If you used a straight through serial cable from your PC to the CNC then 
the CNC is likely DCE, since the PC serial is usually DTE. If you used a 
null modem cable then the CNC could be DTE. You didn't specify which cable 
you used.

DCE is Data Communication Equipment and originally was a device like a 
modem, not a Terminal or PC.  DTE, the Data Terminal Equipment was "in 
charge" and could control peripherals like serial modems or printers. 

A modem or DCE device generally just waits there to be told what to do. DTE 
connects to DCE with a "standard" cable which is just straight through 
since the hardware DCE and DTE ports themselves are wired such that the TXD 
from DTE connects to the RXD of DCE and vice versa. To connect DTE to DTE 
or DCE to DCE you need a "null modem" cable which swaps those pins in the 
cable, otherwise you have both TXD connected to each other = won't work.

What UART adapter are you using on BBB? Did I miss that information? 
Apologies if I just added to the confusion.

Bruce


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