What to do with unused inputs and outputs? - Page 1
Forum section: electronics / beginners

author: sci4me
author_rank: Regular Contributor
timestamp: June 18, 2015, 10:23:52 pm
content: I assume that unused inputs (of logic chips (I'm working with 74LS mainly right now)) can just be tied low (or high depending on the case...) but what about outputs?

author: Yansi
author_rank: Super Contributor
timestamp: June 18, 2015, 10:55:57 pm
content: Pull the unused inputs up or tie ground (will draw more current, I think 2.4mA per input when grounded, if I remember that right). Outputs can be left unconnected.

author: sci4me
author_rank: Regular Contributor
timestamp: June 18, 2015, 11:08:51 pm
content: When using the term "pull", do you mean through a resistor? Or do you mean to just connect the input straight to Vcc?

author: Yansi
author_rank: Super Contributor
timestamp: June 18, 2015, 11:32:51 pm
content: I think TTLs should be tied high only through resistor, but some time ago when I did a small and quick research about this, I haven't concluded anything definitive. If you want tie them high (spare some supply current), put ther eg. 4K7 to 1k resistor. It won't harm anything, just for sure.I remember from a college being told that I mustn't connect the TTL device input directly do Vcc. The reason I was told was the input transistor is being revere biased that way. But if you look on the internal schematic of a TTL logic gate, I can't see any obvious reason why shouldn't I connect the input directly to V+. To breakdown the EB junction you need more than 5 to 7V or so, and that won't happen. So that much for a collegeLuckily these days we have CMOS gates with hig impedance inputs that can be tied directly to ground or V+ (sometimes even higher than V+ for some special types of logic like LVC gates). TTL is totaly OBSOLETE today, simply don't use it if you are not forced to do so.

author: Hideki
author_rank: Frequent Contributor
timestamp: June 18, 2015, 11:40:45 pm
content: From a google groups post:QuoteI also agree with Yansi. Stay away from TTL if you don't absolutely need to use it.

author: sci4me
author_rank: Regular Contributor
timestamp: June 19, 2015, 12:14:55 am
content: This will sound dumb is this the right way to do it:

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: June 19, 2015, 12:29:39 am
content: There are typically three options for unused pins - which is preferred depends one use case.1) Configure as input and pull to ground2) Configure as output and switch to ground3) Configure as input and activate internal pull deviceAt work, we tend to use #1 primarily for ADC inputs, #2 for most NC pins and #3 for pins which are be used in different placement options or as configuration pins.

author: Hideki
author_rank: Frequent Contributor
timestamp: June 19, 2015, 12:35:03 am
content: "you tie all unused inputs that are to be forced high to each other, and then pull them all up with one resistor"There is nothing wrong with using one resistor per input if you have too many resistors and want to use them up.0xdeadbeef: This question is not about microcontrollers

author: 0xdeadbeef
author_rank: Super Contributor
timestamp: June 19, 2015, 12:38:36 am
content: Oops. Should have read the whole posting

author: sci4me
author_rank: Regular Contributor
timestamp: June 19, 2015, 12:39:50 am
content: Quote from: 0xdeadbeef on June 19, 2015, 12:38:36 amHeh no biggie. Thanks guys!

author: slateraptor
author_rank: Frequent Contributor
timestamp: June 19, 2015, 03:40:32 am
content: Quote from: Hideki on June 18, 2015, 11:40:45 pmI don't agree...especially of the end-user is new to electronics.CMOS devices:1.) ...have appreciably faster rise times, which can be real problem in terms of signal integrity for new players.2.) ...are a lot more susceptible to bricking from ESD, especially in dry climates and without proper tools.Quote from: sci4me on June 19, 2015, 12:14:55 amYes.

author: TimFox
author_rank: Super Contributor
timestamp: June 19, 2015, 01:31:50 pm
content: Modern design practice for automatic testing prefers that all unused inputs be connected to ground or high through a resistor, so that board probing can verify full function of all devices (including unused sections). With TTL, the pull-down resistor to ground must be small (check VIL and IIL specs for logic family).

author: tszaboo
author_rank: Super Contributor
timestamp: June 19, 2015, 03:07:39 pm
content: Just connect 13 to ground, and connect 11-12 together. Why would you waste board space and components for a such mundane part of the circuit?

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2015, 06:40:21 pm
content: Although it's not best practise, it's perfectly fine to leave unused TTL inputs floating (disconnected) as they're already pulled high internally. The trouble is it makes it more susceptible to noise pick up but this isn't normally a problem, unless the input is connected to a long piece of wire/PCB trace and is near a source of noise.

author: wraper
author_rank: Unknown
timestamp: June 19, 2015, 06:54:35 pm
content: Quote from: slateraptor on June 19, 2015, 03:40:32 amNot true. TTL (S, AS, LS, ALS, F...) as well as CMOS come in different flavors. Just select the series with appropriate speed. CD4000B is much slower than 74LS, 74HC approximately the same as 74LS, 74VHC very fast.

author: retrolefty
author_rank: Super Contributor
timestamp: June 19, 2015, 10:41:03 pm
content: QuoteI learned this the hard way when I first used a CMOS device (4000 series I believe) back in the early 80s. I was only using two of the four gates of the device and when I touched the device with my thumb it caused a blister burn, from oscillation resulting from un-terminated inputs. Took a long time for me to trust those early CMOS devices as I had so little problems with TTL logic chips.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 20, 2015, 03:27:46 am
content: Quote from: retrolefty on June 19, 2015, 10:41:03 pmI had something similar with 4066 quad analog switches back around the same time period.I left the unused inputs & outputs disconnected & the things got red hot.After connecting all the unused inputs to zero volts they were a bit cooler but still hot.A 6.8v negative supply was jury rigged--------long way from Electronics shops,& 10 PM ,so used whatever zeners were in the Station store.I connected that to the unused inputs & the rotten things were at last turned off!This was an official project,so I was "under the hammer" to get it finished.It had "bugs" I still hadn't ironed out,but the application it was designed for became obsolete,so it was never used,in any case!.