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

[Johnny Billquist]

Pete Turnbull <pete at dunnington.plus.com> wrote:

> On 17/06/2009 16:34, Johnny Billquist wrote:
> 
>> > 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.)
> 
> That won't happen unless both drives are also selected.  Having the 
> write enable signal go to both drives at the same time is normal in 
> ST412 systems, and indeed all the signals go to all the drives at the 
> same time in such systems.

Ok. I haven't checked all the details here, so it's part my research, 
and part information I've recevied from others. The only reason I even 
know about it is because I had to recover the data from a disk that they 
"destroyed" by doing this.

>> > 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.
> 
> The DEC information actually says that it's because of the power 
> requirements.

The note that I read didn't say anything about power requirements, as 
far as I can remember. But maybe I should check the note I read again. 
This was about six years ago... (And as far as I know, the site is still 
running that machine, but they now have a couple of SCSI disks on it 
instead.)

>> > 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?
> 
> Mine works, and I've not cut any wires.

It could be that there are two revisions of the backplane? (Was it you 
who said that?)
I know for a fact that a steel plant in Sweden stood still for five days 
because of this, six years ago. (Well, one part of it anyway.)

	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