Pin-compatibility
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In electronics, a pin-compatible device, such as a logic chip or microprocessor, is one that has the same functions assigned to the same particular pins. Generally this presumes that the chips are also mechnically compatible, that is they can be inserted into the same socket/circuitboard. This differs with being electrically compatible, although chips may be both pin and electrically compatible. Usually if a chip is pin compatible it is also electrically compatible.
Mechanical compatibility measn that the chips may be inserted into the same slot.
Pin compatibility means the same pins, for instance pin 5, perform the same functions on both chips.
Electrical compatibility means that for a given pin function, such as A0, the voltage levels are the same on both devices.
Chips may come in various packaging styles such as QFP, CerDIP, etc. The chips would be pin and electrically compatible, but not mechanically compatible, since the different packing styles are for different socket/circuitboards.
Chips could be machanically and electrically compatible, but not pin compatible. For instance the Athlon and Pentium 4, which have the same voltages, and the same functions, but have those functions assigned to different pins.
Chips may even be mechanically and pin compatible but not electrically compatible if they use different voltages for the inputs and outputs. In some cases, one chip may be compatible with the other, but not the other way around, for instance the 74Cxx family can be used in circuits designed for the 74HCTxx series, but the 74HCTxx series cannot be used in 74Cxx circuits because the 74Cxx series circuit may be designed to use a voltage higher than the HCT family can handle.