TX_UART0 output on ESPrtk follows the message protocol standard:

  • All transmitted data are printable ASCII characters between 0x20 (space) and 0x7e (~)
  • Start the message with 5 bytes “$ESPEL”.
  • Finish the content with a  asterisk (*) and 2 bytes checksum.
  • End the message with 2 bytes <CR> <LF>.

ESPEL meaning : ESPrtk Status Print and Event Log.

Message frame:


Payload:  The content of the message,  after the second comma (,) and before the asterisk (*). The content of the payload does not contain ‘$’ and ‘*’.

Payload_Length: The number of characters of Payload, of type NUMBER (see below), the value in the range [0 -> 32768].

CheckSum  :  a checksum represented as a two-digit hexadecimal number. The checksum is the bitwise exclusive OR of ASCII codes of all characters between the ($ )and  (*).  (does not include $ and * ).

<CR> : “Carriage return”  or ‘\r’ or ‘0x0D in hex’ (13 in decimal) 

<LF> : “Line Feed” or ‘\n’ or ‘0x0A in hex ‘ (10 in decimal)   .

Example for : $ESPEL,28,1,0,0,G-G-G-G,</br> No ERROR*1D<0x0D,0x0A>

<Payload>: <1,0,0,G-G-G-G,</br> No ERROR>

<Payload_Length>: <28>


[CheckSum Range] : [ESPEL,28,1,0,0,G-G-G-G,</br> No ERROR]


Definition of content type representation:

  • Printable ASCII: !”#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~ 
  • [0->9]:  0123456789
  • [a->z]:  abcdefghijklmnopqrstuvwxyz
  • NUMBER: Contains only characters from [0->9], and ‘+’ ‘-‘ ‘.’
  • SENTENCE: Contains only characters [0->9], [a->z],[A->Z], white space, ‘_’ ‘-‘
  • STATE: Contains only characters [0-> 9], [a->z], [A->Z], white space, ‘.’ ‘-‘ ‘+’ ‘_’
  • STRING: All characters range from 0x20 (space) to 0x7e (~), (including commas ‘,’), not including ‘$’ and ‘*’.

Printout ESP32 Status.

Print out information and status about ESP32.

$ESPEL,<Payload_Length>,< TYPE_EL_ESP32>,<Text message>,<<FREE HEAPRAM>*<CheckSum><0x0D,0x0A>

#define TYPE_EL_ESP32  ‘0’

Text message : STRING .

Printout ESPrtk’s error status.


Print out the errors in the stack.

$ESPEL,<Payload_Length>,<TYPE_EL_ERROR>,<Error TYPE  >,<Error in ID  >,<Error in Color>,<Error Text>*<CheckSum><0x0D,0x0A>

#define TYPE_EL_ERROR ‘1’

Error TYPE: NUMBER, [0->9],

  • <1>:UART_error.
  • <2>:INTERNET_error.
  • <3>:MQTT_error.
  • <4>:NTRIP_error.
  • <5>: Other_ERROR_TYPE_1
  • <6>: Other_ERROR_TYPE_2
  • <7>: Other_ERROR_TYPE_3
  • <8>: Other_ERROR_TYPE_4
  • <9>: Other_ERROR_TYPE_5

Error in ID : NUMBER , [0 -> 1024].

Error in Color: SENTENCE – (Contain only : ‘R’,G’,B’,’W’, ‘-‘)

Error Text :  STRING .


$ESPEL,28,1,0,0,G-G-G-G,</br> No ERROR*1D

 Printout a text message .

Print out a text message.

$ESPEL,<Payload_Length>,< TYPE_EL_TEXT>,<Text message>*<CheckSum><0x0D,0x0A>

#define TYPE_EL_TEXT  ‘1’

Text message : STRING .

$ESPEL,21,2,Saved Rtcm:( L1=25)*43

$ESPEL,25,2,State Send to client 0:*35


Printout process.

The process is running on ESPrtk.

$ESPEL,<Payload_Length>,<TYPE_EL_IN_FUN>,<ID_FUN>,<Text message>*<CheckSum><0x0D,0x0A>

#define TYPE_EL_IN_FUN ‘2’


Text message : STRING .

Printout Wifi Scan .

Wifi Hotspot network scan results.

