M8RXD and M8TXD tracks

What happened to the M8RXD and M8TXD tracks (circled in yellow) on arduino uno pcb ? If this is a timing issue, can anyone with arduino uno board hardware design experience explain to me in details ? It looks to me that M8TXD track is longer than M8RXD track in actual board layout.

Besides, what are the purposes of 1K resistors connected in series along the M8RXD and M8TXD signal lines ?

the funny thing is that on the board there are 2 via holes that are not shown on the screenshot. See here: Can anyone help to confirm the difference?

Don't know about the traces - maybe its to help with tracking pirated boards. 1Ks provide some seperation so D0/D1 can be used with external devices. Where are the unexplained via holes?

What happened to the M8RXD and M8TXD tracks (circled in yellow) on arduino uno pcb ? If this is a timing issue, can anyone with arduino uno board hardware design experience explain to me in details ? It looks to me that M8TXD track is longer than M8RXD track in actual board layout.

Impedance-Controlled Routing

CrossRoads: Don't know about the traces - maybe its to help with tracking pirated boards. 1Ks provide some seperation so D0/D1 can be used with external devices. Where are the unexplained via holes?

The via holes are located at the same place inside the yellow circle on the first picture (schematics screenshot).

Regarding 1K resistor separation from external shield, how does it work explicitly ? The main board or the external shield will get hold of the serial lines first and why???

I have made a mistake. M8RXD track is actually longer than M8TXD track. From my understanding, track impedance should be affected primarily by track width. Both M8RXD and M8TXD tracks have the same width of 12. Anything that I have missed out ?

1K resistors - if there is no USB activity, the 1K's act like pullup resistor to 5V. The '328P can drive them high/low, external devices can drive them high low. If there is USB activity, the 16U2 can drive them high low. If there is both USB and external activity, then you have interfering signals.

Re: traces, via: I don't know. We only 1 Uno, it is secured onto a wheeled platform at the moment, I can't see the bottom to see if the vias connect to anything.

Re: the trace wiggle - also not important at the speeds being used, even at 230400 transfer speed the impedance would not be that critical.

It's another of those oddities we should chalk up to "Italian Style." I'd vote "copy detection", if anything.

westfw: It's another of those oddities we should chalk up to "Italian Style." I'd vote "copy detection", if anything.

How would that work? Anyone can download the Eagle file, with the 'wiggles', and copy it. My cloned Uno has exactly the same 'wiggles'.

Yes, exactly. If you simply manufactured the downloaded files, you'd have the squiggles (but not the vias?) If you actually redesigned the PCB at all, they would probably go away. (I'd remove them!) It's like putting nonsense lines of code in your programs, to check for blatant copies, vs code that is similar but re-written.

Re: traces, via: I don't know. We only 1 Uno, it is secured onto a wheeled platform at the moment, I can't see the bottom to see if the vias connect to anything.

Re: the trace wiggle - also not important at the speeds being used, even at 230400 transfer speed the impedance would not be that critical.

I am a bit confused. What is the purpose of having 1K resistors between '328p and 16U2 when we can have interfering signals if there is both USB(-to-serial or vice-versa) and external (shield serial in and out) activity ? Can you explain this in more details ?

Anyone have any further insights into the track wiggle (both tracks are not equal in length and after reading the impedance-controlled routing article, I felt that making tracks/wiggles longer is worsening the signal integrity ??) and the missing via holes along M8RXD and M8TXD tracks?

The 1k resistors are a sort of "wired OR" between the arduino digital pins and the USB serial pins. If you've connected D0/D1 to logic gates that are operating in 0/+5V, the 1k resistors allow those signals to "swamp" the signal from the USB chip, so it works. If they weren't there, the USB might drive +5V (during idle times) while the external device was driving 0V, and that would be ... not good. The 1k resistors have nothing to do with impedance matching.