Booting TX1 with serial cable connected

I have two TX1’s. One purchased a year ago, another just a couple months ago. Both have serial cables connected to UART0 on the J21 header. My newer TX1 will boot but the older one won’t, unless I remove the serial cable. The lights turn on but I cannot ping it and no display shows.

I have read the posts about editing U-boot but I have not done anything like this on my newer TX1, and I am not sending anything on UART0, only listening for the boot messages.

I suppose it is possible that the default “no activity” levels for TX/RX are close to some threshold and noise makes it look like there was activity (any activity on that UART will cause U-Boot to stop at command line). Is there any exposed wiring which is not either shielded or twisted pair? If any wiring is exposed, could you try twisting TX/RX, and perhaps also twisting CTS/DTSRTS?

EDIT: DTS is mistake…is RTS…Clear To Send and Request to Send. These work together if hardware flow control.

Both cables are ~2ft long shielded twisted pair with roughly 5" being exposed (I stripped away the shielding). What do you mean by CTS/DTS, I’m not familiar with those?

If this is 3 wires, those wires will be RX, TX, and GND. If 5 wires, then there will also be CTS and RTS for hardware flow control (these are optional…you might not have them).

Of the 5 inches, can you twist the TX/RX pair?

NOTE: I edited above where I mistakenly wrote “DTS”, it is “RTS” (CTS was correct).