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Old 10 December 2020, 03:24   #44
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Join Date: Jun 2018
Location: Gothenburg/Sweden
Posts: 17
Some general things re the various turns this thread has taken:

The circuit on a regular external drive for an Amiga does two things:
A) Identifies to the Amiga that there is a drive connected, and if it a 40 or 80 track.
B) Latch the motor signal so the Amiga can turn on and off the drive motors individually without having a separate motor wire for each drive

As the interface does nothing else every wire except for the motor signal should be connected directly between the actual drive and the 23-pin connector(s). This makes it easy to measure or visually inspect the pcb and the connectors to tell which pin is which.

For testing purposes you can disconnect the motor signal to the drive and connect the motor pin on the drive to ground. The motor should spin all the time (or in many cases all the ime when a disk is inserted). If your drive works fine with the motor constantly running but won't work with the regular motor wire connected, then there is something wrong with the motor latch circuit. If your drive won't work even with the motor signal grounded, there is something wrong with the pinout, with the drive itself or possible termination of the signals. For a 3.5" drive termination is unlikely to be an issue. However if you connect an old 5.25" drive, like say the huge Tandon drives found in the original IBM PC and PC/XT, you need to replace the 150 ohm termination resistor packs with higher resistance ones (say 1-2k, can be built using a "component adapter" where you solder resitors, or you can push individual resistors in the socket for the terminator pack).

If you need a replacement interface pcb for an external drive, the german language book Amiga Hardware Tuning has a DIY solution using two readily available 74xx chips. Although the book is in german, the schematics and the pcb layout should be usable even if you can't read german.

Btw if anyone decides to order new PCBs based on the Amiga Hardware Tuning book, please please consider changing the layout to support two drives. Only half of each of the 74xx chips are in use, so the chips can already handle two drives but the traces of the PCB is missing. Back in the days three decades ago I used one of those interface PCBs and added a bunch of bodge wires to be able to use it with two drives. IIRC I wired the 34-pin PCB mounted connector the way it is wired on a PC floppy controller, with a twisted flat cable for two separate motor signals (and of course two separate drive select signals)..

Another advice: If you must feed the drive and the computer from different power supplies, make shure the mains power ground of both power supplies are connected to each other. In some countries this statement doesn't make any sense since every socket has been grounded since many years. However in other countries you can still find ungrounded sockets. For example here in Sweden the regulation requiring grounded sockets everywere came into effect in the mid 90's so we still have a lot of homes with ungrounded sockets. Just use a grounded power strip for all your computer stuff.

The reason for the grounding issue is that the mains filer on many switched power supplies leak a small current from the mains to the low voltage side. The current is small enough to not harm humans, but if you connect your drive to your amiga and the signal wires make contact before any of the ground wires this small leakage current can damage the chips in the drive, the drive interface and in the Amiga. This migh be an explanation of why one of the Amigas early on in this thread did display DF1:BAD DF2:BAD and DF3:BAD.
MiaM is offline  
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