In 80s-90s there were popular symbols (specially TTL gates) having power pins hidden. Even now rather not used KiCad allows for such definitions. I suppose that power symbols take something from such symbols but having only the power pin alone. In those symbols the name (like 7400) has nothing to net used by its power pins. So when using this to define power symbol it looks that pin net definition is hidden as changing the symbol name don’t change net.
From what I have read (but not checked) in V8 the power symbols will work little easier. You will be able to place at schematic for example VCC power symbol and rename it to any other name and it will be connected to this new net (now after such modification it is still connected to VCC).
I think the hidden power symbols came from a time where there was one voltage rail on the whole board + 5V.
It’s a good idea to have all that visible, - why ? because you can place next to that power pin on schematic (or , indeed attach to that pin) a bypass cap, so that this explicitly reminds you or the layout person that this cap needs to go on the chip.
You may be right about the situation about only having 5V on the board. Though I have to say I designed boards in the mid 80ies and ever since. I never had such a board. I suppose it depends on the industry you work in. I always worked with mixed signals and thus always had at least two voltages on my boards, typically more.
I can appreciate the aspect of trying to hide away the power pins. They seriously tend to get in the way of conveying the essence of a circuit and add no information of their own. I typically add an extra gate to each chip whose sole purpose is to hold all the power pins. These gates get shoved into a corner (or more typically the bottom) of the schematic and connected to the appropriate supply rails. The bypass caps get then placed next to those gates as you describe. I feel this gets the supply situation taken care of in a neat way and keeps it out of the way of the actual function of the circuit.
In 80s I have designed a TTL frequency counter (for my own use). I have there (it is still working) 3 PCBs having only 5V at them and separate (shielded) input amplifier to allow input up to 100MHz (it was very high frequency for me those time) powered from different voltages.
Those times I have also designed analog oscilloscope (it was the only way for me to have the oscilloscope) and among other PCBs it had a time base trigger system whole made based on TTL ICs. This one PCB there also had only 5V on board.
First serious money I earned being a student (equal to my dad’s two month salary) was for doing chronocomparator (you put electronic watch at its table and it shows how accurate its clock is). It was whole (or at least most) powered from 5V - it was also based on TTL.
All digital those time was powered from 5V so I think in all only (or mainly) digital devices having only 5V at boards was popular.
Cool projects. Sounds like a lot of fun. TTL wasn’t much of a thing anymore in the 80s, but that might depend on the place you worked at. I worked with that stuff in the 70s. In the 80s I worked in the medical device industry. As I wrote, my projects there were mostly mixed signal and I used Motorola 4000 logic a lot, running on 15V and analog circuitry on +/- 15V. Embedded computers were running on 5V (80186 based), so the interface logic was 5V.
Nah TTL never died. It just went CMOS, begatting HC and HCT families but the multi-emitter input is no more. With the single gate variants it’s still used in glue logic. But the 5V supply rail is fading. Even 3.3V is getting long in the tooth.
Back in the 70s, logic chips were mostly very simple and many circuits became a real rats nest. Tracing the connections was bad enough without power wires all over the schematic. Fortunately those days are gone.
or dual triple NOR gates
The schematics for the Apollo Guidance Computers have been redrawn in kicad, BTW
https://www.ibiblio.org/apollo/ElectroMechanical.html#Electrical_Drawings_&gsc.tab=0
1 Like
RTL logic. I have never used RTL or DTL, starting with the original 7400 TTL
Poland - not belonging to the free world at that time and it was early 80s. A currency with no real value. Imported elements were incomparably more expensive than domestic ones, and the domestic industry could barely make TTL.
I always hated that approach, specifically if the sheet has multiple chips, opamps etc. in it, maybe even with different voltage supplies, the schematic gets really hard to read imo. I like my power ports on my main symbol.
2 Likes
I believe that global signals should be discouraged. Power and ground signals should not be handled as a special global class. Rather they should be wired, down through the hierarchy, just like every other signal. This is especially true of newer designs with so many supply voltages. It is nice to see where they go at a glance.
1 Like
I agree.
You ramble use the power symbols.
I use labels and PWR_FLAGs
Thanks @paulvdh for the really clear explanation. I was stuck on things like a Eurorack power connector (which has 10 pins: two for -12V, two for +12V and the rest for GND) and howto declare them without KiCad complaining.
This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.