Front panel switches - what did they do?

Jon Elson elson at
Tue May 24 21:36:55 CDT 2016

On 05/24/2016 05:44 PM, Chuck Guzis wrote:
> On 05/24/2016 02:21 PM, Paul Berger wrote:
>> The CROS cards used in a 360/30 where the same size as an 80 column
>> card on purpose so you could you a keypunch machine to program the
>> microcode.
> But I believe that the CROS cards were mylar, no?
On the 360/30, there were 12 word lines printed in silver 
ink on one side of a .003" thick Mylar card,  and bit line 
boards, with traces running at right angles to the word 
lines.  There were 60 bit lines per board/card.  The  word 
lines had 60 little "pads" hanging below them, and these 
were at the exact location of the holes in standard IBM 
punch cards.  So, if you punched out the pad, one of the 
capacitor plates would be missing.  If the pad was not 
punched out, then it formed a capacitor between the word 
line and the bit line. Air bladders applied even  pressure 
to the stack of mylar card and bit line board.  With only 12 
words/card, it took quite a lot of them to hold the full 

On the 360/50 and /65, the data pattern was etched into a 
bunch of wiggly traces.  For each word, there was a driven 
line and a balance line.  If the driven line was wide across 
from the 1's bit line pad, you got a 1 in the control store 
bit.  if the non-driven line was wide across from the 1's 
bit line pad, you got a zero.
The mylar sheet was not punched, so changing the microcode 
required replacing a whole bunch of etched circuit boards.


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