| > I have a uVAXII with an apparently dead RD53 drive. I found an external RD54
| > drive and installed it in the system case. Does anyone know which jumpers
| > (if any) on the RQDX3 controller have to be changed to use the RD54? I saw
| > some posts referring to W23, but I have know idea which jumper this is. Thanks.
| > 
| You don't have to change any jumpers at all, normally. The RQDX ctlr 
| reads a label on the disk to find out what disk type it is.

Well, except that if you have two drives in the same machine,
you should be really careful about the jumper that determines the
address of the drive if you're swapping drives in and out: if both
drives accidentally end up at the same address, bad things happen.
I don't have the hardware reference here with me or I'd be more
specific.  It was a while ago, but a friend and I managed to
completely munge a MicroVMS install on RD52, by trying to run the
drive as second drive in a uvaxII.  The documentation we had was
for a BA-123, but we had a BA-23, and unfortunately this detail is
opposite between the two as I recall.  Both drive lights came on
in synchrony when we hit the power, and we knew we were screwed.
Fortunately the other RD52 (Ultrix) survived.

Seems to me the jumper was part of the drive cable harness, rather
than on the controller or the drives.

(I'm probably incorrect to call it the "address", too; I suppose
it is just the drive select line, already decoded.)

--ben