Python解析固定格式的串行十六进制字符串

Question

I am successfully communicating with a simple device over serial, with a specific request packet, and a fixed format return packet is received. Im starting with python so I can use this on multiple devices, I'm only really used to PHP/C.

For example, I send the following as hex:

12 05 0b 03 1f

and in return I get

12 05 0b 03 1f 12 1D A0 03 18 00 22 00 00 CA D4 4F 00 00 22 D6 99 18 00 70 80 00 80 00 06 06 00 00 D9

I know how the packet is constructed, the first 5 bytes is the data that was sent. The next 3 bytes are an ID, the packet length, and a response code. Its commented in my code here:

import serial, time

ser = serial.Serial(port='COM1', baudrate=9600, timeout=0, parity=serial.PARITY_EVEN,      stopbits=serial.STOPBITS_ONE, bytesize=serial.EIGHTBITS)

while True:
    # Send The Request - 0x12 0x05 0x0B 0x03 0x1F
    ser.write("\x12\x05\x0B\x03\x1F")
    # Clear first 5 bytes (Original Request is Returned)    
    ser.read(5)

    # Response Header (3 bytes)
    # - ID (Always 12)
    # - Packet Length (inc these three)
    # - General Response (a0 = Success, a1 = Busy, ff = Bad Command)
    ResponseHeader = ser.read(3).encode('hex')
    PacketLength = int(ResponseHeader[2:4],16)
    if ResponseHeader[4:6] == "a0":
        # Response Success
        ResponseData = ser.read(PacketLength).encode('hex')
        # Read First Two Bytes        
        Data1 = int(ResponseData[0:4],16)
        print Data1
    else:
        # Clear The Buffer
        RemainingBuffer = ser.inWaiting()
        ser.read(RemainingBuffer)

time.sleep(0.12)

To keep it simple for now, I was just trying to read the first two bytes of the actual response (ResponseData), which should give me the hex 0318. I then want to output that as a decimal =792. The program is meant to run in a continuous loop.

Some of the variables in the packet are one byte, some are two bytes. Although, up to now I'm just getting an error:

ValueError: invalid literal for int() with base 16: ''

I'm guessing this is due to the format of the data/variables I have set, so not sure if I'm even going about this the right way. I just want to read the returned HEX data in byte form and be able to access them on an individual level, so I can format/output them as required.

Is there a better way to do this? Many thanks.

Answer 1

我建议使用struct模块读取二进制数据，而不是使用字符串函数将其重新编码为十六进制并尝试解析十六进制字符串。

Answer 2

As your code stands now, you send binary (not hex) data over the wire, and receive binary (not hex) data back from the device. Then you convert the binary data to hex, only to convert it again to Python variables.

Let's skip the extra conversion step by using struct.unpack :

# UNTESTED
import struct
...
while True:
    # Send The Request - 0x12 0x05 0x0B 0x03 0x1F
    ser.write("\x12\x05\x0B\x03\x1F")
    # Clear first 5 bytes (Original Request is Returned)    
    ser.read(5)

    # Response Header (3 bytes)
    # - ID (Always 12)
    # - Packet Length (inc these three)
    # - General Response (a0 = Success, a1 = Busy, ff = Bad Command)
    ResponseHeader = ser.read(3)
    ID,PacketLength,Response = struct.unpack("!BBB", ResponseHeader)
    if Response == 0xa0:
        # Response Success
        ResponseData = ser.read(PacketLength)
        # Read First Two Bytes        
        Data1 = struct.unpack("!H", ResponseData[0:2])
        print Data1
    else:
        # Clear The Buffer
        RemainingBuffer = ser.inWaiting()
        ser.read(RemainingBuffer)