Using a 5.25" 1.2Mbyte floppy disk in place of an 8" one

(This page is Don Maslin's message collection on this subject)

Data Cable

8" controller                      5.25" Drive

1  o
2  o
3  o
4  o
5  o
6  o
7  o
8  o
9  o--Gnd-----+
10 o--2 side--+
11 o
12 o
13 o-----Gnd--------------------o 31
14 o-----Head Select------------o 32
15 o
16 o
17 o
18 o
19 o-----Gnd--------------------o 7
20 o-----Index------------------o 8
21 o-----Gnd--------------------o 33
22 o-----Ready------------------o 34
23 o
24 o
25 o-----Gnd--------------------o 11
26 o-----Drive Sel 0------------o 12
27 o
28 o     +----Gnd---------------o 15
29 o     +----Motor On----------o 16
30 o
31 o
32 o
33 o-----Gnd--------------------o 17
34 o-----Direction--------------o 18
35 o-----Gnd--------------------o 19
36 o-----Step-------------------o 20
37 o-----Gnd--------------------o 21
38 o-----Wr Data----------------o 22
39 o-----Gnd--------------------o 23
40 o-----Wr Gate----------------o 24
41 o-----Gnd--------------------o 25
42 o-----Trk 00-----------------o 26
43 o-----Gnd--------------------o 27
44 o-----Wr Prot----------------o 28
45 o-----Gnd--------------------o 29
46 o-----Rd Data----------------o 30
47 o
48 o
49 o
50 o


1) Crimp 34 pin IDC edge connector to one end of 1m length of multicoloured IDC ribbon cable, so that pin 1 is connected to the first brown wire, and pin 34 to the last yellow wire.

2) At the other end of the cable, separate the wires between : 6,7 (blue,violet)
8,9 (grey, white)
10,11 (black, brown)
12,13 (red, orange)
14,15 (yellow,green)
16,17 (blue, violet)
30,31 (black, brown)
32,33 (red, orange)

3) Make the wide section (wires 17-30) about 8 cm long, by tearing back the other sections.

4) Crimp wires 17-30 (violet - black) to pins 33-46 of a 50 pin IDC edge connector. Remove the top part of the connector after crimping so that other wires can be added.

5) Similarly crimp 31,32 (brown and red) to 13,14 of the 50 pin edge connector, 33,34 (orange, yellow) to pins 21,22, 7,8 (violet, grey) to pins 19,20 and 11,12 (brown, red) to pins 25,26.

6) Crimp a short spare length of IDC wire between pins 9 and 10 of the 50 pin edge connector. Fit the top part, and press it firmly into place.

7) Cut off wires 1-6 (brown-blue), 9,10 (white, black), and 13,14 (orange, yellow).

8) Strip wires 15,16 (green, blue), and twist them together. Solder the connection, and insulate it with a short length of Heat-shrink sleeving

Drive links

Set the drive to respond to Drive Select 1 (the default for PC drives)

The system will recognise the floppy if the Ready pin (pin 34 of the 34 pin connector) is held low when the drive is selected, and will not use the drive under any circumstances if the ready pin is high when the drive is selected. Some drives allow a Ready signal to be output on this line, often by setting an internal link. If this is possible, do it. Otherwise, _try_ connecting the ready input on the controller to the Drive Select output via a SPST switch. Close this switch when there is a floppy in the drive


Most machines check for the drive being ready _before_ asserting the Head Load Signal, and thus the head load output cannot be used to drive the Motor On input. By tying the Motor On input low, the drive motor runs whenever there is a floppy in the drive. This is a good compromise, but may cause wear of the disk and heads if left there for an extended period. Do not leave a floppy disk in the drive if it is not being used

The 2-side input of the machine is pulled low, so that the disk is assumed to be double sided.

