How to convert 7f => 02 hex

Question

I'm talking with servo by "Modbus RTU", but to understand what exactly happen I need to print received hex numbers with proper transformation like below.

Convert receiving values in this way:

1. hex 7f convert to bin gives(01111111)
2. next negate the value gives(10000000)
3. rol 2-place gives(00000010)
4. and back to hex gives(02)

This what come from servo after home command:

007f7d5fefff858f007f7d5f2fe0076f0000000000000000000000000000

Here is my code:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import serial
import time
import numpy

port= serial.Serial('/dev/ttyS0', 9600, timeout=2,   
xonxoff=False, rtscts=False, dsrdtr=False) 
port.flushInput()
port.flushOutput()
time.sleep(0.01)
if port.isOpen():
    print(port.name + ' is open...')
    flag = port.isOpen()
    print flag
    #MODBUS_ACTIVE=b"\x01\x05\x04\x27\xFF\x00\x3D\x01"  
    #port.write(MODBUS_ACTIVE)
    #print    
    "sending=>MODBUS_ACTIVE<=>",MODBUS_ACTIVE.encode("hex")
    time.sleep(0.01)
    command=raw_input('Enter your command ie.home: ')
    if( command == 'home' ):
    HOME_INITIAL=b"\x01\x05\x04\x0B\x00\x00\xBD\x38"      
    #command home first time
    port.write(HOME_INITIAL)
    print "sending=>HOME",HOME_INITIAL.encode("hex")
    time.sleep(0.5)
    HOME_FINAL=b"\x01\x05\x04\x0B\xFF\x00\xFC\xC8"   
    #command home second time
    port.write(HOME_FINAL)
    time.sleep(0.5)
    #While True:
    rcv = port.read(30)
    print type(rcv)
    rcv = rcv.encode('hex')
    #rcv = int(rcv,16)
    #rcv = "{0:012b}".format(0xfff ^ rcv)
    #numpy.right_shift(rcv, 30)
    #format(rcv if rcv >= 0 else (1 << 16) + rcv, '016b'
    #binary_repr(rcv)
    #time.sleep(0.1)
    #rcv = bin(rcv^1)
    #print bin(str(rcv))
    #''.join(format(ord(c), '08b') for c in 'rcv')
    #rcv = str(rcv)
    #rcv = rcv >>= 1
    #print type(rcv)
    #print int('rcv')+1
    #print 'rcv'
    #int(''.join(bin(x)[:1:-1]), 2)
    #bin((i ^ (2 ** (i.bit_length()+1) - 1)))[3:]
    #bin((rcv ^ (2 ** (rcv.bit_length()+1) - 1)))[3:]
    #bin((int(s, 2) ^ (2**(len(s)+1) - 1)))[3:]
    #bin((int(rcv, 16) ^ (2**(len(rcv)+1) - 1)))[3:]
    #s.replace('1', '2').replace('0', '1').replace('2', '0')
    #rcv.replace('1', '0').replace('0', '1').replace('2',     '0')
    #bin((rcv ^ (2 ** (rcv.bit_length()+1) - 1)))[3:]
    print ''.join('1' if x == '0' else '0' for x in 'rcv')
    #flip = {'1':'0', '0':'1'}; ''.join(flip[b] for b in s)
    #rcv = {'1':'0', '0':'1'}; ''.join(rcv[b] for b in rcv)
    #print "{0:012b}".format(2**12-1-a)


    #result = int(a, 16) ^ int(b, 16) # convert to integers     and xor them 
    #return '{:x}'.format(result)     # convert back to     hexadecimal
    #result = int(rcv, 16) ^ int(rcv, 16)
    #print rcv.encode('hex')
    #print format('rcv')
    #print (rcv)
    command=raw_input('Enter your command ie.home: ')

As you can see I tried many ways to print on the screen the converted Value, but without good result.

Answer 1

From the inconsistent usage of print in your code (some with and some without parentheses) it appears that you are using Python 2.7. Using encode to convert to hex is probably not a good idea, and it might only work in Python 3.

Instead use the binascii module to get the convert the bytes read from the serial port. See binascii

>>> binascii.hexlify('test')
'74657374'

If you wish to do the bit manipulation yourself, you could use the example code below. Note that the bits are shifted right six bits rather than left 2 bits with wrapping, so that you don't have to worry about integer conversion from a single byte.

>>> abyte = 0x7f
>>> bin(abyte)
'0b1111111'
>>> b = abs(~abyte)
>>> bin(b)
'0b10000000'
>>> c = b >> 6
>>> bin(c)
'0b10'
>>> hex(c)
'0x2'