Ways to implement a selectable 3rd drive
#1
I am working on an enclosure for my SCP and one thing I want to do is have a means to easily connect an external drive. I have found the SCP makes for a nice way to test drives when repairing them.

Thinking of the various ways to do this the most straightforward would be to mount two IDC type, DB37 connectors on the case breaking the path of the cable to Drive1. A simple loop back cable would normally be in place to use Drive1 and when removed you could plug in the external drive. This is simple but takes up a bit of panel space for two large DB37 connectors. I suppose a small PCB holding two boxed male pin headers could be used instead of the DB37s.

Another option would be to just intercept the Drive1 select line and use a 74LS153 to toggle where it is routed, to internal or external drive. Looking at the SCP PCB (a picture only as I am at work) it looks like Huey and Lewy are buffering the signals to/from the bus so this would probably work OK. It would require a PCB, some 34pin headers, but only one external DB37. Thinking about it now two of the free pins on the DB37 could be used to sense when an external drive is connected so you would not need a select switch. is there any electrical downside to this I have not considered?

Thanks!
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#2
You can't drive more than 2 sets of bus signals at the same time. So, even having the drive select high (unselected) for a 3rd (or 4th) drive it is still going to be a problem for the bus. It's balanced literally for two disk drives only. You would need to disconnect ALL of the lines on the bus, or at least the RD_DATA, WR_DATA, WR_GATE... probably STEP and DIR as well. I don't see any good solution. When I designed SuperCard Pro I looked for an option to allow up to 4 disk drives to work and the only way was to add latches/buffers for each extra drive. That was too much hardware, so I opted to support just two drives.
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#3
Thanks. I will go with a pair of 34 position shrouded pin headers poking out the back panel with a very short cable between them for normal operation. OR, maybe I really just need one male shrouded pin header with the female connector/ribbon poking out above it. Plugged in for normal operation or unplugged to use an external drive. I will still need an external power plug, but that is easy.
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#4
You could go with a single 37 pin output connector wired from inside your case, and use the extra three pins tied to +5v, +12v, and ground.  On the external drive side you would use a 37 pin connector wired to a 34 pin ribbon cable, along with the power wires (molex/berg connectors?) to the drive.  This would allow for 5.25" and 3.5" drives to be used.
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#5
(05-21-2020, 04:49 PM)admin Wrote: You could go with a single 37 pin output connector wired from inside your case, and use the extra three pins tied to +5v, +12v, and ground.  On the external drive side you would use a 37 pin connector wired to a 34 pin ribbon cable, along with the power wires (molex/berg connectors?) to the drive.  This would allow for 5.25" and 3.5" drives to be used.


 Ha! Great minds think alike. That is what I laid out last night in Fusion. I also want to add a USB panel mount B-to-mini in the square power switch filler section. I could use the rounded fuse filler for a power connector.

I thought about using the extra 3 wires in the DB37 for power. I was a bit worried about the voltage drop across the 28AWG ribbon cable. Looking at the actual resistance per foot now though even with a 1A current draw there would be less than 0.2V drop over three feet. I searched quite a bit yesterday for a good reference as to how much current at 5V and 12V a single 5-1/4" drive might draw but was not able to find anything authoritative. The 3-1/2" drive draw is nothing to worry about.


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#6
For a 5.25" disk drive you can expect to need 350mA for the 5v side and about 1A for the 12v side. These are the peak currently, and the 12v side will vary depending on how tight the disk is in the sleeve. What you could do is use 2 of the 3 extra wire for 12v. The 3rd wire is 5v. There are numerous grounds on the 34 pin ribbon cable, so you don't actually need an extra ground.
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#7
Thanks for the info about current draw, that helps. After looking through my parts bins I found I have some panel mount 8-pin DIN connectors (C64 power connector). I was able to sneak one in the filler for the fuse and conveniently the hole left by the top of the power switch is the perfect size for a USB-B connector. 

I guess now this leaves the extra three pins on the DB37 free. I get really motivated to make a small PCB to switch between and internal and external drive via a wire loop on the external FDD harness acting like a switch.

Thanks for your help!


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