How does the 2nd drive connector work (J5)?

[Tim Radde]

I'd like to use a 2nd disk, but am not sure how this works.  How would the second drive even know it's been selected? The 20 pin cable is the data cable.  I am sure I am just missing something to connect the dots.

[David Gesswein]

The 34 pin cable handles both drives. Most systems daisy chain the drives on the same cable so it works. If you have an unusual system that uses two 34 pin cables that will cause complications.

There is a separate jumper for each emulated drive to set which select line it should use.

[Tim Radde]

This is for a DEC Pro-350.  I believe I have read that the controller can support 2 drives.  The cable has two 34 pins and two 20 pin connectors.  I was going to try to use a real drive as the

2nd drive because some of the installs I am trying require 2 drives. Don't know if this will work or not.  Right now I have BSD 2.9 on it but there is not much extra space.  The install for

this for the PRO only recognizes RD50s and RD51s (5MB and 10MB).  Yes, I see that on xhomer they have a way to patch in other drives.  Not sure how that works with the actual

drive.  It generates a patch boot diskette image.  And a patch unix, but I don't know how I then get that image to the disk.  Documentation is scarce for this system.

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[David Gesswein]

Tried to look at schematics to see what they do. Looks like need to find
mp-01483-00 rcx50 which didn't seem to be in the print sets.

Some of the DEC controllers that use two cables the signals need are on both
some aren't and you would need to make modifications.

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[Tim Radde]

These are two cables (34 pin and 20 pin) both with two connectors for drives.  I do not know if the controller can support two drives.  I have read somewhere that it can.

Also know the previous owner hooked up a switch to the power plugs so he could power one or the other drive.   P/OS was on one and RT-11 was on the other. Never

could get the P/OS drive to boot so it may be faulty.  These are the old MFM drives DEC used back then.

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[David Gesswein]

That setup would work for switching between drives. The 20 pin cable will be
double terminated but should still work ok. That would be unusual configuration
for connecting two drives. If the pro does support two drives and will generate
two selects you could make a 20 pin cable that can connect to both connectors.
Or modify the code to use just the one.

If your just wanting to select between two images like selecting one of the two
drives you can connect a switch to J7 connector and modify startup scripts
to use the switch to select image.

Did you try reading with the emulator the P/OS drive to see if it has errors?

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[Tim Radde]

I think I have a way to use a larger image rather than just an RD50 or 51. Gonna make an RD53 of 70M and go from there. Xhomer has instructions on how to do this and conversion scripts to change the code on the boot disk and the kernel code to use a larger disk. Appears BSD for the Pro was limited to only those 2 disk types. I did not try a read on the Pro disk.

Sent from my iPhone

> On Oct 23, 2025, at 7:39 PM, David Gesswein <d...@pdp8online.com> wrote:
>
> That setup would work for switching between drives. The 20 pin cable will be

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