Floppy Disk Drives / 8 inch / General



The most important thing in advance. Yes, it is possible to operate 8 inch drives under DOS. Looking back, it was not that difficult to do this. From concept to implementation, it took only 3 months. Most of the time took the search for a suitable drive (Shugart 860) on ebay and shipping from the US. The actual implementation was then done on a weekend. But first things first.

This introduction sounds pretty casual, as if running an old 8 inch drive on a DOS PC is a breeze - no, that's not - it's not plug and play like most people in the Windows age are used to.

As stated in the introduction, it is possible to run 8 inch drives in DOS, but ... Basically, I have not found a single hint in any old textbook of a true operation of these drives in DOS PCs. I have several issues of Scott Mueller "Upgrading and Repairing PCs ", also the first two editions from 1988 and 1992. Mark Minasi: "The Complete PC Upgrade and Maintanace GUIDE, 1991". Corey Sandler: "Fix your own PC, 1997." Result: Nothing! See [Literature]

IBM PC, XT, AT and the other later compatible PCs were equipped only with 5.25" floppy disks (single-sided and double-sided) and later with 3.5" floppy disks. Not with 8 inch drives!

Although Microsoft's support page (Q75131) and "The MS-DOS Encyclopedia" explicitly mention 8 inch drives (Version 1.0 - 2.0) the direct file access with IBM compatible PC's is NOT possible. I have started MS-DOS version 1.25 from floppy disk (A: QUME 142) with access to my Mitsubishi M2896 63 as drive B:. But it only works in mode 320 KByte (DS) or 160 KByte (SS). There is definitely no parameter in the FORMAT command for 8 inch floppys, see [dr88], p.865.

The only sector size the standard MS/PC-DOS FORMAT command supports is 512 byte/sector. Not 128 and not 1,024; these both are the standard sector sizes for the 8 inch floppies!

8 inch floppy drives were only supported by special OEM versions like MS-DOS 2.0 for the Seattle Computer Products (SCP) with the S-100 bus and special controllers (Cromenco / Tarbell 1771, 1791, 1793 or 16FDC).

According to Dave Dunfield, the NEC APC is one of the few computers that runs with 8-inch drives under MS-DOS. In Wikipedia the operation of an 8 inch drive under MS-DOS 2.0 is mentioned. But this is the Xerox 8/16, a dual CPU computer. These both are not IBM PC compatible computers!

The trick

But there is a solution using 8 inch drives with MS-DOS from version 1.25 through 6.22 on IBM compatible computers. I found it on the internet at Dave Dunfield, the programmer of Image Disk (an ingenious program). He runs the site Daves Old Computers - Disk/Software Image Archive. Right at the beginning there is a link to the operation of 8 inch drives on DOS PCs:" Notes on connecting 8 "and external floppy drives."

The simple trick is to fool MS-DOS and output an 8 inch drive as a 5.25 inch drive (1.2 MByte, DS/HD). That's it. Of course, this is only possible because the characteristics of both drives are almost identical. The difference is only in the number of tracks. A 5.25 inch DS/HD disk has 80, an 8 inch DS/DD disk has 77 tracks. All other parameters, such as revolutions per second (360 RPM), data transfer rate (500 Kbps), sectors (15), ... are the same.

... Two years later IBM introduced the Personal Computer AT, based on the Intel 286 microprocessor. With it came the high-capacity 5” floppy disk. It basically gave the 5” disk the same specifications as the older 8” disks – doubling the data rate and spinning the disk at the faster 360 RPM. Capacity increased to 1.2 MB, the same as the 8” disks.

Tim Patterson: http://dosmandrivel.blogspot.com/2011/04/all-those-floppy-disk-formats.html

Update 06.02.2019: Today I simply connected my Mitsubishi M2896-63 as drive A: to my test PC. Afterwards I set the drive A: as 1.2 MByte 5.25 inch drive in the BIOS, inserted an 8 inch boot diskette and restarted the PC and ... The PC starts perfectly from the 8 inch floppy disk.

The second way to access 8 inch floppies - in their native format - is via special programs only. Names such as Anadisk, Teledisk and the already mentioned ImageDisk by Dave Dunfield appear here. ImageDisk bypasses the operating system and accesses the drive directly from the floppy controller. Here you can set each parameter quite specifically and read and write disks with foreign formats.

But attention! Although these programs are capable to access 8 inch drives in the whole (as images), they are not capable to copy or delete a single file! Too bad I have not yet found a program that can copy, ... files like 22disk (CP/M) or PUTR. Have you?

Problem no. 1: Power supply

8 inch floppy drives always need +24V and +5V. The usual PC power supplies deliver only +12V and +5V. You therefore always need a separate +24V DC power supply. Who owns a laboratory power supply owns luck, appropriate socket or table power supplies are available to buy. Only get the right plug and a few adapters and problem no. 1 is solved.

Attention: 8 inch floppy disk drives with full height usually need an additional 220V or 110V AC connection for the drive, sometimes even a -5V line! The newer drives like the Shugart 860 and the Mitsubishi M2896-63 with half the height only need the two above-mentioned connections!

