Intel 386 manual PLD Devices, Common Logic Families

	MEMORY INTERFACING
	Table 6-1. Common Logic Families*
74xxx	The original TIL family. Now obsolete.
74Lxxx	Low-power version of standard TTL. Very slow and now obsolete.
74LSxxx	Low-power Schottky TIL. Lower power and higher speed than standard TIL. Widely
	used in microprocessor systems.
74Sxxx	Schottky TIL. High speed and high power consumption. Now obsoleted by newer
	families.

74ALSxxx Advanced low-powe'rSchottky TIL. An improved version of LS TIL, providing faster

	speed, lower power, and better-defined specifications.
74ASxxx	Advanced Schottky TIL. A replacement for Schottky TIL, with higher speeds and
	lower power.
74Fxxx	Fairchild Advanced Schottky TIL. A competitor of AS.

There are also several families of CMOS logic that are pin-compatible with the TIL families and use the same type number designations. CMOS logic has the advantage of very low power consump- tion. Unlike TIL, however, the power consumption of CMOS logic increases linearly with switching frequency.

There are several major families of CMOS logic, as follows:

74Cxxx	CMOS equivalents of standard TIL devices, but considerably slower. Obsoleted by
	newer families.
74HCxxx	High-sPeed CMOS logic. Speeds comparable to LS TIL.

74HCTxxx High-speed CMOS logic with TIL-compatible input thresholds. Widely used as a low- power LS TIL replacement.

74ACxxx Advanced CMOS, logic, with higher speeds than HC. This nomenclature is used for several families, including Fairchild'sFACT family and the ACL family from TI, Signet- ics, and Phillips.

74ACTxxx Same as 74AC, with TIL-compatible thresholds

*Microprocessor Based Design. Michael Slater; © 1987PLD Devices

6.2.2 PLD Devices

Many design examples in this manual use PLDs (Programmable Logic Devices) and EPLDs (Erasable Programmable Logic Devices), which can be programmed by the user to implement. random logic. A PLD device can be used as a state machine or a signal decoder, for example. The advantages of PLDs include the followjng:

1.PLD pinout is determined by the designer, which can simplify board layout by mov- ing signals as required.

2.PLDs are inexpensive as compared to dedicated bus controllers.

6-3