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XMODEM protocol implementation

Documentation available at

Python Package Index (PyPI) page is available at


Create a function to get and put character data (to a serial line for example):

>>> import serial
>>> from xmodem import XMODEM
>>> ser = serial.Serial('/dev/ttyUSB0', timeout=0) # or whatever port you need
>>> def getc(size, timeout=1):
...     return or None
>>> def putc(data, timeout=1):
...     return ser.write(data)  # note that this ignores the timeout
>>> modem = XMODEM(getc, putc)

Now, to upload a file, use the send method:

>>> stream = open('/etc/fstab', 'rb')
>>> modem.send(stream)

To download a file, use the recv method:

>>> stream = open('output', 'wb')
>>> modem.recv(stream)

For more information, take a look at the documentation.


  • bugfix: Abort send on EOT in startup-sequence Issue #34.
  • enhancement: Include LICENSE file in the source distribution.
  • bugfix: Remove bogus assert False code in recv() that resulted in AssertionError introduced in version 0.4.0 commit-id 9b03fc20, PR #29.
  • bugfix: Large file transfers in send() were more likely to fail for small values of retry: This value should be the maximum failures per block transfer as documented, but was improperly implemented as the number of failures allowed for the total duration of the transfer, PR #21.
  • bugfix: send(retry=n) and recv(retry=n) should retry n times as documented, was retrying n - 1.
  • bugfix: putc() callback was called in series, 3 times for each part of xmodem block header, data, and checksum during block transfer. Now all three data blocks are sent by single putc() call. This resolves issues when integrating with microcontrollers or equipment sensitive to timing issues at stream boundaries, PR #19.
  • bugfix: documentation files missing from the release tarball Issue #16.
  • bugfix: re-transmit in send() on NAK or timeout, previously re-transmissions (wrongly) occurred only on garbage bytes. PR #12.
  • enhancement: support for python 3 PR #8.
  • bugfix: CRC failures in XMODEM.recv() were not renegotiated correctly PR #11.