Justin Jansen

UART stop bit framing issue

Discussion created by Justin Jansen on Aug 18, 2016

We are having compatibility trouble with a modbus device we are trying to interface with a KE06 processor over RS485 2-wire.  It appears that the Kinetis UART samples the RX line 3 times and if one of the other samples doesn't match, it throws away the byte due to noise in the processor expert generated drivers.  The reason that this is a problem with us with this particular modbus device is that they are turning off their RS485 driver before the last stop bit in their message is complete.


So, we have modified the drivers to not throw away the byte, but I guess my question would be should the byte really be flagged as "noisy" if the data and parity were fine?


Also, it appears as though changing it to 2 stop bits also works around the issue.  They are effectively transmitting 1.5 stop bits in 2 stop bit mode, but the kinetis UART appears to be ok with that, so it must only be checking if the first stop bit is "noisy" in 2 stop bit mode.