$ESPEL,<Payload_Length>,<TYPE_EL_RESULT_SCAN_WIFI>,<Text message>*<CheckSum><0x0D,0x0A>


Text message : STRING .

Printout queries the file by webconfigure.

To know which file the WebConfigure has queryed.

$ESPEL,<Payload_Length>,<TYPE_EL_WEBSERVER_READ_FILE>,<Text message>*<CheckSum><0x0D,0x0A>


Text message : STRING .


Printout MQTT Broker’s  Status.

Print the MQTT Broker status (MQTT Cloud Server).




<TYPE BROKER_STATUS in ID> : NUMBER trong  [1,7].

  •  <1>:The total number of bytes received since the broker started.
  • <2>:The total number of bytes sent since the broker started.
  • <3>:The number of currently connected clients.
  • <4>:The number of disconnected persistent clients that have been expired and removed through the persistent_client_expiration option.
  • <5>:The maximum number of clients that have been connected to the broker at the same time.
  • <6>:The total number of active and inactive clients currently connected and registered on the broker.
  • <7>:When bridges are configured to/from the broker, common practice is to provide a status topic that indicates the state of the connection. This is provided within $SYS/broker/connection/ by default. If the value of the topic is 1 the connection is active, if 0 then it is not active. See the Bridges section below for more information on bridges.



(Base) Printout Rover’s NMEA  sent to.: 

Notice there is a message NMEA from Rover will print out. Following the message is 1 (or more) messages of NMEA of Rover.

$ESPEL,<Payload_Length>,<TYPE_EL_NMEA_FROM_CLIENT>,<Device ID of Rover>*<CheckSum><0x0D,0x0A>

…<NMEA 1><NMEA 2>..


Device ID of Rover: SENTENCE.

(Base) Printout Rover’s Status sent to: 

Print out other Rover’s status information.

$ESPEL,<Payload_Length>,<TYPE_EL_STATUS_FROM_CLIENT>,<Device ID of Rover>,<State 1> ,<State 2>,<State 3>,<State 4>,<State 5>*<CheckSum><0x0D,0x0A>


Device ID of Rover: SENTENCE .

State 1, State 2, State 3, State 4, State 5 : STATE.

(Rover) Printout Base’s Status sent to: 

Print other information status of Base.

$ESPEL,<Payload_Length>,<TYPE_EL_STATUS_FROM_MASTER>,<Device ID of Base>,<State 1> ,<State 2>,<State 3>,<State 4>,<State 5>*<CheckSum><0x0D,0x0A>


Device ID of Base: SENTENCE .

State 1, State 2, State 3, State 4, State 5 : STATE.

Printout IMU data from MPU9250 sensor.


  • GX,GY,GZ : Raw Gyroscope 3 Axis (Degree/second)
  • MX,MY,MZ :  Raw Magnetometer 3 Axis (uT) (micro-Tesla)
  • AX,AY,AZ : Raw Accelerometer 3 Axis ( scale 0.1 m/s^2)
  • T :  Temperature sensor (°C)    (degrees Celsius)
  • MXc,MYc,MZc : Magnetometer 3 Axis + Calibrated.

$EPIM2, QW,QX,QY,QZ, Yaw,Pitch,Roll, H, FQW,FQX,FQY,FQZ, FYaw,FPitch,FRoll, FH,*  <CS><CR><LF>

  • QW,QX,QY,QZ : DMP-Quaternion.
  • Yaw,Pitch,Roll: DMP-Euler 3 Axis. (Degree)
  • H :Compass Heading (using magnetometer calibrated) + Tilt compensation. (Degree)
  • FQW,FQX,FQY,FQZ : DMP-Quaternion + Kalman Filter.
  • FYaw,FPitch,FRoll : DMP-Euler 3 Axis + Kalman Filter. (Degree)
  • FH :Compass Heading (using magnetometer calibrated) + Tilt compensation + Kalman Filter (Degree)


Calculate checksum in C:

#include <stdio.h>

int checksum(const char *s) {
    int c = 0;
        c ^= *s++;
    return c;

int main()
    char mystring[] = "ESPEL,28,1,0,0,G-G-G-G,</br> No ERROR";
    printf("Checksum: 0x%02X\n", checksum(mystring));
    // result  -> Checksum:0x1D 
    return 0;