The 8" drive rotates at 360rpm, as do 1.2Mbyte drives. However, tests have shown that a 300rpm drive will also work, although not all of its capacity will be used. Therefore, a 3.5" 1.44Mbyte drive is another possibility. -tony ard at

The following table is extracted from the CompatiCard manual:

      Card            34   37              50      8 Inch Drive
Signal Name          Pin  Pin  Direction  Pin     Signal Name
Programmable          2    3       --->    2       Low Current
Index                 8    6       <---    20      Index
Drive Select 1/3      12   8       --->    28      Drive Select 2
Motor Enable 1/3      16   10      --->    18      Head Load
Step Direction        18   11      --->    34      Direction Select
Step Pulse            20   12      --->    36      Step
Write Data            22   13      --->    38      Write Data

Write Enable          24   14      --->    40      Write Gate
Track 0               26   15      <---    42      Track 0
Write Protect         28   16      <---    44      Write Protect
Read Data             30   17      <---    46      Read Data
Select Head 1         32   18      --->    14      Side Select

Connect odd number pins of 34 pin connector to odds of 50 pin connector
Connect pins 21/37 of the DB-37 to all the odd pins on 50 pin connector

The interface on 8" drives and 5 1/4" drives are essentially the same. The 34 lines on a typical 5 1/4" controller are sufficient to control most 8" disk drives using soft-sectored disks.

Here, then, is a diagram for a basic conversion cable to allow con- nection of an 8" drive to an IBM-compatible, AT-style (high density) controller.

This diagram also works in the other direction--that is, to attach high-density 5 1/4" drives to an 8" controller.

                                                   8" disk drive
PC-AT style controller                         Based on Shugart SA-851

Grnd.  Sig.  Sig. Name                             Sig Name  Sig  Grnd
  1      2   Double/High Density ->>
                                  >>- Write Current Switch/    2     1
                                 Active Read Compensation
                                 User Customizable I/O pins    4     3
                                  "         "       "    "     6     5
 33     34 **Ready ---------------<<------------ True Ready    8     7
                                  <<-------------#Two Sided   10     9
 33     34 **Disk Change ---------<<----------- Disk Change   12    11
 31     32   Side 1 Select ------->>-----------#Side Select   14    13
  3      4   In Use/Open --------->>---------------- In Use   16    15
 15     16  *Motor On ------------>>------------- Head Load   18    17
  7      8   Index ---------------<<----------------- Index   20    19
 33     34 **Ready ---------------<<----------------- Ready   22    21
                                  <<---------------##Sector   24    23
  9     10   Drive Select 0 ------>>-------- Drive Select 1   26    25
 11     12   Drive Select 1 ------>>-------- Drive Select 2   28    27
 13     14   Drive Select 2 ------>>-------- Drive Select 3   30    29
  5      6   Drive Select 3 ------>>-------- Drive Select 4   32    31
 17     18   Direction Select ---->>------ Direction Select   34    33
 19     20   Step ---------------->>------------------ Step   36    35
 21     22   Write Data ---------->>------------ Write Data   38    37
 23     24   Write Gate ---------->>------------ Write Gate   40    39
 25     26   Track 00 ------------<<-------------- Track 00   42    41
 27     28   Write Protect -------<<--------- Write Protect   44    43
 29     30   Read Data -----------<<------------- Read Data   46    45
                                  <<------##Separation Data   48    47
                                  <<-----##Separation Clock   50    49


* - This has not been tested by me.  It seems to be a logical
    substitution since the vast majority of 8" drives have continuously
    running spindles and instead of MOTOR ON require a HEAD LOAD signal.
    Also, I assume that a controller sends MOTOR ON before a DRIVE
    SELECT (my 8" controller will send HEAD LOAD before DRIVE SELECT).

**- Most 5 1/4" disk drives do not provide a READY signal but send a
    DISK CHANGE signal on line 34 of the interface.  An 8" drive has
    provisions for both signals.  Likewise, most AT-style controllers
    expect a DISK CHANGE signal on line 34, so lines 33 and 34 should
    be connected to lines 11 and 12 of the 8" disk connector.  Also,
    some 8" drives provide a TRUE_READY signal which is more useful
    than the standard READY.

# - Unused on single sided drives (SA-800/801).

##- Used only on hard-sector configured drives (SA-801/851).

Some 5 1/4" disk drives have the option of providing _either_ DISK CHANGE _or_ READY on line 34 (in particular, the TEAC FD55R series). Some 8" disk controllers do not care about the DISK CHANGE signal, but must have the READY signal. If you are attaching a high-density 5 1/4" drive to an 8" controller, you may get away with making the drive always ready by shorting lines 21 and 22. I have heard that this may cause a few re-tries when switching sides, but it works fine for me. If your drive offers a READY signal that your controller can deal with, by all means use it.

