>>13447034>>13447006It gets fun with printed circuit boards.
Standard IC chips have pins 0.1" apart. That's ONE TENTH of an INCH. But when you want to put something in between, that's one-twentieth of an inch, or .05", and it gets worse the more you divide it by two. Many other chips have pin spacing based on dividing one tenth of an inch by (usually) a power of two.
Except it's not all that easy. An inch is 2.54mm (defined exactly), which is almost but not quite 2.5mm. Russians made their copies of western IC chips using 2.5mm spacing. It's not so bad for small chips, but for the bigger ones it adds up.
I recently found out that Texas Instruments made their TMS9900 chip (the one used in the crappy TI-99) with a metric pin spacing, just like the Russians. Which wouldn't be so bad except it's a fat 64-pin chip, and those extra half millimeters add up. So forget using a standard socket for it, or replacing it with some kind of riser board for a mod. Pic related is a similar size chip, but not metric.
