ttltypes.tut
-------------

A Short Tutorial on TTL Chip Types 74xxyyyy Where xx is the Type and yyyy
Is the Logical Construction of All 74 Series TTL of that Final Number.
--

From: sloman@sci.kun.nl (Bill Sloman)
Subject: Re: Which chips: S,LS,F,HC, or CHT?
Date: 29 May 1997 14:11:06 GMT

Someone said:
>Generally speaking, as long as everything is operating from a 5-volt
>power supply, all CMOS circuits can drive all TTL circuits. Also
>generally speaking, TTL can drive CMOS but only when a pull-up
>resistor is used to pull the TTL output up to the 5V supply voltage.
>Use a 10K or 20K resistor as the pull-up.
>
>S, LS, and AS are all TTL circuits. C, HC, and HCT are all CMOS
>circuits. (I can't remember about F.)
>
>HCT is rather unique. It is made to accept TTL drive signals without
>the need for a pull-up resistor.
>
>A few words about "fanout." Fanout is the term used to describe how
>many inputs can be driven by a single output. A TTL circuit can
>typically drive 10 inputs of the same type; that is, an S device can
>drive 10 S devices, an LS device can drive 10 LS devices, etc. TTL can
>drive a bunch of CMOS inputs. CMOS, too, can drive a bunch of CMOS
>inputs. But CMOS can only drive one or two TTL inputs.

The original 7400 logic, and 74L, 74H, 74S, 74LS, 74ALS and 74F are all 
bipolar logic, built with NPN transistors. The nominal logic theshold is 
1.4V, and a signal has to be less than 0.8V to be guaranteed to be 
recognised as a "0" and higher than 2.0V to be guaranteed to be 
recognised as a "1".

The output stages guarantee less the 0.4V when set to "0" when sinking
a defined load current or less, and more than 2.4V when set to "1" when 
sourcing a defined load current, or any lesser current.

You need to sink appreciable current when setting a bipolar input to "0",
varying from 0.4mA for 74LS to 2mA for 74S, but takes very little - about
40uA - to set ti to a logical "1".

74C, 74HC, 74AC, 74HCT, 74ACT, 74ACL, 74FCT etc are all CMOS logic and
have the common property that their inputs present essentially zero DC 
load, and their outputs pull up to +5V (whatever the supply rail you
have chosen) and pull down to 0V. 

The output stage on 74C, 74HC and 74HCT parts can't sink a great deal of
current, so they can't drive many bipolar inputs in parallel and still
guarantee to pull them lower than 0.8V - the exact number depends on the
flavour of TTL being driven. The more modern, faster 74AC, 74ACT, 74ACL
and 74FCT parts can sink a lot more current, so they can drive a 
respectable, but still finite number of bipolar loads.

74C, 74HC, 74AC and 74ACL logic use a nominal threshold voltage of about 
2.5V (with Vcc at +5V - it might be better to say Vcc/2) with a low
threshold at about Vcc/3 and a high threshold at 2.Vcc/3. They drive 
bipolar TTL without problems (provided they can sink the current), but
the guaranteed 2.4V "high" output from TTL isn't high enough to be
reliably recognised as a "1" by the CMOS. Adding a 1k pull-up resistor to 
a biplar TTL output is usually enough to solve this problem.

74HCT, 74ACT, and 74FCT logic all use the same threshold voltages as 
bipolar TTL, and can be driven by bipolar TTL without benefit of pull-up
resistor (this feature hasn't been unique to 74HCT since the late 1980's).

Hope this fills in some of the background.


Bill Sloman (sloman@sci.kun.nl)        | Precision analog design
TZ/Electronics, Science Faculty,       | Fast analog design and layout
Nijmegen University, The Netherlands   | Very fast digital design/layout
                                       |  e-mail for rates and conditions.