A Practical Guide to Custom ESP32 Digital Walkie-Talkies (Part 3): Deconstructing the Serial Control Protocol

Sep . 2025

By sdga:

The key to effective communication with the module is to correctly implement its serial control protocol. This protocol uses a binary frame format, and all parameter configuration and status queries are accomplished by sending and receiving specific data frames.

Frame Structure Analysis

Serial Protocol Frame Structure

Serial Protocol Frame Structure Example

Each data frame follows a fixed structure, consisting of a header, command, data, and tail, among other parts.

Table 3: Serial Protocol Frame Structure

Offset (Bytes)	Field	Length (Bytes)	Description
0	Head	1	Frame header, fixed at 0x68.
1	CMD	1	Command word, defines the function of the frame, such as setting frequency, sending SMS, etc.
2	R/W	1	Read/Write flag. 0x00=Read, 0x01=Write, 0x02=Module-initiated report.
3	S/R	1	Set/Response flag. When sent by the host, it is a set request; when replied by the module, it is a response status.
4-5	CKSUM	2	16-bit checksum. Covers all bytes from CMD to the end of DATA.
6-7	LEN	2	Data length of the DATA field (in bytes).
8...	DATA	n (determined by LEN)	Data payload. The specific content is defined by the CMD.
8+n	TAIL	1	Frame tail, fixed at 0x10.

Checksum Calculation Method

According to the protocol document, the checksum is calculated as follows: each pair of bytes in the data to be checked is formed into a 16-bit number and then added sequentially. If the data frame has an odd length, 0x00 is appended to the last byte. The carry-over part (exceeding 16 bits) from the accumulation process is added back to the lower 16 bits of the result until the sum is less than or equal to 0xFFFF. Finally, the resulting 16-bit number is XORed with 0xFFFF to get the final checksum.

The reference C code provided in the protocol document is as follows:

uint16_t PcCheckSum(uint8_t *buf, int16_t len) {
uint32_t sum = 0;
while (len > 1) {
sum += 0xFFFF & ((*buf << 8) | *(buf + 1));
buf += 2;
len -= 2;
}
if (len) {
sum += (0xFF & *buf) << 8;
}
while (sum >> 16) {
sum = (sum & 0xFFFF) + (sum >> 16);
}
return ((uint16_t)sum ^ 0xFFFF);
}

Checksum Calculation Flow Example

A noteworthy engineering detail is that the protocol document mentions that if the user does not want to calculate the checksum, the CKSUM field can be sent as 0x0000, and the module will ignore the checksum process. This can be very useful during the initial debugging phase, but for the sake of communication reliability, the correct checksum should be implemented and used in the final product.

Complete Command Set Reference

The following table categorizes and organizes all the commands supported by the module, providing a more structured reference than the original documentation.

Table 4: DMR858M Command Code (CMD) Complete Reference

CMD (Hex)	Function Description	R/W Support	Scope	Persistent	Notes
Configuration Commands (Saved on Power-off)
0x01	Change Channel	Write	Current	Yes	Switch to the specified channel.
0x02	Set Receive Volume	Write	All	Yes	Set the audio output volume level.
0x0B	Set Microphone Gain	Write	All	Yes	Adjust microphone sensitivity.
0x0C	Set Power Save Mode	Write	All	Yes	Enable or disable low-power mode.
0x0D	Set TX/RX Frequency	Read/Write	Current	Yes	Set the receive and transmit frequencies for the current channel.
0x12	Set Squelch (SQ) Level	Read/Write	Current	Yes	Set the squelch threshold for analog mode.
0x13	Set CTCSS/CDCSS Mode	Read/Write	Current	Yes	Set the sub-audio mode (e.g., receive only, transmit only, both).
0x14	Set CTCSS/CDCSS Value	Read/Write	Current	Yes	Set the specific sub-audio code.
0x17	Set High/Low Power	Read/Write	Current	Yes	Switch the transmit power for the current channel.
Operational Commands (Effective Immediately)
0x03	Scan	Write	Current	No	Start or stop channel scanning.
0x06	Initiate Call	Write	Current	No	Initiate a group or private call.
0x07	Send SMS	Write	Current	No	Send a DMR text message.
0x09	Emergency Alarm	Write	Current	No	Trigger the emergency alarm function.
0x15	Monitor Switch	Write	Current	No	Force open the squelch to monitor channel activity.
Status Query Commands
0x04	Check TX/RX Status	Read	Current	No	Query if the module is in receive, transmit, or idle state.
0x05	Read Signal Strength	Read	Current	No	Get the RSSI value of the current received signal.
0x24	Read ID	Read	All	No	Read the module's DMR ID.
0x25	Read Firmware Version	Read	All	No	Read the module's firmware version number.
0x28	Check Encryption Status	Read	Current	No	Query if encryption is enabled on the current channel.