Editing Apple IIc (section)

===Serial port timing fix===
Serial ports on Apple IIc motherboards manufactured before December 1984 are 2.9% slower than the [[Electronic Industries Association]] standard, greater than the permitted plus or minus 2% variance. Non-Apple modems with speeds greater than [[300 baud]] are incompatible with the ports.<ref name="linzmayer198412">{{Cite magazine |last=Linzmayer |first=Owen W. |date=December 1984 |title=Apple cart; using a parallel printer with the IIc, an RGB interface, a speed-up card, and the problem with modems. |url=https://www.atarimagazines.com/creative/v10n12/205_Apple_cart_using_a_paral.php |access-date=2025-01-31 |magazine=Creative Computing |page=205}}</ref> The motherboards derive the serial timing through a 74LS161 [[Transistor-transistor logic|TTL]] logic chip. This causes some third-party modems and printers, which operated at 1200 baud or faster, to function improperly. Slower serial devices operating at 300 baud or less are unaffected, as well as some faster devices which can tolerate the deviation. The solution to ensure all devices are compatible is to replace the TTL chip with a 1.8432&nbsp;MHz crystal oscillator (next to capacitor C19) during manufacture. Apple swapped affected motherboards for users who could prove they had an incompatible serial device (e.g. a third-party 1200-baud modem which presented problems; not all do).