Floppy Replacement

You can still find some spare parts on eBay, but prepare to pay a lot of money. Known compatible floppy drive models are:

Mitsubishi MF355C-599MA
P/N: 1057103
P/N: 72X6068
P/N: 64F0162
P/N: 1619618

Mitsubishi MF355C-599MQ1
P/N: 1619636

Floppy drives for PS/2 models 25 (286), 30 (286), 55 SX, 60, 65, 80, 90, 85, 95, and possibly others, should also be compatible.

Some say that you can even install later PS/2’s 2.88M drives. Surprisingly enough the PS/1 2011 has a Intel 82077AA controller which supports perpendicular recording, so maybe it’s worth a try…

How to replace the custom floppy drive with a standard one

Author: paperinik77pk

This guide is specific to the PS/1 type 2011. It should be the same for the PS/1 2121 (and the PS/2 line as well) but I have no means to verify.


Like many IBM computers of the same era, the PS/1 also suffered from the IBM policy to be “a standard on its own”, therefore using some parts specifically designed for this machine. Floppy disk drive and hard disk drive are a typical examples.

Back in the day, the failure of an IBM part was easy to solve, since you simply had to pay a lot of money and get a new part from IBM.

But today it’s a little more complex to find replacement parts for PS/1 systems since many of these computers were thrown away when obsolete or did not survive. And don’t even try to ask to to IBM, obviously.

So the solution could be installing a modern floppy drive and reworking it a bit to made it compatible with the proprietary PS/1 floppy connector.

3.5" floppy drive


This guide was created with the help of information retrieved from this site (IBM PS/1 Technical Reference Manual) and on the VINTAGE-COMPUTER forum. The discussion started some years ago and was referred to the IBM PS/2 Model 30, but it fits perfectly the PS/1 as well.

When a floppy drive breaks down

The error I got when my Alps drive broke down was “sector 0 not found” at any attempt to read/write on the floppy disk. This probably is caused by heads alignment, dirt, failing capacitors, or whatever.

Since I did not want to pay a lot of money for an original spare drive, I tried to understand how to connect a cheap, modern (1.44mb 3.5”) floppy disk drive to the PS/1.

Standard PC Floppy vs PS/1 Floppy Pinout Comparison

So, let’s compare the PS/1 pinout to standard PC floppy connector pinout:

1 GND GND 2 Density Select 1=Low/0=High Density
3 GND 5v+ 4 Reserved Drive Type 1
5 GND GND 6 Reserved 12V+
7 GND GND 8 0=Index Index
9 GND GND 10 0=Motor Enable Drive 0 Open
11 GND GND 12 Drive Select 1 Drive Select
13 GND GND 14 Drive Select 0 Open
15 GND GND 16 0=Motor Enable Drive 1 Motor Enable
17 GND GND 18 0=Direction Select Direction
19 GND GND 20 0=Head Step Step
21 GND GND 22 Write Data Write Data
23 GND GND 24 Floppy Write Enable, 0=Write Gate Write Enable
25 GND GND 26 0=Track 00 Track0
27 GND GND 28 0=Write Protect Write Protect
29 GND GND 30 Read Data Read Data
31 GND GND 32 0=Head Select Head Select 1
33 GND GND 34 1=Disk Change/0=Ready Disk Change

The PS/1 floppy drive has no power plug connection, since +12v and +5v are connected using the 34-pin floppy connector.

For this reason, NEVER CONNECT A STANDARD, UNMODIFIED FLOPPY DRIVE TO PS/1 CONNECTOR. You will damage it, since there’s a lot of electricity where it shouldn’t be any…and the drive’s logic board won’t be happy.

Looking at the above comparison table, at a first glance it’s possible to find the power pins: #3 and #6.

Also the pin #4 has a specific role, but we’ll talk about it later.

Apart from the power pins, the other PS/1 pins seem to be the same as the standard floppy connector.

Powering the floppy is not a problem if you have the serial expansion board, which is installed in international models (can anyone confirm that also the modem board installed on US models has a power connector?). There is a standard floppy connector on it, so we just need to create a small power cable from scratch.

If you don’t have the serial/modem expansion board, it is still possible to get power. You’ll have to reroute pin #3 and pin #6 outputs to a standard floppy disk power connector and do some soldering.

