External Floppy Issue

Hardware topics for all 68000 Ataris (260/520/1040ST(FM), Mega ST, 520STE/1040STE, MegaSTE, STBook and Stacy)
Post Reply
Posts: 2
Joined: 13 Nov 2023 05:04

External Floppy Issue

Post by jdunlap »

Hello all!

I recently acquired an Atari STf from Ebay. I have installed a Gotek internally and am looking to use a PC floppy drive externally. I have an adapter for the 14 pin DIN to a floppy cable, and the drive is set up properly as when I run the tests on the test cart for Drive B:, the external drive spins up. However, when I try to run the Quick Test or the Disk Exerciser, I used to get a Write Protect Error. I have since applied this fix that I found over on Exxos. Now, I do not get the Write Protect Error, but I do get a bunch of bad sectors. When trying to format through GEM, I just get an error that the drive is not responding.

I have tried several different drives and have tested the floppy on a PC, so these should not be the issue. I am using a Panasonic JU-256A216P drive.

Anyone have any ideas?
User avatar
Site Admin
Posts: 41
Joined: 03 Jan 2016 17:23

Re: External Floppy Issue

Post by Administrator »

Have you tried a 'real' DD (aka 720KB) DD FDD as external drive?

Some people use HD-Disks, close the HD-Detection hole and use them in DD mode. This is not recommended!

The "fix" might be needed if there are either too big pull-ups on the drive or no pull-ups at all. Did you check that? The "fix" is never necessary if you use a 74F04 (PP outputs) instead of the 7406 (OC outputs). Just a side-note.

Other than that I cannot help - I'm not into FDDs that much.

Did some measurements and checked the pull-ups on different FDDs on pins 20 and 22 of the Shugart Bus.
  • Epson SMD-380 - no pull-up
  • Sony MP-F11W-5DU - 1K
  • unknown Chinon and TEAC 235HF - 1K

If you look at the signals STEP and WRITE_DATA coming from the 7406 you will detect a 47R resistor in series.
I don't know how the OC output can output reliable high signal without pull-up but with a 1K pull up this will happen:

Code: Select all

                                       VCC              VCC
                                        +                +
                                        |                |

                                        |                |
                                       .-.              .-.
                                       | |              | |
                                       | |  1K          | |  1K
                                       '-'              '-'
      7406                              |                |
                     47R                |                |
        |\            ___               |                |
       -| >O---------|___|--------------+----------------+
                                       FDD #1          FDD #2

If only FDD #1 is connected then the voltage drop over the 47R resistor is around 0.22V but with two drives (and each having a 1K pull-up) this increases to 0.43V. Still enough for a general TTL input circuit but maybe the falling flank isn't steep enough to be low at the desired point-in-time.
I had a look at a Hypertext which describes common errors on Ataris. It does state this problem - but with HD-Circuits. And recommends a 74F04 to replace the 7406 and decrease the 47 Ohms to 10 Ohms, but this will increase reflections on the other hand. So my best guess is: you have to try & error yourself.
But first I would check with a scope if these signals with a 47 Ohm resistor can get close to GND or not.
Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest