by Posix driver I meant the Linux implementation in the serial.nim library. You could ask the maintainer to add your baud rate but is probably a bad idea.
1) You say the serial device "is locked" at this rate. Which serial device: your RPi or the linked peer ? 2) I doubt is actually locked, after all is just a value sent to the UART hardware/chipset supporting driver. Let's suppose you run Python on RPi and talk over serial (from your test script) to a Windows box, like TerraTerm or Putty serial port, Kermit, miniTerm or whatever have you on the client end. What's the baud rate of the serial port reported/set on that box? (if using a USB to serial adapter dongle, things may be hidden or rather irrelevant). 3) A wrong or non standard value (250.000 is a bit unusual) will probaby be ignored by the Python implementation (not very sure what it does/details). It may fallback to default value, or round down, or actually use it, or ignore the UART driver response on a stange value? Again, is implementation specific. This may give the caller the "working" state result. 4) advice at this point is try using _a standard value_ (ref: [https://en.wikipedia.org/wiki/Serial_port#Speed](https://en.wikipedia.org/wiki/Serial_port#Speed)) and make sure both ends are set the same. Then try increasing it (again using standard values) till peers no longer able to talk to each other. P.S. watch out that "Handshake.None", it may lead to discarding data if the other side pumps data like crazy. Perhaps use wanted hardware hand-shaking instead? Good luck. Lucian