bjork@navajo.UUCP (Steven Bjork) (01/15/88)
Here's what I know off the top of my head about some of the TTL
(and other logic) families around these days.
Jargon: TTL = Transistor Transistor Logic
7400 The original series. Evolved from DTL (diode transistor logic)
giving higher speed.
74H This is TTL with gold doping and decreased internal resistances
in the output stage. This creates a faster, higher powered device.
74S This family uses the Schottky transistor. These devices do not
have saturated output stages, and thus can switch much faster
than normal TTL.
74LS Here the device uses the Schottky transistors, but the internal
resistances are increased to reduce power consumption. The parameters
are juggled to generally give slightly better speed at reduced
power levels as compared to 7400.
74F This is the best of the wonder chips. Fairchild applied their
expertise to bipolar, giving near-ECL speed at near-LS power
levels to TTL. Motorola junked the ALS/AS stuff to follow
Signetics and Fairchild/(National) with this family.
74ALS TI's attempt at bettering LS. Obsolete with 74HC/HCT.
74AS TI's attempt to combat the 74F series. It loses.
74C National brought this family out to give designers the familiar
TTL pinouts in a CMOS logic family. RCA had a CMOS line, but the
brain-damage of figuring out the 4000 pinouts was a lose...
74HC This family is CMOS upgraded with modern processing techniques
to attain higher speeds than regular 74C.
74HCT This is 74HC fully spec'ed to replace 74LS. Actually, at higher
frequencies, HCT dissipates >>more<< power than LS, so beware...
--Stevebill@sigma.UUCP (William Swan) (01/16/88)
In article <2040@navajo.UUCP> bjork@navajo.UUCP (Steven Bjork) writes: >[...] >74HCT This is 74HC fully spec'ed to replace 74LS. Actually, at higher > frequencies, HCT dissipates >>more<< power than LS, so beware... Also note that 74HCT parts will consume *much* more current when driven with TTL levels (0.8/2.4 V) than with CMOS (rail-rail) levels. I'd have to dig out the books to tell whether it's more than 74LS - I seem to recall that in at least some cases it is. Oh, for the good old days of 15V TTL! :-) -- William Swan {ihnp4,decvax,allegra,...}!uw-beaver!tikal!sigma!bill
mbe@dde.uucp (Martin Berg) (01/19/88)
In article <1457@sigma.UUCP> bill@sigma.UUCP (Willian Swan) writes: > Also note that 74HCT parts will consume *much* more current when driven... There is another problem when using this family of TTL.: Last when I read TI's specifikations on the 74HCT family, it was made clear that this family needed at least 4.5 V supply voltage. Therefore you should not use these parts (for reliable operation) in applications with a low-voltage, low-power standby mode (f.ex. for preserving contents of CMOS-RAM). (Ok, Ok: I'am just reading the specifications ;-) ); I have some times seen this done in memory-makers application notes as a way to seperate the CMOS-memory (with standby power) from the rest of the system (with more-or-less standard bipolar TTL parts). My question is: Has the specifications changed or are these parts better than promised ? -- Martin Berg mcvax!diku!dde!mbe or mbe@dde.UUCP
bill@sigma.UUCP (William Swan) (01/20/88)
In article <143@Giraf.dde.uucp> mbe@dde.uucp (Martin Berg) writes: >> Also note that 74HCT parts will consume *much* more current when driven... >There is another problem when using this family of TTL.: > >Last when I read TI's specifikations on the 74HCT family, it was made >clear that this family needed at least 4.5 V supply voltage. > >Therefore you should not use these parts (for reliable operation) >in applications with a low-voltage, low-power standby mode (f.ex. >for preserving contents of CMOS-RAM). >[...] >My question is: Has the specifications changed or are these parts >better than promised ? The specifications are as you mentioned. I took a look at some other manufacturer's specs, and they are the same. It appears that HCT is fairly limited (probably as a result of whatever is done to shift the threshold on these devices). For an example, the specifications given by Universal Semiconductor are as follows: VCC: Min Max 74HCxx 2.0V 6.0V 74HCTxx 4.5V 5.5V -- William Swan {ihnp4,decvax,allegra,...}!uw-beaver!tikal!sigma!bill