Preparing the drive

This procedure allows to get the new drive ready for installation.

  1. First, remove the plastic IBM sled from the broken drive, keep the bolts.
  2. Get a new drive, (1.44mb 3.5”) and simply remove (cut away) from the rear 34-pin connector pins #3, #4, and #6 using a small plier (just turn them right/left a bit, they’ll come off easily)
  3. Carefully remove the front plastic cover and the eject button from the new drive using a small flat screwdriver to unlock the locking plastic lids.
  4. Bolt the IBM sled to the new drive.
  5. Time to get some cables: cut two female floppy disk power connectors leaving 6/7cm of wires attached to them.
  6. Usually the +5v wire is the red one, the yellow wire is +12v, and the two remaining black wires are GROUND.
  7. Now connect the wires and solder them (red with red, yellow with yellow, black with black) and you’ll get a brand new, two-headed female floppy power connector.

Floppy connector

I was lucky, since I got from an old AMIGA 500 an already working two-headed female-female connector that looks very nice.

Installing the drive

Simply put it in the PS/1, connect the 34-pin connector and the two-headed power cable on serial expansion board. Do not invert the cables.

Verify the drive installation on the PS/1

Turn on the PS/1, it should present the error 601 and 162. Press F1 and go on.

Run CONFIGUR.EXE and look at the floppy drive.

Does it show as 5.25”, 1.2Mb? Well, it is CORRECT, at least for the moment.

Just know this strange behavior is BECAUSE OF PIN #4.

Incorrect floppy detected

Pin #4 and “DRIVE TYPE 1”

Remember the table shown above? The pin #4 on PS/1 is described as Drive Type 1 and according to the Technical Reference Manual, it is output only.

This means that specific pin is used by PS/1 logic board to understand what kind of floppy drive is connected to the system.

Original 3.5” original floppy drives were designed to send some calculated output to the logic board, in order to determine the 720k or 1.44mb format. The bigger 1.2mb 5.25” did not send any output on that specific pin, so the PS/1 bios is “educated” to recognize it immediately.

Our new, freshly installed floppy drive has no pin #4 (we cut it), therefore does not send any output. The logic board understands there’s NO OUTPUT ON PIN #4 and recognizes our drive as 5.25.

New floppy drives do not send any output on PIN #4, therefore it’s practically useless. Even without pin #4 cutting, the drive would be recognized as 5.25” because it was not designed by default to “describe himself” to the BIOS.

Dealing with pin #4

Now the interesting part.

Floppy power and pin #4

  1. Get a small electric wire (5cm). Use a thick cable in order to prevent bad contacts, resulting in errors and misconfigurations (I experienced this in the past, working on this modification)
  2. Peel it on both sides.
  3. Put one end on the floppy power connector inside one of the black ground cables. Solder it if needed.
  4. Put the other end inside PIN #4 of the connector. Then, attach the connector to the floppy. The connector itself will block the wire in its correct position.
  5. Turn on the PS/1.

Error 601 and 162 should appear and drive should do a seek.

Press F1 and go on.

Run CONFIGUR.EXE. If everything is done properly, THE DRIVE WILL FINALLY SHOW AS 1.44MB 3.5”.

Correct floppy detected

Test the drive a bit

Close the CONFIGUR.EXE program saving the shown configuration, reboot the PS/1 and do the following:

  1. Insert a floppy in the drive
  2. Go in Dos mode
  3. Format it using the command FORMAT A: /F:1.44
  4. Wait for the end of the format, then use the command SYS A: to create a boot disk
  5. Wait the boot disk preparation
  6. Copy something already on PS/1 hard drive on the floppy, a small game, configure.exe and customiz.exe, what you like the best … something that can be run directly from floppy.
  7. Eject the floppy, reboot again
  8. Insert the floppy, try to run the game or what you copied on it from the floppy.
  9. Make some test copies from/to the drive.
  10. Try to boot from floppy: it is a very important test to do!

If everything works, well… you’re done.

The final touch

You have to adapt the original blue 1.44 eject button to the drive. You can glue it, modify the metal eject lid on the drive to fit the button… there are a lot of solutions. Be creative (you have to, since there are a lot of types of eject buttons on floppies)!