reflections and standing waves in transmission lines. CoCo drives typically are (and should be) terminated in the proper manner. The Wintel crowd, on the other hand, decided to eliminate that little additional bit of required knowledge (along with drive selection elimination by twisting the interface cable). What they do is terminate ALL drives; that way, one cannot forget to install the terminator at the proper place. Of course, with only two possible drives in the first place, it does not cause as much of a problem as four terminators would. ;)

Another issue to be aware of is that all drives in a CoCo system must spin up when told to do so by the 'motor on' line, and not by their drive select lines. PC drives may be jumpered wrong for CoCo operation. Problems may arise when copying information from one drive to the other, as the CoCo assumes that all drives are spinning while any drive is accessed. No delay is inserted between reading from one drive and writing to another. The name of the jumper which controls this behavior depends on the specific drive.

5.25" 80-track 720K ('quad-density') and 3.5" 720K drives are also CoCo-compatible, within the preceeding constraints. They are electrically identical to each other. Disk BASIC will only access the first 35 tracks on the first side of any disk, however, so they will still only hold about 160K of information each. Various patches exist to Disk BASIC to allow access to the extra space, and the OS-9 operating system can make use of it as well. Note that 720K 5.25" drives were virtually unknown on the PC platform (except in the Tandy 2000); they were fairly popular in the CP/M and OS-9 communities.

5.25" 1.2 meg drives are virtually useless on the CoCo. They are reportedly more like the old 8" drives than either other 5.25" or 3.5" drives. They also spin at a different rate (360 RPM rather than 300). Some may reportedly be jumpered to spin at 300 RPM and function as either 360K or 720K drives, but they would still not be reliable for use with true 360K media (just as they are not in PCs).

3.5" 1.44 meg drives can sometimes be jumpered to function as 720K drives; one should always use true 720K media when doing so. The previously-mentioned hack for the original 26-3022 controller may make these drives useful (under patched Disk BASIC or OS-9, of course) as true 1.44 meg drives; I have not attempted to implement that hack. UPDATE: supposedly, just the act of using true 720K media (without the high-density hole) will make a high-density 3.5" drive function as a double-density drive. I intend to confirm this.

Something else should be noted about 1.44 meg 3.5" floppy drives. I encountered such a drive which appeared to be nonfunctional. Further testing revealed that it worked with 720K disks, but not 1.44 meg disks. I checked the density hole sensor switch, and found it to be good. I then traced the circuit from that switch and found that it connected to one conductor in a small mylar ribbon cable connecting the two halves of the drive's logic circuitry. It turned out that the ribbon cable was not making solid contact. Disconnecting it from the small connector on the one PC board and reseating it fixed the drive. I have since repaired other bad drives in the same manner.