The alphasign protocol calls for 7 data bits per byte, not the default 8 (See Alpha Sign Communications Protocol, Section 4.4, Protocol version comparison, on page 8)

alphasign/interfaces/local.py

@@ -28,6 +28,7 @@ class Serial(base.BaseInterface):
                                baudrate=4800,
                                parity=serial.PARITY_EVEN,
                                stopbits=serial.STOPBITS_TWO,
+                               bytesize=7,
                                timeout=1,
                                xonxoff=0,
                                rtscts=0)