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

[Johnny Billquist]

Ethan Dicks <ethan.dicks at gmail.com> wrote:

> On Sun, Jun 14, 2009 at 4:14 AM, Johnny Billquist <bqt at softjar.se> wrote:
>> >
>> > ...and if you wanted to use more than one hard drive
>> > on the RQDX3 you also need the RQDXE, since the BA23 distribution board is buggy, and
>> > can't actually connect two hard drives, even though you have two connections.
>> > I think it's worth pointing out that people should never even try connecting two hard drives in a BA23.
>> > You'll definitely wreck whatever contents you have on the disks if you try.
> 
> I thought there was a way to do it, but you had to start moving drive
> select jumpers around and it was still easy to get a double-select and
> trash your contents.

It's actually worse. The write enable signal apparently always gets to 
both drives no matter what else you are doing. So when you write to one 
drive, the other will also start writing, even if the head is moving 
right at that time. I've had to recover an RD53 which was destroyed that 
way. (Salvage as much data as I could, and then reformat the drive.)

The funny thing is that DEC actually do write in the documentation that 
it is not permissible to have two hard drives in a BA23, but that note 
is not so easy to find, and if you don't have the documentation, it even 
easier to assume that you can, since you do have connectors for two drives.

I believe it's a hardware "bug", which can't be fixed without cutting 
wires, and adding new ones. But maybe someone knows better here?

> I remember trying to put a pair of RD52s or an RD53 and an RD52 on one
> machine 20+ years ago.  I also remember not succeeding.  Clearly an
> RQDXE would have made the job simpler (possible?), but we didn't have
> one handy.

The RQDXE will definitely do the trick. If your "not succeeding" just 
went as far as you didn't get it to work, without trashing the conents 
of a drive, you were lucky.

	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