The MOTOR ON/HEADLOAD dilemma may also have an alternate solution if you are connecting 5 1/4" drives to an 8" controller. Some 5 1/4" drives permit motor turn-on by means other than the MOTOR ON signal. For example, the TEAC FD55R series of drives may be configured to turn the motor on based on the state of the IN USE light. The IN USE light can, in turn, be set to turn on only on drive select. Thus selecting the drive automatically turns on the motor and neither a MOTOR ON or IN USE signal need be present.

John D. Baker ->An Apple ZCPR3 nut //
8 January 1991
Revised 12 March 1992
Annotated (#,##) by Don Maslin
17 Oct 1995

Date: Sun, 9 Feb 1997 00:10:04 -0800
Subject: list-tcj V1 #178

list-tcj                  Sunday, 9 February 1997      Volume 01 : Number 178

Subjects in this issue:
[list-tcj] RE: 3.5" drive formats.
[list-tcj] 1.2m floppy drives (cont.)


From: (John D. Baker)
Date: Sat, 8 Feb 97 03:12:15 -0600
Subject: [list-tcj] RE: 3.5" drive formats.

I use 1.2MB PClone-style 5.25" floppy drives VERY successfully with
systems originally designed for 8" floppy drives.

Yes, it takes a little cable-jockeying to get them hooked up
and running.  There are signal-sequence issues to contend with,
but I've made it all work just fine.

The '765-based floppy controllers no-doubt handle this better
than the WD177x and WD179x controllers, but all of my machines
use the WD179x and I've been using PClone 1.2MB drives on them
for the last 6 years.

First, short the READY* line to ground at the controller.  This may cause
some re-tries, but I've not had a problem with it.  Chances are, your
floppy drive doesn't have the option to return READY* anyway.  Even
if it does, it doesn't operate in a fashion that the WD179x FDC can use.

Second, connect HEADLOAD* to MOTOR_ON*.  The delay is almost
insignificant. (READY* must be asserted or the WD179x will
not assert HEADLOAD*.  The first hack ensures that this
condition exists.)  Most 5.25" drives do not multiplex the
MOTOR-ON signal, so all drives' motors will run.  This actually
has the advantage of speeding up disk-to-disk copies.

Third, leave pin 2 on the floppy drive unconnected or pull it
high.  This forces the drive to operate in high-density mode.
(360 RPM, R/W head tuned for high-density floppies).

Fourth, all the PClone drives I've used work very well with
the 3ms step rate that is obtained from the WD179x when using
the 8" clock rate.  If you have the option to reprogram the
step rate, some performance may be gained here.

Fifth, ALWAYS use the high-density rated floppy disks.  I never
even _considered_ trying to use ordinary double-density disks
in the drive when spoofing an 8" drive.  I did run one just
to see what would happen and as Allison noted, you can only
get single-density to work at the 8" data rate.

Given the above hacks, the problem I've been trying to tackle is
how to get 8" drives and 1.2MB drives to cooperate when attached
to the same controller.  Mainly, how to do the READY* hack when
addressing the HD5.25" drive and let READY* function normally
when addressing the 8" drive...

So far, all I've been able to come up with is to simply tie
an open-collector driver to the READY* line and pull it low when
the DSx line of the 5.25" drive is asserted (double-inversion,
typically).  I had been trying to come up with a system to
mimic the behavior of a real 8" drive, but I don't have
enough information.

John D. Baker ->A TransWarp'802'd Apple //e CardZ180 Z-System nut //
BBS:  JOHN BAKER on PIC of the Mid-Town [(281) 326-5890] 1:106/31,
Z-Node #45 [(713) 937-8886], The Vector Board [(716) 544-1863]


From: (rzh)
Date: Sat, 8 Feb 1997 12:24:57 -0500
Subject: [list-tcj] 1.2m floppy drives (cont.)

