> They don't get told "Mr Copper trace, you are going to do i2c and send XYZ data! and you are going to be a pci express and send ABC!" at the fab. Jeez.
I do only very basic electronics, so I'm asking here in honest confusion: Why can't you figure that out? I'd have expected that part footprints, high-level topology, fine details of individual traces, and the general need for everything to make sense at all would tell you everything you needed to know.
RF optimizations (squiggles to control propagation delay, weird shapes in corners, notches) (edit: wouldn't these give you the bus's frequency too?) and thermal tweaks (wider traces, bigger vias) would give you information about the pinout of whatever's plugged into each footprint (fast, slow, low-power, high-power, etc). Basic topology (is a line connected to two pins or thirty, is a line isolated or is it part of a sixteen-wide or ninety-wide bus) would narrow that down even more. More important parts will almost necessarily have more lines coming out of their footprints so you'd have more information about the stuff you care about.
Even if we assume there're no standards in use ("941 lines, power here here and here, high-frequency bus with 288 lines here, so that's socket AM3, which means these are..."), I wouldn't have surprised if you'd told me you could figure out what part numbers they're dropping into the slots for microprocessors and similar. What am I missing? Why can't you figure out which lines are i2c to misc peripherals and which lines are PCIe between CPU and key coprocessors?
> > They don't get told "Mr Copper trace, you are going to do i2c and send XYZ data! and you are going to be a pci express and send ABC!" at the fab. Jeez.
> I do only very basic electronics, so I'm asking here in honest confusion: Why can't you figure that out?
You can. OP is trying to be snarky, but he's wrong.
I don't think it's worth getting into an argument with someone with such an obviously demeaning attitude ("Dear God," "Jeez," etc.), but a few points:
1. It's not just bare copper, silkscreen frequently tells you what's going on, even in secretive government airplanes.
2. Even without silkscreen, e.g. on assembled boards where the chip part numbers have been lasered off for "secrecy," you can often uniquely identify a chip family just by the pins connected to power/ground, certain passives like crystals or filters, etc.
3. If it's a chip with highly remappable pins where you can't just look up which ones are i2c in a datasheet (e.g. FPGA), you can often find them routed together with obvious strategies for impedance control and/or shielding.
4. For i2c in particular, you will likely see two wires routed together, each with a resistor-like footprint pulling the line high.
Amateurs regularly do stuff like this... It's silly to assume a state level actor couldn't do the same or better.
The parent commenter is being naive. Of course you can make good guesses at which components fit where, and from there, which lines carry which signals.
Most boards in the plane are probably designed to have as little RF sensitivity as possible, and are also shielded.
However, if you control the copper traces, you can definitely influence characteristics of the circuit. Presumably, those would be picked up in testing, because they are looking for those kinds of defects.
But a semi-passive component underneath a trace, constructed to act a resistor/capacitor/inductor once every million pulses might not be.
I do only very basic electronics, so I'm asking here in honest confusion: Why can't you figure that out? I'd have expected that part footprints, high-level topology, fine details of individual traces, and the general need for everything to make sense at all would tell you everything you needed to know.
RF optimizations (squiggles to control propagation delay, weird shapes in corners, notches) (edit: wouldn't these give you the bus's frequency too?) and thermal tweaks (wider traces, bigger vias) would give you information about the pinout of whatever's plugged into each footprint (fast, slow, low-power, high-power, etc). Basic topology (is a line connected to two pins or thirty, is a line isolated or is it part of a sixteen-wide or ninety-wide bus) would narrow that down even more. More important parts will almost necessarily have more lines coming out of their footprints so you'd have more information about the stuff you care about.
Even if we assume there're no standards in use ("941 lines, power here here and here, high-frequency bus with 288 lines here, so that's socket AM3, which means these are..."), I wouldn't have surprised if you'd told me you could figure out what part numbers they're dropping into the slots for microprocessors and similar. What am I missing? Why can't you figure out which lines are i2c to misc peripherals and which lines are PCIe between CPU and key coprocessors?