>>> from mock import patch
>>> 
>>> p = patch('serial.Serial')
>>> m = p.start()
>>> m.return_value.readline.return_value = (
...     b'\x10\xa8\x00\x00'
...     b'\x00\x00\x00\x00'
...     b'\x10\x00\x00\x01'
...     b'\x00\x00\x00\x01'
...     b'\xca'
... )
>>> 
>>> import serial
>>> 
>>> def get_rpm_speed(ser, cmd):
...     query = bytearray(cmd)
...     query.append((0x100 - sum(query)) & 0xFF)
...     ser.write(query)
...     resp = bytearray(ser.readline())
...     crc = resp.pop()
...     if sum(resp) & 0xFF != crc:
...         raise ValueError('incorrect crc')
...     resp.pop(0)
...     resp.pop(0)
...     h0, h1, h2 = resp[:3]
...     rpm = (h0 * 256 + h1) / 4
...     spd = h2
...     return rpm, spd
... 
>>> ser = serial.Serial('COM3', 9600, timeout=1)
>>> 
>>> rpm, speed = get_rpm_speed(ser, [0x20, 0x05, 0x00, 0x10])
>>> print 'Rpm', rpm
Rpm 0
>>> print 'Speed', speed
Speed 0
>>>