trying to upgrade old schematics with ttl to cmos...

[loss1234]

I dont know much about this but i was wondering if i could use cmos instead of ttl for a few of these parts. i am going to try and build a "semi random" cv generator from an old schematic and it asks for lots of ttl. i wrote down what it asks for and also looked for substitutes based on my limited information (or disinformation)

any help appreciated!!

ttl ////// possible replacement?

7490-decade counter- 4017???
7441-bcd decoder- 4028???
74ls193-up down counter- 4029?
74154-4 to 16 decoder/demultiplexer-- no idea
74ls00-quad nand gate--- 4012 maybe?
74ls164-shift register-- 4015???

[françois]

Hello loss1234,

You certainly can redesign a TTL schematic to use CMOS instead, but you do have to redesign. Every (or so) TTL IC has a CMOS equivalent. But the pinouts are not the same, unused CMOS inputs must be grounded... I guess you already knew that.

Attached is a reference guide for finding equivalents. I can't remember where I found it, probably on Texas's site.

EDIT : the file is too big for an attachment (over 4Mbytes). You may find it at Texas's site, it is entitled "Digital Logic Pocket Databook". Sorry for that.

Yours,

-- françois

---

Digital Logic Pocket Databook (Texas).pdf
Description:		Download (listen)
Filename:	Digital Logic Pocket Databook (Texas).pdf
Filesize:	4.68 MB
Downloaded:	254 Time(s)

---

[loss1234]

i knew about the pinout differences..i figured i could attack some datasheets

thanks for that awesome doc!!!

[Pehr]

[richardc64]

74C (which operates up to 15V but is getting harder to find) and 74HC (5V only) are pin-for-pin identical with TTL. Every TTL IC you listed should be available in 74HC somewhere -- even the '154.

The 7490 is a 4-bit decade counter: it has only 4 outputs. the 4017 decodes the 4-bit count into 10 discrete outputs -- which might eliminate the need for the 7441-bcd decoder. I'd have to see the circuit.

One thing to watch out for is the values of any pull-up and, to a lesser extent, pull-down resistors, which of necessity are lower values in TTL than in CMOS. One-shots and clock oscillators might need some tinkering of RC values.

[loss1234]

looks like the 74ls164 is not any old shift register that i have in cmos. its an 8 out model. hmmm. i guess i need to save a tiny bit of money for shipping and make another big logic order.

[etaoin]

I'd opt for the easy option and replace all the 74LS by 74HCT which is readily available everywhere and fully compatible with TTL so most likely to work as a direct replacement without redesigning. Don't be fooled by the '74'. 74HCT is CMOS.

[loss1234]

ok then
i just need to order some of those chips!!
i always thought (incorrectly) that cmos had to start with a 4.

thnaks

[etaoin]

Note though that using HCT will give you the original circuit in cmos. I.e. it will still operate only at 5V!

[State Machine]

[loss1234]

so 74hct are faster but can only run at 5 volts?

i like the 4000's because i dont have to run them at any odd voltage....

[etaoin]

[fluxmonkey]

nice overview of various logic families of chips:

http://www.interfacebus.com/Design_Logic_Family_Selection.html

also links to handy charts outlining various trigger voltage levels, if you wind up needing to mix families.


b