5.25"/8" FDD pinouts (from CPM-L digest)



8"      description     pc floppy

2       side select     32
10      seek complete   n/c
12      restore         "
14      eject           "
20      index           8
22      ready           34
24      motor on        16
26      ds1(drv select) 10
28      ds2             12
30      ds3             14
32      ds4             6
34      step dir        18
36      step pulse      20
38      write data      22
40      write gate      24
42      track 0         26
44      write protect   28
46      read data       30

all other even pins are not connected
all odd pins are earth/ground

With some 'wire weaving' in the ribbon cable - going from 50 to 34 - it is quite possible to substitute a 5.25" high density drive for an 8". That is what they were originally designed for, to be a functional replacement for the 8" DSDD.

********************************************************************

  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

Notes:

* - 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


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