Driving input pin from open collector pin?

I’m using KiCad 6 and as you can see in the schematic, I’m failing the rules check because the output pin from the inverter is setup as open collector and isn’t driving the input pin of the '244. I’m not sure how to fix this problem other than editing the “built in” components and changing it to an output. But that doesn’t seem like a clean solution considering these are the standard components.

Any ideas on how I can handle this?

BTW, I’m taking this schematic from the excellent book Microprocessor Systems Design. I thought about maybe putting a pull-up on that line but the original schematic doesn’t have it.


No you’re not. There is no such thing as a KiCad V6. The closest you can get at the moment is KiCad-nightly V5.99.

About the error: “Input pin not driven by any output pins.” KiCad is right here. To fix it, you need at least a passive pullup resistor to have a defined “High” level when the 74ls05 is “off”.

After you’ve done that, you can right click on the ERC violation and then select Exclude this Violation from the popup menu.

The excluded violations turn grey, and if you want to completely hide them, then un-check the “Exclusions” checkbox near the lower right corner.

At least, that should work for both ERC and DRC if you’re using KiCad-nightly V5.99. Your KiCad version might behave differently.

No you’re not. There is no such thing as a KiCad V6. The closest you can get at the moment is KiCad-nightly V5.99.

Ah, a rounding error on my part. lol

I was thinking the pull-up was the way to go but I’m not sure why the original source didn’t do that. I will look more in the book. Maybe I missed something.


Those rounding errors are quite important, as messages posted on this forum will be kept, be visible and searchable like “forever”. KiCad V6 was expected early this year, but at the moment it would make a lot of people happy if it’s released before the end of this year.

What source? What book?
The theory is pretty simple. Open collector outputs switch between “floating” and pulling to GND, and TTL inputs should never be left floating, and there must be plenty of other sources where you can verify this.

Books often have uncorrected errors.

Un-driven TTL inputs generally pull themselves high due to the internal structure of the IC. This is not a reliable design, however, especially at higher switching rates. Modern ICs based on CMOS technology have unpredictable behavior when an input is not driven. If you substituted a 74HCT244 for the 74LS244, the circuit would likely fail without the pull-up resistor.

I did some experiments using the textbook “Digital Computer Electronics” by Malvino and Brown. Originally published in 1977, and updated in 1983 and 1993, this book is out of print but still available on the used book market. The design works as described, but when configured to load a program into the RAM it enables two tri-state outputs to drive the data bus at the same time. It’s good enough for an experimenter’s project, but it’s a serious error nonetheless.

Here’s a link to my blog post describing this design error in detail: click

74LSxx had around 20k internal pullups,so the open collector driver would work.
CMOS inputs are high impedance, so a pullup resistor is essential


From what I remember of those old things this was considered a bad habit. It works on your bench, and later in the field you have intermittent problems. A partial blame on PCB design habits of those day’s with dual layer PCB’s (horizontal and vertical layer) and no proper GND plane.

My own experiences with AVR controllers are also not great. The reset pin of an ATMEGA8 has an internal pullup, and especially if when wires are connected (reset button in accessible location, ISP connector (not even a cable!) and build the thing on Matrix board, you can get occasional / random resets. After I discovered that I always added an extra pullup and a small capacitor to suppress glitches (So it’s not the RC constant to create a long reset time which used to be common). Just like regular decoupling caps, I put this capacitor close to the IC (although less critical).

1 Like

At Racal Milgo 30 years ago, our digital boards were two layer, with a peculiar two layer bus bar on each row of ICs. This bar carried GND and 5V, so the boards were a pseudo 4 layer.

74LS05 Fairchild data sheet explains the necessity for a pull-up resistor and gives the formula for calculating, but why not use a normal output 74LSxx inverter?


Alright, got off my posterior and did a one minute search: 74LS04 has the same package and pin connections but the six inverters are standard TTL outputs… not open collector. Substitution will solve the problem?

I think the voltage at such input even ‘seeing’ 1 stayed very close to level of changing the state so small voltage disturbance could change the input state.

Now I have slept on it, from memory, (or was it a dream or nightmare? :astonished:) I think the only time I ever used a 78LS05 was for voltage translation.

Probably find the original designer made a careless mistake by typing an 05 instead of an 04.

I think I see what the problem is. The schematics I was looking at were on two different chapters. On the first, it was a “high level” schematic that indeed used a 74LS05. But that 74LS05 DID have pull-up resistors like it should have.

Then, several chapters later, there is a more detailed schematic. However, it’s still a “high level” and not all parts are even identified. This schematic shows an inverter symbol but doesn’t label it an 05 or 04. So I assumed I could just use the leftover gates.

Long story short, it’s quite possible the second schematic would probably use a 04 instead of an 05.

So now I’m wondering why they used the 05 in the first place and not an 04. I’ve got lots to learn and this is a pretty large book. :slight_smile:

You used the open collector drivers either when you had multiple drivers on a bus or when you wanted higher level than a TTL “1”

As davidsrsb says, active high wired AND or active low wired OR. Also used to drive indicators. Before LEDs incadescent displays were driven by OC outputs with higher voltages than the main supply. Look at the 7406/07/16/17 ICs. The concept carries over to open drain for FETs. Look at the TPIC6B595 for example.

The most important part here is that KiCad’s ERC caught a real error that very likely would have made it into manufacturing.

Davidrsb gives you two reasons for using open collector outputs, but the reason you stated is also a valid and often used reason. You have some left over gates.

So the question becomes: What are those other 5 gates doing, and are any of the open collector outputs needed?

You also have to consider that a passive pullup has slower timing on the rising output flanks.

(I used to work at Racal Milgo in UK, back in 80s): yes, I remember those busbars, and back when you had rows of 0.3" chips, were a good way to get the power distributed… and yes, probably a lot cheaper than going to 4 layes - especially back then!

Why use a 74LS chips at all, because those ICs are available in 74HC series and much more, like 74HC40 and 74HC45 series. Or they were available because world has moved on, and those chips are used less and less every year.

Or they were available because world has moved on, and those chips are used less and less every year

The world moved away from the 68000 too but I still like designing around it. :slight_smile:

Caveat: if you replace only some LS with HC check all the places where LS drives HC because LS logic 1 isn’t quite high enough resulting in lower noise margin. So either replace all or if must mix use HCT which is designed to cope with LS levels.

1 Like

Indeed. You always have to be careful when mixing logic families.

Recently (euhm few years ago) I used an STM32 (at 3V3) driving 74LS244 buffers which drive optocoupler leds of stepper driver controllers on the other end of a cable. (Sink to GND to turn them on)

Input levels of 74LS fits nicely with output levels of the STM32, and these old chips are less susceptible to overloads & ESD, if wires get damaged or a connector gets loose or other weird stuff.