Pin compatibility

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In electronics, a pin-compatible device, such as a logic integrated circuit (IC), memory module or microprocessor, is one that has the same functions assigned to the same particular pins.[1]

There are several levels of compatibility in integrated circuits (ICs). Strictly speaking, "pin-compatible" refers only to two chips that have the same functions assigned to the same pins. For instance, two logic chips would have their inputs, outputs, power supply, and ground connections on the same pins.

Generally the term "pin compatibility" (or "pin-for-pin replacement") also presumes mechanical and a significant degree of electrical compatibility, and the expression "pin-compatible" is often used as shorthand to refer to compatibility in all three of these senses, although terms such as "drop-in replacement" or "equivalent device" or "exact replacement" imply more certain compatibility.

ICs are mechanically compatible if they can be inserted into the same socket or soldered to the same footprint. This is generally true only if they use the same packaging standard (e.g. DIP, TO-220, TSOP, etc.). Small details can be important, such as a ceramic dual-in-line package being slightly higher than the plastic counterpart may mean devices that are pin-compatible and electrically compatible still not being able to be always be used interchangeably in some high-density equipment.

Electrical compatibility implies that the ICs work with the same supply and signaling voltage levels; that one is a suitable replacement for another (perhaps in a particular circuit, or in general). What constitutes electrical compatibility (and indeed pin compatibility) varies considerably in popular usage; be aware there may still be differences in:

  • operating temperature range and the guarantee of various characteristics over a wide temperature range
  • speed (for logic circuits; bandwidth for analog circuits)
  • power supply current demand or working voltage range; the 74HC family of chips can run on 2–6V supplies, while the 74HCT family only tolerates 4.5–5.5 V

and so on. One integrated circuit may be able to replace another in a particular circuit, with a particular supply voltage for instance, yet the two are not perfectly interchangeable (e.g. a 74HCT00 could not safely be used in a 6V circuit designed for a 74HC00 or 74C00). Careful checking of manufacturer's datasheets remains important even when one device is said to be a pin-for-pin replacement for another.

The most widely used and recognized family of pin-compatible chips is the 74'xx series of logic ICs (although there are a small number of pin-out incompatibilities). The tick-mark here represents the omitted subfamily designator – most commonly HC (high-speed CMOS), but can also be blank (original TTL), C (CMOS), LS (low-power Schottky), and many others. The "xx" represents the actual part, and ranges from 00 (quad 2-input NAND) to 40103 (8-bit synchronous binary down counter). All of the various families have remained pin-compatible over several decades of production by numerous different companies, including some versions that were not electrically compatible. Exceptions are few, but include the 7400N (or 74LS00J/N/W) vs the 7400W; the 7454(J or N) compared with 7454W, 74H54, 74LS54 and 74L54W (five different 4-wide AND-OR-INVERT gate combinations of pin-out and logic function for the same device number). Ironically (given the history of competition between the two logic series), the 74HC series has grown to include pin-compatible versions of several members of the 4000 series chips, such as the 74HC4051.

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