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
}
}
}

}