Why are logic inputs sometimes tied to rail through resistor?

[jrmymllr]

I've noticed on occasion some designs connect a logic gate input through a resistor to a power rail instead of directly. What is the purpose of this? Is it to avoid some sort of transient short during power-up?

I'm sure this is a super easy question for someone. But in all my years of playing with this stuff I still don't know, probably because I encounter it so infrequently.

[Benta]

Are we talking otherwise unconnected gate inputs?
In that case there's no real electrical reason to do it (ECL logic excepted).
On bidirectional I/Os it may be necessary.
Or, very simply, on old single-sided PCBs it made routing easier.

[langwadt]

Are we talking otherwise unconnected gate inputs?
In that case there's no real electrical reason to do it (ECL logic excepted).
On bidirectional I/Os it may be necessary.
Or, very simply, on old single-sided PCBs it made routing easier.

and it makes it easier to use the gate for a bodge without cutting tracks

[jrmymllr]

Are we talking otherwise unconnected gate inputs?
In that case there's no real electrical reason to do it (ECL logic excepted).
On bidirectional I/Os it may be necessary.
Or, very simply, on old single-sided PCBs it made routing easier.

The specific example I'm thinking of is the schematic for an old Compaq ISA controller card, so it's from the 80s era and double sided at least. They have a 150 ohm resistor network with a single common to ground for multiple gate or other logic IC control inputs. So they are input only, and being a resistor network, definitely doesn't help with routing.   

[artag]

The main reason I know of is for ATE (automatic test equipment).

In the 80s, these were commonly bed-of-nail fixtures that probed every circuit node and exercised the components as individual parts. They were tested against a library so the ATE programmer could mostly just assign the pins and call up a library, but 2 things stopped this : the presence of other signals, especially clocks and feedback, and pins which are tied high or low. If other gates drove them they could be overdriven, but not power and gnd. So a passive pullup or pulldown would allow the part to be fully tested with the standard library.

So you'd go through the board, ensuring that each logic component was in a passive, undriven state. if you couldn't do it, you'd have to use a partial test. True, you'd be testing functionality you didn't need - but a chip with some faulty pins is likely to be shortlived.

I was also told that some generation of TTL could tolerate higher voltages on the supply pins than on the logic pins, and pulling up with reistors was safer. Don't know how true that was.

Neither of these are very good reasons to do it today.

[TimFox]

It was necessary for the original 7400 series of TTL, due to a low breakdown voltage of the input transistors.
Since we don't use that series much anymore, the real reason to include appropriate resistors there is to allow in-circuit test.
If a gate input is directly grounded or connected to the positive rail, the ATE equipment can't apply a test signal to that node.

[james_s]

I've encountered this occasionally on old arcade boards, it's handy sometimes to be able to force a pin low or high to test something and if it's pulled up or down through a resistor you can do this.

[SiliconWizard]

The main points have been given above.

Note that beyond this, it can still be useful 1/ in case we're dealing with an IO that can act either as an input or output, to prevent any potential short-circuit, even if unlikely (if, say, the direction, depends on a software configuration, unlikely is never really unlikely), and 2/ to protect the system in case said IO gets damaged for any reason and the damage causes a permanent short to either rail, which would cause a short on the power supply.