So back in the day ducks under thrown bottles and tomatoes we had the standard 4000 series on 3V-18V and life was grand. The CMOS chips, less grand.
Then we had the 7400 series around then too, and boy howdy did that product line proliferate. The BJT TTL families got in so many things, but it demanded Vcc of 5V +-5% and I think it is safe to say anything expected to work with those chips standardized on 5V. Great, everyone is agre— ah.
74C is a CMOS line that happily uses 3V-15V. Ok, we can ignore that one it's just a... Nevermind, 74HC and friends do 2V-6V, neat. But it still works with the 5V so we're great.
So, CMOS got better, we got gates down to 3V or less with competitive propagation timing, and it turns out that less power use is good for clocking, but we can't actually feed anything 3V because of voltage drop so it's 10% higher cause... someone figured that was a nice round number or something?
As for why it keeps dropping... Thinner conductor channels, thinner insulation, you do the math. Yes, power consumption with more gates is a reason too, but I'm pinning the biggest blame badge on decreasing sizes in the process nodes.
Oh, you know all that already and want to know about just the sensor/interconnect split? My bad. Line drop, afaik. 3.3V signalling is fine on a PCB or when your wires are short. Want to move your sensors further away? Maybe 5V will be a tad more reliable. That sensor over 20m away? Sure, 24V sounds appealing. I don't know the exact lengths where voltage drop on the wiring is enough to cause issues but I'm pretty sure that kind of reasoning in why it persists beyond simple logic compatibility.
As a secondary concern of the same form, non-differential protocols are voltages you have knowledge and control of, the various kinds of line noise less so. A data line offset by a volt is more problematic at 3.3V than 24V
Oh and if I'm driving a small industrial motor or actuator, or a 400V rated relay, I definitely want to be doing so with more enthusiasm than 5V signifies. I also want less enthusiasm than electrocution cause I touched a logic line. 24V will (just about) generally not give you a meaningful shock unless you try to lick it.
Then we had the 7400 series around then too, and boy howdy did that product line proliferate. The BJT TTL families got in so many things, but it demanded Vcc of 5V +-5% and I think it is safe to say anything expected to work with those chips standardized on 5V. Great, everyone is agre— ah.
74C is a CMOS line that happily uses 3V-15V. Ok, we can ignore that one it's just a... Nevermind, 74HC and friends do 2V-6V, neat. But it still works with the 5V so we're great.
So, CMOS got better, we got gates down to 3V or less with competitive propagation timing, and it turns out that less power use is good for clocking, but we can't actually feed anything 3V because of voltage drop so it's 10% higher cause... someone figured that was a nice round number or something? As for why it keeps dropping... Thinner conductor channels, thinner insulation, you do the math. Yes, power consumption with more gates is a reason too, but I'm pinning the biggest blame badge on decreasing sizes in the process nodes.
Oh, you know all that already and want to know about just the sensor/interconnect split? My bad. Line drop, afaik. 3.3V signalling is fine on a PCB or when your wires are short. Want to move your sensors further away? Maybe 5V will be a tad more reliable. That sensor over 20m away? Sure, 24V sounds appealing. I don't know the exact lengths where voltage drop on the wiring is enough to cause issues but I'm pretty sure that kind of reasoning in why it persists beyond simple logic compatibility.
As a secondary concern of the same form, non-differential protocols are voltages you have knowledge and control of, the various kinds of line noise less so. A data line offset by a volt is more problematic at 3.3V than 24V
Oh and if I'm driving a small industrial motor or actuator, or a 400V rated relay, I definitely want to be doing so with more enthusiasm than 5V signifies. I also want less enthusiasm than electrocution cause I touched a logic line. 24V will (just about) generally not give you a meaningful shock unless you try to lick it.
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