Modifying a RXV21 for an 11/83

Chris Zach cz at
Sun Jan 24 20:11:38 CST 2021

Ok. My guess is it throws off the cache on the 11/73 and above as it 
works in an 11/23 (which is soooooo slow!).

I looked at the images, cut the traces, then soldered in two of the 
wires. Board did not work on either the 11/83 or the 11/23+ CPU. However 
when I looked closer I saw the tiny little wire down by the crystal and 
the Q bus fingers. Put that in, and the board now works on the 11/23+, 
will take the whole thing apart (again) and plug in the 11/83.

Fortunately I have the pdp11/83 configured with a Q bus memory board 
(2-4mb) and a PMI memory board from an 11/84 placed in slot 2 (so the 
11/83 will access it as Q Bus memory). Thus I could pull the 11/83 CPU 
in slot 1 and pop the 11/23+ in. Aside from the exceptionally slower 
speed and long boot times it works pretty well for testing.

Still need to get a 11/83 compatible PMI board, but the 11/84 ones work 
as normal Q bus memory.

Thanks! Now I can read some of these RX02 disks and image them. I'm 
guessing my BRU floppy that I generated with the RXV11 controller will 
not boot on this RXV21 controller so I'll have to re-make that. And so 
far it doesn't look like the disks made on that third party MXV21 
controller are compatible with the RX02, should they be?


On 1/24/2021 7:41 PM, Jerry Weiss wrote:
> On 1/24/21 11:27 AM, Chris Zach via cctalk wrote:
>> So I found an RXV21 board in the pile of stuff here and popped it into 
>> my 83. Crashes the system. It's an M8029 D board, and I see from this 
>> site that you have to add a few traces and cut a few others to make it 
>> work on an 11/73 or better.
>> I'll pop in an 11/23+ CPU just to test it, but is it really just 
>> putting in two wires on the top of the board and cutting a total of 
>> three traces? Looks simple enough...
>> C
> My M8029 stamped Rev F  has three jumpers and three trace cuts. It 
> matches the images posted in the link and works with an 11/73 ot 11/83. 
> But yes, it should be that simple.
> There is a sharpied H on my card, which it is not the way DEC marked 
> these boards. The available field maintenance prints refer to this (no 
> G).  I don't know what the change was.
>     Jerry

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