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Provisions for the B: drive has been provided from the first day the +3 was born. Except for a (minor ?) error, in that the drive’s connector pinout description from the manual is reverse numbered, there is no major problem for connecting an external 3.5” drive.
Yes, the power supply may be a problem, but I have seen a few external 3.5” drives powered from the +3’s original power supply and ... no problem so far.

That was for the B: drive. But how about a 3.5” floppy disk drive as the A: drive ?

I have done this to a few +3s, including my own original. This example shows my original +3 with an internal 3.5” drive mounted in it.

However, before going to change the internal original 180K 3” drive with a 720K 3.5” drive, you should be aware of the following related problems:

  • you agree with the fact that you can no longer use original protected software on Amstrad CF-2 floppy disk(s), or, if you really need it, any software stored on CF-2 floppies must be first copied to 3.5” DSDD floppies; there are a few alternatives:
    • for one-side protected 3” disk, you must copy the entire floppy side to a blank 3.5” floppy
    • for two-sided protected 3” disk, you must copy each entire side to separate blank 3.5” floppies (two 3.5” blank disks required);
      both situations may be solved by using a dedicated copy program that can handle protected disks; however, for the purpose of this technical note, these days a better alternative might be to make a disk image on PC (or use an existing one) and then use that image to create back an “original” content on a 3.5” floppy
    • you can hack the disk boot loader and make all the software +3DOS compatible (i.e. no longer protected); that way the program will load from either logical drive like any “normal” software (I have done this already, a few are listed on the +3 Software page; e‑mail me directly if you are looking for help regarding some particular original protected CF-2 floppy disk – my e‑mail address can be found at the bottom of any of my web pages)
  • Although the original Amstrad 3” disk drive may be connected as the external B: drive (with a few modifications) and you can use CF-2 disks with it, protected disks will most likely not load, because of the following limitations:
    • they have a boot sector that can be loaded either via +3’s Loader menu, or the +3 Basic LOAD"*" command; the problem is that the +3 can only load a boot sector from the A: drive
    • once a boot sector is loaded into memory, the program execution is passed to that code; the problem is that the boot sector routines are using direct µPD768 floppy disk controller commands (bypassing the +3DOS ROM) and are considering the current drive as drive0 (µPD768 sees the drives as physical devices; physical drive0 is the logical drive A: and physical drive1 is the logical drive B:)
  • Note: 720K DSDD floppy disks may be used as 180K disks with only one side available; this seems to be a stupid idea, but for some, this may be the only chance for copying and running protected software.
  • DKIT720K screenshot
    DKIT720K, a +3DOS based disk utility;
    the "Duplicate" command is not yet implemented
  • The CP/M Plus for +3 can be copied to any other disk format (it is not protected), however, the original CP/M package has no provisions for making a bootable 720K disk. You will need my DKIT720K utility in order to make CP/M bootable disks. Supported sizes for CP/M bootable format are 180K, 720K and 800K. Although the source code for this utility is available in HiSoft GEN80 for +3 format, because at the time of this writing the “Duplicate” command is not yet implemented and the “Format” command is not complete (everything is functional except the “Mask bad sectors” option), I am not making available for download the compiled program. At present, you still have to e‑mail me directly in order to get (freely) the compiled DKIT720K utility.
  • !! the power supply connector of the original Amstrad 3” disk drive has the +5V and +12V inverted in respect to any 3.5” disk drive; the red and yellow (orange in some cases) wires from this connector must be reversed on the connector side before connecting to any 3.5” disk drive !!
  • the 26 pin connector must be changed with a 34 connector (see below, electrical considerations paragraph)
  • the +3 Basic FORMAT command cannot format 720K floppy disks, neither A: or B:; the DKIT720K utility (or any equivalent program) will be required

Mechanical considerations

As you might expect, the (new) 3.5” disk drive has different positions for the mounting holes in respect with the original drive. Fortunately, the modification required is quite easy to be done.

Spacer
Hexagonal nickel plated brass spacer, with M3 tapped hole and M3 threated stud

There are four spacers needed, as seen in the picture here on the right; two of them must have 15mm in lenght, the others two 20mm in lenght.

Look at the bottom side of the 3.5” drive; there are four M3 tapped holes at standardized distance each other (94mm on the width axis, 70mm on the depth axis) . The spacers will screw up into these holes, the 15mm ones on the left side (direction: center of the +3), the 20mm ones on the right side (direction: +3 border).

 

Front view floppy disk drive mounting dimensions
Front view of drive positioning and spacers dimensions

The 15mm left-front spacer is fixed into a hole that was previously used by the original 3” drive. The rest three mounting holes must be given with reference to this existing one.
This example may give an idea about how a real drive is mounted.

 

Electrical considerations

First thing, already mentioned before: although the power supply connector is mechanically the same for both 3” and 3.5” drives, !! the +5V and +12V (the outermost pins) must be reversed before plugging into the 3.5” drive !!

Fortunately, this can be easy done, by hacking the power connector with a help of a needle and reverting the two outermost pins.

The 26 ribbon cable is a more delicate issue. The original 26-way cable mounting socket cannot be directly connected to the 3.5” drive. You must build a new cable with a 34-way cable mounting socket.

The new connector should look like in this example and the final result should result as shown in this example . An alternate crimping solution is shown in this example ; this alternate solution is usually the most reliable, but the limited space may force the first solution shown (it merely depends on the physical floppy model).

