The timing of switching from the network transparent transmission mode and the protocol transparent transmission mode to the "AT command mode":
1. The serial device sends “+++” to the device continuously. After the device receives “+++”, it will send an ‘a’ to the device. No data can be sent during a packing cycle before "+++" is sent.
2. When the device receives ‘a’, it must send an ‘a’ to the device within 3 seconds.
3. After receiving the ‘a’, the device sends “+ok” to the device and enters “temporary command mode”.
4. After receiving the "+ok", the device knows that the device has entered the "temporary command mode" and can send an AT command to it.
Switching from AT command mode to network transparent transmission and protocol transparent transmission mode:
1. The serial device sends the command “AT+ENTM” to the device.
2. After receiving the command, the device sends “OK” to the device and returns to the previous working mode.
3. After the device receives “OK”, it knows that the device has returned to the previous working mode.