On 2016-Oct-13, at 10:39 AM, Jim Brain wrote:
> I thought the question about the prevalence of .156" connectors in early 
> systems was interesting and I assume someone here has the detail on the 
> rationale.
> Jim
> -------- Forwarded Message --------
> Subject:      CBM edge connectors pitch?
> Date:         Thu, 13 Oct 2016 19:31:47 +0200
> From:         silve...@wfmh.org.pl
> Reply-To:     cbm-hack...@musoftware.de
> To:   cbm-hack...@musoftware.de
> Do we know what is the norm used in the CBM edge connectors? Like the IEEE, 
> I found out mentions that it uses a 0.156" pitch. Where the heck does that 
> come from? Nothing "round" in either metric or imperial..
> -- 
> SD!
>       Message was sent through the cbm-hackers mailing list

The 0.156" is presumably the oldest PCB edge connector, I have an example in 
tube equipment going back to 1959.
Very common in all sorts of equipment through the 60s and 70s.

As Tony suggests, it matches up with 5/32" spacing, just as 0.125 is 1/8".
Where the rounding of the 1/4 thou (0.00025) took place - in the name or the 
implementation - I'm not sure.

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