For all this to happen, you have two possibilities:

  1. make a short adapter, that consists of one 26-way cable mounting plug, a short 26-way ribbon cable and a 34-way cable mounting socket; see below table 1 for pins correspondence;
    this solution has a few problems:
    • cable mounting plugs are difficult to find; alternatively, you can use a normal 26-pin header connector and solder the ribbon cable to it; however, the ribbon cable is a crimping model and is usually difficult to solder
    • the resulting assembly may be not fully isolated
    • the resulting assembly may be too large for the available space; special care should be taken if the 3.5” drive model used has its motor drive open (not mechanically protected under the drive)
  2. replace the original 26-way cable mounting socket with a 34-way cable mounting socket; see below table 1 for pins correspondence;
    this solution has a major problem:
    • the original 26-way ribbon cable may be too short and, most likely, it must be replaced; the problem lies on the printed circuit board side: you have to unmount the original 26-way PCB-type socket and then re-crimp the new ribbon cable on it

Note: take care, a new 26-way PCB connector may be hard to find; you should pay attention when handling it !
Just in case you broke the PCB connector, there is a solution to replace it: change it with a (boxed) 26-way header; the 26-way ribbon cable must then have a 26-way cable mounting socket on it. It is a bit complicated, but it works and it is not bad at all !


Table 1: 26-way to 34-way floppy disk drive connection correspondence

26-way
ribbon pins:
34-way
cable socket pins:
signal
1 7 GND
2 8 n/INDEX
3 9 GND
4 10 n/DRIVE SELECT 0
5 11 GND
6 12 n/DRIVE SELECT 1
7 15 GND
8 16 n/MOTOR ON
9 17 GND
10 18 n/DIRECTION SELECT
11 19 GND
12 20 n/STEP
13 21 GND
14 22 n/WRITE DATA
15 23 GND
16 24 n/WRITE GATE
17 25 GND
18 26 n/TRACK 0
19 27 GND
20 28 n/WRITE PROTECT
21 29 GND
22 30 n/READ DATA
23 31 GND
24 32 n/SIDE SELECT
25 33 GND
26 34 n/READY

Floppy drive considerations

Modern floppy disk drives are, in general, not compatible with the +3. For a drive to work with the +3, it has to be also compatible with the old PC-XT computers.

PC-XT computers allowed 4 floppy disk drives. The XT drive connector has 4 “disk select” signals and 1 “motor on” command signal which is common for all drives. When the motor signal becomes active, the motors from all present drives are turned on, regardless of whether a particular drive will be used or not.

On the other hand, the PC-AT computers only allows 2 floppy disk drives. The AT drive connector has separate “disk select” and “motor on” command signals for each of the two drives. The “ready” signal has been replaced by “disk change” signal.

See below table 2 for XT/AT drive connector function differences. Please note that the +3 uses pins 10, 12, 16 and 34 of the drive, functionally similar to the PC-XT connector.


Table 2: PC-XT/PC-AT  floppy disk drive connector function differences (only the relevant pins shown; all other pins have the same function on both)

cable pin: PC-XT function: PC-AT function:
2 N.C. or LOW CURRENT N.C. or DENSITY SELECT
6 n/DRIVE SELECT 3 N.C.
10 n/DRIVE SELECT 0 n/MOTOR ON DRIVE 0
12 n/DRIVE SELECT 1 n/DRIVE SELECT 1
14 n/DRIVE SELECT 2 n/DRIVE SELECT 0
16 n/MOTOR ON n/MOTOR ON DRIVE 1
34 n/READY n/DISK CHANGE

For +3 floppy disk drive implementation, either internal (A:) or external (B:), you have to find an XT-only drive, or a drive that has XT<->AT switching capabilities. Most likely, there are no 3.5” XT-only drives, but chances are, you can find a (new) one that has XT<->AT switching capabilities.

The switching between the two categories is made by means of jumpers, DIP switches, or 0 ohm SMD bridges.

Below is an example of a few drives that are +3 compatible. For all drives in this example, the drive select should be DS0 for drive A: (the internal one) or DS1 for drive B: (the external one).

Chinon FZ-354 or FZ-357
has a jumper block
- jumper for motor driving, two positions, can be MD or MM
- jumper for the ready signal, two positions, can be RDY or DC
- jumper for drive select, four positions, can be DS0, or DS1, or DS2, or DS3
For +3 use, the jumpers have to be set to MM and RDY.

Panasonic JU-257-203P
has DIP switches
- switch for motor driving, two positions, labelled MO or MS
- switch for the ready signal, two positions, labelled RY or DC
- switch for drive select, four positions, labelled DS0, or DS1, or DS2, or DS3
For +3 use, the switches have to be set to MO and RY.

Panasonic  JU-257A-606P
Samsung (not sure about exact model, presumably SFD-321B)
have 0ohm SMD resistors that have to be (re)soldered
- 0ohm SMD resistor for motor driving, two positions, can be MO or MS
- 0ohm SMD resistor for the ready signal, two positions, can be RY or DC
- 0ohm SMD resistor for drive select, two positions, can be DS0 or DS1
For +3 use, the 0ohm SMD resistors have to be set to MO and RY.

If a floppy disk drive connected to a +3 has the “ready” selection left on DC, the result is that the drive will not work at all. If the floppy disk is not inserted, the computer waits forever for the floppy to say something.
Floppy disk drives that are usual in today’s PCs have no more the “ready” and “motor” jumpers, more likely not even the “disk select” jumpers, they are PC-AT locked and DS1 locked. These drives cannot be used on +3 at all.

Note: a search on the internet for Amiga or Amstrad CPC6128 drive replacement might offer a compromise solution for using modern (PC-AT) floppy disk drives, that may be applied also on +3. The solution involves forcing the nREADY signal to GND, thus fooling the system with regard to the real drive ready status; the drawback is that some +3DOS drive commands might give impredictible results in case the drive is actually not ready for an otherwise mandatory ready requirement (for example when trying to write some data on disk).

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