Jump to content

Making a 26-pin to 50-pin SCSI adapter


Recommended Posts

  • 68kMLA Supporter

My BlueSCSI has finally updated to departing regional facility after ~2 weeks of sitting in the same place about 15 minutes from my house and I am starting to think about how I am going to connect it to my Dove MacSnap SCSI adapter on my 512k.  I have a 26-pin header (f) to DB25 (f) that came with the SCSI adapter and a 50-pin header (f) to 50-pin header (f) that I took from a broken external SCSI drive.  I plan on using the BlueSCSI internally and want something that can connect it directly to the MacSnap board. I'm going to try and cobble together a 26-pin header (f) to 50-pin header (f) that will fit right into both components.  Is the image that I have below the correct pin out diagram I need to do this? The MacSnap has a 1 on one of the pins as shown in the attached image, I'm assuming that row goes 1-13 and the top row is 14-26. Is cutting the ribbon cables up and soldering the correct pins together the way to do this, or is there something better? I'm trying to use only scraps that I already have :smiley:. I'm just beginning to play with DIY electronics so any advice is appreciated.  If there's a super cheap option to do this that I've missed online, let me know.

 

Bonus question: Why would they switch to 50-pin if they just use half of them for ground anyway?

 

post-2560-0-99339000-1487302362.jpg.e12a228deec2b55ff9b67c5b7385049c.jpg1388402052_DoveMacSnapSCSICablePin1.jpg.202b39e0ff5beb6c93b111d08405e6ed.jpg

 

 

Link to post
Share on other sites
Quote

Bonus question: Why would they switch to 50-pin if they just use half of them for ground anyway?

 

Interleaving the signal lines with grounds reduces crosstalk. 

 

ATA/66 accomplished this without giving every ground wire in the cable its own pin on the board.

Link to post
Share on other sites
  • 68kMLA Supporter

Yeah, in general, the bigger the ground you have and the more it is interleaved with the signals, the better-behaved those signals are going to be with respect to one another, and often to noise as well, and the higher-frequency the signals, the more it matters.  There's a lot of stuff goes into grounding design if you want your signals to actually survive, and I don't understand any of it properly other than 'more ground usually better' :-)

Link to post
Share on other sites
  • 68kMLA Supporter
Posted (edited)

Ahhh that makes sense, thanks for the info!  I found this guide that will actually help me more. I made my own guide thats easier to read. I got my BlueSCSI today and got it all put together. Tomorrow I'm going to go to Microcenter and get some Dupont connectors and wire it up that way to test it outbfphdodhz.thumb.jpeg.d5a28a4efaa0756710d3ef43dfff4612.jpeg

Edited by Stillwell
Link to post
Share on other sites
  • 68kMLA Supporter
Posted (edited)

Here's what I ended up with. Any chance this will work or will this cable probably have too much interference?

 

I haven't got it to work yet. I had to plug in power externally for the power light to come on, but I haven't made the revision c mod for the term power yet so its not unexpected. I got a log file on the SD card saying it recognized my hdd file and loaded it after I turned it on once, but nothing was recognized in SCSI Director when I booted from floppy emu. 

 

IMG_0268.thumb.jpeg.ed4b207d600d138285c41b9aa36497d0.jpeg

 

Edited by Stillwell
Link to post
Share on other sites

Are you sure you have the BlueSCSI end connected correctly? By my understanding the entire row of (odd numbered) pins (closest to the edge of the board with the notch) are ground. There should only be 6 of these in your ribbon but I count more like 12! Unless I'm missing something this suggests you've shorted some of your signals to ground.

 

Given there are exactly 6 ground pins on the even row of pins there should be no need for any wires on the odd row, as shown in the colourful adapter diagram posted above.

 

Some other thoughts:

 

You can double check the pinout of the 26pin header by beep testing with a multimeter to the unpopulated 50p footprint next to it (note pin 1 at the opposite end).

 

Also with your dupont cables attached, but the BluePill and termination resistors removed form the BlueSCSI you should be able to beep test a 1:1 connection between the 50p connector on the BlueSCSI and the unpopulated 50p footprint on the MacSnap SCSI.

 

Is there 5V available on pin 25 of the 26p header? I see there is an unpopulated diode (D2), there could be 5V on the other side of that, in which case it may just be a matter of populating D2 with a schottky diode (but I get it if modifying vintage hardware is not part of the plan).

 

If you do get it working then I suggest picking up some 2x13 and 2x25 Dupont shells and swap the pins from the individual shells into those. I often do this with smaller looms, the pins are better supported and it allows reconnection without worrying mixing up pins.

 

I found some by searching "dupont plastic shell 50p 26p" on aliexpress.

 

1606723794_ScreenShot2021-04-20at11_18_55AM.png.b278dc45937bbcd196a2b8814f9c4ffe.png

Link to post
Share on other sites
  • 68kMLA Supporter

Yeah I had made the mistake of assuming the entire bottom row was 1-25 and the top was 26-50 in that picture. Not sure how I made that mistake when I had a diagram haha. I need to test for 5V and wouldn't mind adding a diode if it made it have power. I really want an internally powered hdd so I can just plug the computer in and use it without external drives. I was plugging the BlueSCSI into 5V power from an outlet to test. I'm definitely going to get some of those shells. Plugging in that many wires is a nightmare. I made the modification to the BlueSCSI as suggested in the development thread for a Mac Plus. Not sure if I got it right though. I'll have some time to do some testing tonight I think.

 

Thanks for all the help @mogs !

Link to post
Share on other sites
  • 68kMLA Supporter
Posted (edited)

I actually just decided to mess with it now. There's no 5V power on pin 25, but there was 5V power on the other side of the diode that you pointed out, so thats good. I hooked up the Dupont connectors correctly (I think) and still no luck with being recognized. 

Edited by Stillwell
Link to post
Share on other sites

That sounds positive (no pun intended) for populating the diode to get power from p25 on the MacSnap SCSI.

 

For the BlueSCSI Yes there should be continuity between p26 and one side of D5, what did you do for the modification? Looking at the schematic I'm guessing maybe you reversed (or jumped) D5?

 

 

Link to post
Share on other sites
  • 68kMLA Supporter

https://github.com/erichelgeson/BlueSCSI/blob/main/docs/1.0b-to-1.0c.adoc


I did that modification so that 5V power from the blue pill can be used instead of term power from the SCSI bus. I started checking the connections between the 50 pin and 26 pin header to make sure I have their arrangement correct and it seems like I do. Pin 25 on the 26 pin had no continuity with pin 26 on the 50 pin header of the MacSnap. I might just take the MacSnap out and give it a once over and test it a bit. I don't have any SCSI Device that I know works that I could plug into it to make sure its functioning properly.

Link to post
Share on other sites

Okay, that modification looks good, you should be able to test that by checking for 5V at the jumper.

 

On the MacSnap does p26 on the 50pin already have 5V on it? If so, this suggests the diode when populated is only intended as protection against external devices back powering the MacSnap.  It also suggests you could populate a nice box header in there and use a regular 50-way flat cable.

 

I'm assuming the BlueSCSI is finding the virtual drives in the log file now?

 

Link to post
Share on other sites
  • 68kMLA Supporter

Theres 5V at the jumper and the BlueSCSI reports that its finding the virtual drives and mounting them in the log file.  

 

Theres no 5V on pin 26 on the 50 pin, so the diode is necessary for power apparently. 

 

I've played around a bit with using different system versions and trying to see if it would recognize blank hdd images or system 6, 7, etc. and no luck. 

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...