Making a +3 compatible FDD interface

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tsm
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Making a +3 compatible FDD interface

Post by tsm » Sat Jan 19, 2019 6:14 pm

Hello
I have a 128 +2B so I figured I could replicate the +3 FDD interface exactly, provided I found the parts needed. The idea is to make a detachable cartridge rather than hacking my Speccy.

Problem is, not all necessary signals are available at the expansion connector: I need the NRS signal from the gate array, but I only have NRST, which comes after the RC circuit R57 / C27. Here is the schematic.

How can the circuit be changed to accomodate for this?

I tried to understand how the reset signals are used throughout the circuit, and it seems like the designer devised a way to get the devices ready at different times, that is, keep both drives unselected while the '765 resets, then let the controller take over them, then wake up the rest of the computer. Just guessing, though, as I'm no expert.
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Re: Making a +3 compatible FDD interface

Post by Guesser » Sat Jan 19, 2019 10:05 pm

other people have already done this, so it must be possible. I'd be inclined to just use the filtered NRST and see if it works. (or use RESET to avoid having to invert it)
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Re: Making a +3 compatible FDD interface

Post by tsm » Sun Jan 20, 2019 12:19 am

Thank you guesser!
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Re: Making a +3 compatible FDD interface

Post by tsm » Sat Jan 26, 2019 4:29 pm

By the way... I see that the controller IC has hardware provision for supporting up to 4 drives. Is this allowed by the computer's architecture and ROM, or are 2 drives the maximum possible?
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Re: Making a +3 compatible FDD interface

Post by zxbruno » Sat Jan 26, 2019 7:14 pm

I wonder if this old page would have any useful info for your project:

http://www.worldofspectrum.org/BackToThePlus3/
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Re: Making a +3 compatible FDD interface

Post by Guesser » Sat Jan 26, 2019 10:30 pm

Nope, the ROM has no way of using 4 drives, and I doubt there'd be much point patching that ability in other than "because you can".
You'd also have to invent your own 4-drive floppy interface to be able to connect four PC drives.
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Re: Making a +3 compatible FDD interface

Post by tsm » Sat Jan 26, 2019 10:49 pm

Got it, @Guesser, thanks!
Yeah @zxbruno, I also bought some Tesla PROMs, then I realized that the integrated data separator can be had and its usage greatly simplifies the circuit.

Can anyone confirm that R76 is there to keep the drives unselected during reset?
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Re: Making a +3 compatible FDD interface

Post by Guesser » Sun Jan 27, 2019 12:24 pm

tsm wrote:
Sat Jan 26, 2019 10:49 pm
Got it, Guesser, thanks!
Yeah zxbruno, I also bought some Tesla PROMs, then I realized that the integrated data separator can be had and its usage greatly simplifies the circuit.

Can anyone confirm that R76 is there to keep the drives unselected during reset?
I don't know what R76 was intended to do, but as far as I can figure out it does nothing at all and IC15f and N_DRIVE_1 will always see the output of IC18a.
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Re: Making a +3 compatible FDD interface

Post by 1024MAK » Sun Jan 27, 2019 1:50 pm

I would say that it is best to ignore R76. It does not provide any useful function at all. IC18a will ALWAYS overpower the very weak current from R76. So R76 could be removed from all the +3 / +3B machines in the world and nobody would notice any difference in use whatsoever.

I would also not worry about what happens during reset or power on, as long as the correct logical reset signal is used it should be fine. The delay between the different reset signals is not of any consequence.

Mark
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Re: Making a +3 compatible FDD interface

Post by tsm » Sun Jan 27, 2019 6:25 pm

Many thanks to all who replied.
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