Troubleshooting RX02

From: Vintage Computer Festival <>
Date: Fri Feb 4 14:41:06 2005

In my continuing saga to get my PDP 11/03 working, I hooked up an MXV21
(RXV21 clone, RX02 controller with formatting capabilities) with a
standard set of 8" floppies. I'm getting effectively the same results as
with the DSD 440 though. The heads load but nothing happens, and the
system halts at 177204.

I was fortunate to find the MVX21 manual with the various RT11 system
disks I'd be trying. Upon reading it, I find the halt location of 177204
to be odd, and possibly a clue as to why things aren't working.

Roughly, when 173000 is executed, the controller hands instructions to the
processor (the processor thinks it is pulling words from memory) that
ultimately set up the first disk boot stage. After a few instructions are
executed, the processor then hands control of the bus over to the disk
controller, but not before the controller passes a Clear PC instruction to
the processor. So now the controller inserts a Branch to Current Location
instruction at location 0. It then releases the bus back to the
processor, which continues execution at location 0. Since there's a BCL
there, it loops infinitely. The disk controller then goes on to read the
first stage DOS boot sectors from the disk into location 2 and onwards
through DMA. The last step the controller takes is to put a NOP at
location 0, releasing the processor from its infinite loop and into the
boot code.

So I should be seeing a 777 (BCL), or perhaps even a 240 (NOP), after
halting the processor when the disk doesn't boot. Instead, I see whatever
byte I deposited there from the get go. So it doesn't look like any
sectors are being loaded. In fact, I put a series of numbers from 0 to 17
and nothing gets changed after a boot attempt.

What's now really strange is why the computer is halting at 173204. How
the hell did it get all the way up there?

Is it possible that some other hardware in the system is conflicting with
the disk controller?

DEC hackers?

Sellam Ismail                                        Vintage Computer Festival
International Man of Intrigue and Danger      
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Received on Fri Feb 04 2005 - 14:41:06 GMT

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