Allison ( writes:

>The 1.2m drive on a good day is a bag of worms.  I've used them at 780k on 
>kaypro and ampro systems at 300rpm.  The 8" emulation however is bogus.  The 
>8" SSSD format is fm 250kbits and the DD format is 500kbits MFM format.  
>I've tried the 1.2m drives at 8" SD and 8" DD, it works ok.  At the 8" DD 
>rate the media must be the special HD 1.2m stuff as the drive uses a 
>different write current at higher bit packing.  It would appear and 
>experience says that the 3.5" is a far better replacement as it can accept 
>most all of the data rates.  For 720/1.44 3.5" drives that's a lot closer to 
>8" emulation.  No matter which you chose to replace an 8" drive with the 8" 
>format will not work or if it does, not well.

My experience with 1.2m drives appears to run completely counter to this.
I replaced 8" drives on my Cromemco S-100 system (16FDC controller) without
making ANY changes to the O.S.  The 1.2m drives have run flawlessly in place
of the 8 inchers.  In fact, I have to be careful to remember what I did, and
reply to diskette-type prompts with 8" (NOT 5.25").  The O.S. and the applic-
ations DO NOT KNOW that they are NOT dealing with 8" drives.  This is also
true of my diskette formatting software.  What is even more curious, is that
(as many of you all know) Cromemco writes track 0 side 0 in single density
on ALL media for backward compatibility with their older hardware.  So when
I format a 5.25" 1.2m floppy as DSDD, the format program switches density,
track layout (track 0 side 0 looks like any track from a SSSD 8" diskette),
and all....thinking that the floppy drive and diskette are 8 inchers.  The
1.2m drives have no problem with any of this!  Of course, by doing this, I
am "wasting" a small part of each diskette, because the 8" drives only format
77 tracks, and the 1.2m drive and diskettes are capable of 80.  It was worth
it to me to have a hassle free replacement, and some day when I get ambitious,
I MAY go back and alter everything to utilize the drives more fully.  For now,
it will do.  There are too many other fish to fry.

>to newer and smaller floppies.  Some rewrite of the bios or patching is 
>required to correct this.  In some cases signals like motor on are not 
>available and the smaller drive will have to run the motor continously.
>Obviously is it not quite plug and go.

Again, this was not my experience with Cromemco.  All I had to do was jumper
the 1.2m drive to return "drive ready" on pin 34 (I think!), and wire pin 34
on the 34-pin header of the controller to pin 22 (?) of the 50-pin header.
I may have the pin numbers screwed up, but all this did was provide "drive
ready" from the floppy drive to "*READY" of the 8" interface.  Everything
else worked without modification.  Was I just lucky???????

On the standard format question:

>> The main argument for going to 1024 is to increase capacity and 
>> performance - if compatibility with existing formats is not an issue.  
>> The other factors you mention are of vanishingly small significance.
>That runs counter to seeking to _not_ create yet another format on yet 
>another media.  The slight increase in performance or space is not 
>significant enough to warrent it.  The second stament is a question as going 
>from one media size to another is by default incompatable.

I do NOT have a strong opinion one way or the other on the format question.
It is obvious that there are advantages to both approaches.  However, some-
thing to consider here is the issue of perpetuating a format that is
inefficient w.r.t. a newer one.  We see so much of this in the CP/M
world.  A good example is the Cromemco diskette format discussed above.
By trying to retain backward compatibility with their older hardware, they
created a "monster" that haunts us to this day.  Maintaining track 0 side 0
as single density makes this format a nightmare.  Even the very wonderful
22DISK program cannot deal with this horrible format without a special
diskette controller, because (I guess) it is not designed to ignore the
contents and format of track 0.

There is something to be said (sometimes) for "moving on" to better things,
even if it means loss of compatibility.

Just something to think about.

[Please don't take this as a "flame", Allison.  It is not.  You have helped
me with hardware design questions in the past, and I value your input on
hardware (particularly floppy hardware) VERY highly.  I can only wonder if
I got extremely lucky on this Cromemco floppy drive replacement project!]

      roger         Alexandria, VA.

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End of list-tcj V1 #178


Also check Frank Durda's excellent page on connecting an 5.25i drive to the 8i TRS-80 Model II:

How to connect 5.25 drives to your Model II/16