Power supply: plug + adapter
Power supply: plug + adapter
Stromversorgung: +24V und +5V Netzteile
Power supply (from Mean Well): +24V (2,5A) und +5V (1,2A) DC

Problem no. 2: The controller cable

The cable, however, is a real problem. If you do not want to or can not solve this, then you do not need to look for a suitable 8 "drive. Both ways suggested here are no plug and play solutions!

Background: 8 inch drives have a 50 pin (50p) connector (Shugart-50), but today's floppy controllers (PC-34) use a 34 pin cable (34p). How do I connect a 34p ribbon cable to a 50p and, for example, connect line 34 (35p) to line 12 (50p)?

You have three solutions to choose from:

  1. The 1st is only for "real" electricians with a lot of soldering experience and is well priced. Daves Old Computers - Disk/Software Image Archive.
  2. The 2nd, however, requires only minimal crafting skills. I worked it out.
  3. Update 11.06.2018: The 3rd is a really plug 'n play solution. See John Wilson (D bit): ersatz11 - PDP-11 Emulation System / FDADAP floppy disk adapter (en) Very easy to use. I bought one and it is in action.
Interface PC-34 to Shugart-50
Interface PC-34 to Shugart-50 / Looks easy! It is.

Insert the 34p ribbon cable from the PC controller on the left, then make the correct cable bridges and connect the 50p ribbon cable to the 8 inch drive on the right again .. Voila! It's that simple, no more than 15 minutes of tinkering. This solution has the advantage that you are completely flexible with the cable bridges.

In addition to the obligatory 35-pin ribbon cable, you also need a 50-pin SCSI cable and a card edge connector with 50 contacts as well. The corresponding terminal blocks and connectors can be obtained at Farnell GmbH or TMU.

My solution with the clamping blocks has the advantage that you can change the screw bridges at any time, because, as it should be otherwise, the Shugart 50 interface is sometimes slightly different in the 8 inch drives. It may also be necessary for you to connect an input signal to two output signals. Unfortunately, there were no uniform standards back then! The 8 inch drives are from the pre-PC era, see the movie "War Games" from 1983 and the IMSAI 8080.

The result

And so it looks like in the finished state.

Establish connection
Establish connection

Please do not disturb the imprint of the 34p cable. This is originally from a tapedrive and was used the other way around. Only red cable 1 on pin 1 is important. You can also see the attachment plug here. Here a second floppy cable can be connected. So two floppy drives can be used!


Problem no. 3: The signal TG43

Dave Dunfield (see link above) described this in detail and presented also possible solutions. Briefly. The signal TG43 (track greater 43) reduces the Write Current Low from track 43 (at the inner tracks). However, this signal never occurs with the PC interface! Either HD disks are written with a high write current (coercivity 600) or DD disks with a low write current (coercivity 300). It never follows a change on a floppy disk depending on the tracks. There may be problems with the operation of 8 inch drives in PCs, but it does not have to!

The TG43 signal is expected at the Shugart interface on line # 2. Since the PC controller does not know this signal, there can be problems during the writing process, but reading is no problem. The Shugart 860-1 is a newer drive and can control this signal internally via its own logic. There is an extra jumper for this.

Other drives with internal TG43 jumpering (Write Current Low) are the Tandon 848-1E and 2E (1983), please do not confuse it with the 848-1 or 2 (1982) and the Mitsubishi M2896-63-02M. I also have the M2896 in operation. Works flawlessly with my terminal block / interface solution.

I only listed the TG34 problem at the very end, because it is not clear if this is really a problem. All the authors I've researched are also not sure. Can be or can not be! In case of doubt only trying out helps.

So, if you're seriously considering running an 8 inch drive, then study the manual carefully before you buy it! You can easily find the appropriate manuals on the Internet.

Update 29.04.2018

I've just tested a Qume 142 (Shugart-34) as drive A: and the Shugart 860 (Shugart-50) as drive B:. For this I had to change two signal cables at the terminal blocks again. This means that I have now linked three different controller types. PC-34 (from the computer controller) with Shugart-34 and Shugart-50. This is only possible with my terminal block solution. Mind you PC-34 is not Shugart-34, even if the signals are transmitted almost the same. But only almost! The whole thing is possible because the 34p cable from the controller is a so-called tapecable. This has a branch connector. Simply attach the appropriate 35p cable for the floppy drive and you're done! Test System # 2 now features two Seagate ST-225 MFMs and the two retro floppies (Qume 142 + Shugart 860).

Update 06.10.2018

CompatiCard I: If you own this controller card, you are able to read and write 8 inch floppies under MS-DOS with the original 77 tracks, but ...

Even with the CompatiCard I it is NOT possible to read and write the official logical 1.2 MB 8 inch DOS floppy format (8 sectors/track, 1024 bytes/sector)! The controller supports only a pseudo logical 1.2 MB format (16 sectors/track, 512 bytes/sector). Too bad!

Here is a list of the logical floppy disk formats. Look for IBM compatibles.