PDP-11 Controller needed to use a couple of RX33 drive

[Johnny Billquist]

tiggerlasv at aim.com wrote:

> I'm not certain of the mechanics involved, but,
> you shouldn't be able to trash the format of either drive,
> unless the Drive Select jumpers are set incorrectly.

There are no drive select jumpers. That could, of course, be the 
problem. :-)

> The only thing that might have an impact on this would be
> if you try it in a BA23 with an un-modified 4-button front panel.

This was definitely in a 4-butten front panel BA23. However, I'm not 
sure the front panel is the only place to blame.

> *Maybe* the fact that the Ready/Write Protect lines
> for the 2nd drive are left floating, is somehow confusing
> the RQDXn controller, but that seems doubtful.

The RQDX wasn't confused. It was the drives... When the RQDX started 
writing to #0, the write circuitry of #1 also went active. Or if it was 
writing to #1 which also activated the circuitry of #0.

> Those lines merely report status of the 2nd drive.
> The Drive Select and Head Select lines don't go
> anywhere near the front panel, and there are
> separate signal lines for drives 0, 1, 2, and 3.

Exactly. Which is why I wonder if the front panel really is to blame for 
the problem.

>>From a modification standpoint, there are NO CHANGES
> required to the disk drive distribution panel; all of the
> signal lines are there, and ready to use for 2 drives.
> 
> DEC sold the "upgrade" kit for 2 hard drives;
> it's called the BA23-UC.   It consists only of a 6-button panel.
> This is the same 6-button panel that was shipped with
> later models of the MicroVax, which clearly supported
> 2 internal RD-series disk drives.
> 
> All the 6-button panels did was to add the extra
> ready/write protect buttons.   There is no extra logic involved.
> 
> The Write Protect LIGHTS for the RX50's were deleted,
> as the newer RQDX3 controller doesn't provide any outputs
> for the floppy write protect lights.
> 
> Most of this information is documented on-line,
> in the document "third-party disks.txt".
> 
> All that need be done to the 4-button panels is to
> solder 2 resistors onto the boards, to pull the 2 signal lines
> to their appropriate logic levels.   The RQDX3 only uses these
> signals to establish that the drive is on-line, and write-enabled.
> It has nothing to do which drive is selected when it goes to write.
> 
> Have I trashed the format of a drive in a 2-drive BA23 configuration?
> Yes, I have.   But only because I wasn't paying attention,
> and didn't set the Drive Select jumpers to "3" on both drives.
> 
> Remember:  The distribution panel shifts the signal lines around,
> and all hard drives need to be set to "3".
> 
> This could easily trip you up if you weren't paying attention,
> as some of the drives had DS markings of 0 - 3, and some
> were marked 1 - 4 . . .
> 
> Thus, depending on what DS settings you have,
> you could in theory have 2 drives responding to the
> same drive select signal, which could trash your format.

Hmm. As far as I can remember, there are no DS jumpers on the drives. If 
you have an RQDXE, you have a bunch of jumpers on that in order to set 
up which drive is which, and all that.
But the backplane of the BA23 don't have any jumpers, as far as I can 
remember.

And in this case, both drives were RD53s. Properly formatted, DEC 
branded and everything.

And no, it wasn't me who tried putting this together. I was called in 
after the fact, to rescue as much as I could from the trashed drive.
Which is why I suspect it was writing to #1 which trashed #0, since I 
believe they were in the process of making a copy of #0, which was their 
only copy of the system, and which they booted from.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol