Front panel switches - what did they do?

Jon Elson elson at
Tue May 24 20:41:51 CDT 2016

On 05/24/2016 11:56 AM, Swift Griggs wrote:
> On Tue, 24 May 2016, Jon Elson wrote:
>> The early PDP-11s had a diode matrix ROM for the boot memory.  You could
>> change the boot code with a wire cutter and soldering iron.
> Is that similar to "wire wrap" ? I remember my grandmother talking about
> having to snip wires connected to diodes. I think this was in the 50's but
> it might have been the 60's, too. She mentioned something like that.
> -Swift
Here's some wire-wrap.  And, yes, you did need to change the 
wire wrapping on some systems to configure different 

But, the DEC diode ROM boards were printed circuit boards 
with a matrix of diodes diagonally connecting between 
vertical traces (on one side of the board) and horizontal on 
the other.  Diodes were removed to change zeroes to ones.  
(or vice versa.)

Here's a homebrew one:


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