More PDP 11 hacking (help!)

From: Vintage Computer Festival <>
Date: Tue Feb 8 23:12:30 2005

On Tue, 8 Feb 2005, joe heck wrote:

> Typing any location with a "G" will make the heads clunk, that is a bus
> reset.

I've at least verified with the DSD-440 controller that I can poke a 40000
at 777170 (the "RXCS" register of the controller) which sends an INIT
to the drive and I do get a head clunk. So it's at least somewhat

> Sure you have the modules in the backplane correctly? A broken interrupt
> chain will usually hang the system, but I guess it's possible to get a
> halt.

The chain is complete. There are bus grant cards in otherwise empty

> Sounds more like single density disk in a double density system.

Inconclusive, but I don't think this is the case. Upon further
examination, my boot disks DO identify themselves as either RX01 or RX02
disks. I supposedly have two RX02 system disks. Whether they are
actually good is another story, but I have reason to believe they are.
Still, I need to get a known working boot disk (couldn't get one tonight).

Also, the DSD 440 controller automatically retries a boot at single
density when it detects a density error (ie. it expects double density,
sees single, retries with single density).

> Non bootable disks usually came up with a message like "not a bootable
> floppy" and other errors out of the bootstrap (disk error) gave
> boot-u-i/o error. Still, in far past times, I seem to remember some
> systems bombing out like yours.

Tis truly strange.

> Yes, you can use the P for proceed, and not G for go. If you poke the PC
> with the address you want to start at, and then use P, you can execute
> code without doing a bus reset or changing the status bits.

Yes, I've played with this, but due to my limited knowledge of PDP 11
systems I don't know what to look for. The DSD 440 manual says to look in
R0, R4 and R6 for the definitive error when a disk doesn't boot. However,
I haven't figured out how to make sense of the error codes yet.

Here's something I didn't notice before: the drive light on the DSD 440
blinks after I try to boot a disk (and it bombs out at 165426. It keeps
blinking until I re-initialize the bootstrap (773000G).


Sellam Ismail                                        Vintage Computer Festival
International Man of Intrigue and Danger      
[ Old computing resources for business || Buy/Sell/Trade Vintage Computers   ]
[ and academia at  || at  ]
Received on Tue Feb 08 2005 - 23:12:30 GMT

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:37:36 BST