import socket
import struct
import time

# 配置信息
SERVER_IP = '127.0.0.1'  # 模拟本地回路
PORT = 502
UNIT_ID = 1
REG_ADDR = 30  # 逻辑 40031

def poll_register():
    # 1. 构造 Modbus TCP 请求 (Function Code 03)
    tid = 0x0001
    pid = 0x0000
    length = 6 # 后续字节长度 (UnitID 1 + PDU 5)
    uid = UNIT_ID
    
    fc = 0x03
    addr = REG_ADDR
    qty = 1
    
    # [MBAP Header] + [PDU]
    request = struct.pack('>HHHB', tid, pid, length, uid) + \
              struct.pack('>BHH', fc, addr, qty)
              
    try:
        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
            s.settimeout(2.0)
            s.connect((SERVER_IP, PORT))
            print(f"[TX] 发送请求: {request.hex(' ')}")
            s.sendall(request)
            
            # 2. 接收响应
            response = s.recv(1024)
            if not response:
                print("[!] 服务器关闭了连接")
                return

            print(f"[RX] 收到响应: {response.hex(' ')}")
            
            if len(response) >= 9: # MBAP(7) + FC(1) + ByteCount(1)
                # 解析响应
                header = response[:7]
                pdu = response[7:]
                
                res_tid, res_pid, res_len, res_uid = struct.unpack('>HHHB', header)
                res_fc = pdu[0]
                byte_count = pdu[1]
                reg_value = struct.unpack('>H', pdu[2:4])[0]
                
                print(f"[+] 解析成功!")
                print(f"    - 事务ID: {res_tid}")
                print(f"    - 寄存器 {REG_ADDR} 的值: {hex(reg_value)} ({reg_value})")
            else:
                print("[!] 响应长度不足")
                
    except Exception as e:
        print(f"[!] 连接错误: {e}")

if __name__ == "__main__":
    print("[*] Python Modbus TCP 客户端模拟启动...")
    poll_register()