Setting up and sending data over a serial connection in Python

Question

I have code written in C++ that I am translating to Python. The C++ code is:

    WriteBuffer[0] = unsigned char (0xc0 + (Steer & 0x1F));         // change the signal to conform to jrk motor controller
    WriteBuffer[1] = unsigned char (Steer >> 5) & 0x7F;             //      two-word feedback signal

    if(!WriteFile(hSerial[1], &WriteBuffer, 2, &BytesWritten, NULL)){
        //error occurred. Report to user.
        cout<<"error writing T2 \n";
        cout<<BytesWritten;
    }

The code I have written in Python, which does not work, is this:

rightWheel = bytearray(b'\xc000')
rightWheel[0] = rightWheel[0] + (rightWheelSteer & 0x1F)
rightWheel[1] = (rightWheelSteer >> 5) & 0x7F
rightWheel = bytes(rightWheel)
ser2.write(rightWheel)

The first byte of rightWheel seems to contain the correct data, but the second byte does not. I want rightWheel[1] to contain a single byte, but it does not.

What Python code will let me set up a single byte containing a variable shifted right five bits and then bitwise-anded with 0x7F?

Can you provide example input and output? Your code looks correct and produces sensible results for me. — MisterMiyagi
– MisterMiyagi, Commented Jun 30, 2016 at 13:52
Thank you very much for checking the code. When I went to create example input and output, I found a problem with my test code that was producing the problem. So my code is indeed correct. — Daanii
– Daanii, Commented Jul 9, 2016 at 0:27
Oops, there still was a problem, as noted in my answer below. — Daanii
– Daanii, Commented Jul 9, 2016 at 3:15

Daanii · Accepted Answer · 2016-07-09 03:23:16Z

1

The problem was in my first line of code. I had this:

rightWheel = bytearray(b'\xc000')

But that was making rightWheel into 3 bytes, taking the rightmost two 0s as one-byte ASCII characters for 0 each. So rightWheel became b'\xc0\x30\x30'.

This modified first line of code works correctly:

righWheel = bytearray(b'\xc0\x00')

answered Jul 9, 2016 at 3:23

Daanii

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