Apple IIc "Rev. B" (nov 1, 1984 – aug 1, 1988)
Description:
The original Apple IIc motherboard (manufactured between April and November 1984) derived the timing for its two serial ports through a 74LS161 TTL logic chip. It was later found that this method's timing was 3% slower than the minimum requirement specified and caused some third-party (i.e. non-Apple) modems and printers, which operated at 1200 bits per second (baud) or faster, to function improperly. Slower serial devices operating at 300 baud or less were unaffected, as well as some faster devices which could tolerate the deviation. The solution to ensure all devices were compatible was to replace the TTL chip with an oscillator during manufacture. Apple would swap 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 did). It is important to note the problem did not affect all owners; it was more or less a hit-or-miss issue depending on the specific device connected.
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