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.
Making a +3 compatible FDD interface
Re: Making a +3 compatible FDD interface
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)
Re: Making a +3 compatible FDD interface
Thank you guesser!
Re: Making a +3 compatible FDD interface
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?
Re: Making a +3 compatible FDD interface
I wonder if this old page would have any useful info for your project:
http://www.worldofspectrum.org/BackToThePlus3/
http://www.worldofspectrum.org/BackToThePlus3/
Re: Making a +3 compatible FDD interface
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.
You'd also have to invent your own 4-drive floppy interface to be able to connect four PC drives.
Re: Making a +3 compatible FDD interface
Got it, [mention]Guesser[/mention], thanks!
Yeah [mention]zxbruno[/mention], 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?
Yeah [mention]zxbruno[/mention], 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?
Re: Making a +3 compatible FDD interface
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
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
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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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: Making a +3 compatible FDD interface
Many thanks to all who replied.