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Daedalus · 11 October 2017, 22:17

Yep, it's standard practice for TTL stuff. They require more current to drive the output low than high, so tying high reduces power consumption, which can be substantial if you're talking about a lot of inputs. And the resistor is required because 5V is fairly close to the breakdown voltage of the junctions in the original designs, meaning that tiny high voltage spikes from switching or other electrical noise can, over time, damage the gate resulting in an always on or always off input. A resistor ensures the input can deal with a higher input voltage and so the spikes have less impact. It's not needed when connected to another logic output because the output isn't normally as high as 5V, and carries less noise. For this reason, a common technique is to drive inputs from other, unused outputs.

The value of the resistor isn't hugely important - anything between 1K and 50K is probably fine. Original 74 series logic probably needs 1K, but they're long obsolete, and even the old LS series uses 10K as a rule of thumb for pull-ups.

CMOS chips work totally differently and function more like op-amps that are saturated, so their input impedance is extremely high and they don't require much current at all to drive in input to either state. So tying either high or low is fine. And no resistor is needed because there are no bipolar junctions at close to breakdown voltage, so they can tolerate a wide range of input voltages. Many TTL-style chips are actually using CMOS technology in their cores (e.g. 74HC series), so they don't actually need the resistor or the pulling high, but it's no harm to follow those guidelines, especially when you're not sure what technology the chip uses.

I'm sure there are plenty of references online for it, but I'm a fan of old-school technical books. I'm pretty sure I got that information from my 20-year-old and very well worn copy of Practical Electronic Design Data. It's a Babani book, and they're really quite excellent references.

Edit: Here it is. Out of print but 2nd hand copies appear to be cheap. Can't recommend it enough as a handy reference for all your electronics tinkering needs.

11 October 2017, 22:17	#16
Daedalus Registered User Join Date: Jun 2009 Location: Dublin, then Glasgow Posts: 6,334	Yep, it's standard practice for TTL stuff. They require more current to drive the output low than high, so tying high reduces power consumption, which can be substantial if you're talking about a lot of inputs. And the resistor is required because 5V is fairly close to the breakdown voltage of the junctions in the original designs, meaning that tiny high voltage spikes from switching or other electrical noise can, over time, damage the gate resulting in an always on or always off input. A resistor ensures the input can deal with a higher input voltage and so the spikes have less impact. It's not needed when connected to another logic output because the output isn't normally as high as 5V, and carries less noise. For this reason, a common technique is to drive inputs from other, unused outputs. The value of the resistor isn't hugely important - anything between 1K and 50K is probably fine. Original 74 series logic probably needs 1K, but they're long obsolete, and even the old LS series uses 10K as a rule of thumb for pull-ups. CMOS chips work totally differently and function more like op-amps that are saturated, so their input impedance is extremely high and they don't require much current at all to drive in input to either state. So tying either high or low is fine. And no resistor is needed because there are no bipolar junctions at close to breakdown voltage, so they can tolerate a wide range of input voltages. Many TTL-style chips are actually using CMOS technology in their cores (e.g. 74HC series), so they don't actually need the resistor or the pulling high, but it's no harm to follow those guidelines, especially when you're not sure what technology the chip uses. I'm sure there are plenty of references online for it, but I'm a fan of old-school technical books. I'm pretty sure I got that information from my 20-year-old and very well worn copy of Practical Electronic Design Data. It's a Babani book, and they're really quite excellent references. Edit: Here it is. Out of print but 2nd hand copies appear to be cheap. Can't recommend it enough as a handy reference for all your electronics tinkering needs. Last edited by Daedalus; 11 October 2017 at 22:29.