openmv和arduino通信，为什么能接收到信息但无法发送信息

[4u1f]

//openmv端代码

import time
from pyb import UART

uart = UART(3, 9600,timeout=10000000,read_buf_len=1)

while True :
     receive = uart.read(1)  #读到一个数据，把一个数据赋值给receive
     print(receive)  #读到8会变成b'8'
     time.sleep_ms(200)
     if receive == b'8' :#与arduino通讯
        print("receive")
        time.sleep_ms(2000)
        uart.write('n')#发送完成信号
        print("transmit")
        break

//arduino端代码
#include <SoftwareSerial.h>
SoftwareSerial BTserial(5,6); // 建立SoftwareSerial对象，RX引脚5, TX引脚6
int b=0;
void setup() {
    Serial.begin(9600);
    BTserial.begin(9600); // HC-06 默认波特率 9600  
  
}

void loop() {
  int a = 1;
  if (a = 1) //a区，收到A板发出的消息，左采摘
  { switch (b) {
      case 0:
        delay(3000);
        BTserial.write("8"); b = 30; //给openmv发一次消息
        Serial.println("transmit");
        delay(1000);
    }
    if (BTserial.available())  //读取open mv的消息
    { char c = BTserial.read();
      Serial.println(c);
      delay(1000);
      if (c == 'n')         
      { 
        delay(1000);
        Serial.println("end");
        b = 0; //b恢复
      }
      }
    }
 
  
}