Replacing 5.25 inch drives with 3.5 inch drives - 2003

Contents copyright Herb Johnson 2006. Last update Sept 20 2006. Quoted material is copyright by the respective authors of that material and used with permission. For more info or for reuse or questions, email me via this Web link. Corrections are appreciated.

Documents, links

Old computer newsgroups often discuss converting old CP/M systems from 8-inch and 5.25-inch floppy drives, to 3.5" drives and media. Issues raised included support of FM or single density; finding old media; choices of formats. I decided these discusssions were worth preserving, as these issues come up over and over. For more technical information on floppy drives and floppy disks, go to our technical page on floppy drives and media. For specific discussions about converting from 8-inch, 5.25 inch, and 3.5 inch, look for links on this section of that page.

Replacing 5.25 inch with 3.5 inch drives - 2003

In comp.os.cpm during June 2003, there was a discussion about use of 3.5 inch floppy drives in systems designed with 5.25 inch 360K or 720K (40 or 80 track) floppy drives. It was acknowledged that 720K (double density, not 1.4M high density) 3.5" diskettes would be supported, operating as if they were the 360K or 720K 5.25" disks expected by the floppy drive's controller. I've summarized that discussion and put it in this document as below.

The following considerations were posted by Larry Sonderling (quoted here with permission and with my comments in square brackets):

"You _can_ use a new 3.5 inch floppy drive [in a system designed for 5.25" drives]. I have just done so in my Ampro LB sytem. It works great. The only caveat is that pins 2 and 4 _must_ be left open between the drive and the cable. A 3.5 inch drive outputs a signal on pin 2 according to whether a HD or DD disk is in the drive (this is signaled by the second hole in the floppy casing). If this pin is connected to something else in your system's drive controller it can cause problems.

"On some older [3.5"] drives pin 4 is an input for ejecting the disk (MacIntosh, for example), and on others it is an output signaling "in use." On most modern drives it is indicated as "reserved.""

There were follow-up comments posted by Amardeep S Chana (quoted here with permission):

Well... there *was* one difference between the older 1.4M [3.5"] drives and the modern ones, but it wasn't the spindle speed. It was the fact that the older drives received their operating mode as an input on pin #2 instead of sensing the media hole. If pin 2 was low, it was a 250Kbps [DD] 720K drive. If pin 2 was high, it was a 500Kbps [HD] 1.4M drive. What complicated this even more was that some drives could be jumpered to reverse the behavior of this input. These drives caused all manner of problems with media and controller compatibility since some controllers don't drive this pin, some drive it one way, some drive it the other, some drive it only for certain data rates, etc. This might have been what caused your early problems and was the chief reason why the next generation drives ignore this pin and simply use the media sensor."

"There is also the issue of bus termination. Older drives typically terminated with 150 Ohm pullup resistors because the controller used an open collector driver (7416). Modern controllers have CMOS totem pole drivers so most newer drives terminate to 1KOhm. Normally this is not a big deal but if you have problems with your setup try keeping one older drive on the end of the cable with the 150 Ohm terminator."

In private discussion, Chana provided more details on pin 2 and pin 4:

"On 1.2MB [5.25" HD] drives. pin two is called out as /RWC (reduce write current -- active low). When left disconnected, the signal will typically default to the high state if the drive is terminated (i.e. defaults to high capacity mode).

"For 1.4MB [3.5" HD] drives its function varies greatly. Some drives use it exactly the same as 1.2MB drives do. Others provide jumpers that allow it to be active high, active low, ignored (use media sensor), or even output the media sensor state. I had a Toshiba ND356 with all these options back in 1988. IBM PS/2 1.4MB drives define pin #2 in the opposite sense than PC/AT (reduce write current -- active high). Most any modern PC/AT type 1.4MB drive will simply ignore pin #2. I don't know anything about Mac drives."

"I do not know what problems Larry experienced with pin #4. In my numerous systems it is a no connect at the controller and drive. But I don't own any Macs. The only time I've ever seen pin #4 defined on a mini floppy drive bus is in the Shugart SA400 documentation where it is indicated as an input for "in use". That signal would light the activity LED instead of drive select if the drive was so jumpered."

I discussed with Chana the 40 track use of 3.5" diskettes:

To me [Herb wrote] there is a question about where "should" the 40 tracks be on a 360K formatted 3.5" diskette. A 5.25" 80-track drive must skip tracks to create a compatible 360K 40-track diskette. So a drive controller expecting a 360K 40-track 5.25" drive MIGHT be reprogrammed to provide that "double stepping" of tracks when using an 80-track 3.5" drive. But in private discussion, Chana commented: "Double stepping is NOT necessary. If a 360K format is used on an 80 cylinder 3.5" drive it will use the first 40 cylinders single stepped. There is no purpose in double stepping since there is no 40 cylinder 3.5" drive to maintain compatibility with."

Also I want to ask you about a statement you made recently in that thread:

"Only 8" and 5.25" High Capacity (1.2M) drives operate at 360rpm. Some 5.25" 1.2M models had the spindle speed step down to 300rpm for DD media compatibility but later a 20% faster data rate of 300Kbps was used instead of a slower spindle speed."

What were the details of that use of 300Kbps? Was that use specific to some particular brand of 5.25 inch drives? An issue there might have been the analog (read/writer) circuit design compensation, presumably set for 250Kbps, versus the slightly higher 300K. And was that 300Kbps particular to any well-known computers or FDC chips? I'm looking for anything that suggests how common or specific this option was.

Herb Johnson
New Jersey, USA
to send email @ me, follow this link.

Copyright © 2006 Herb Johnson