Send data with Serial.write() hex values

Question

I'm using ESP32-C3 as hardware and Arduino IDE as my software core.

I receive a data from MQTT server and want to send it to another board using serial port as a hex number. However, it gets sent over serial port as ASCII characters.

Here is the code

unsigned char messageTemp; //data received from MQTT server
unsigned char cups_qty; //data to be transferred to serial port
 
cups_qty = messageTemp;
Serial.write(cups_qty);

For example if I receive messageTemp = 2 and send it to variable cups_qty and then send cups_qty to serial via Serial.write(cups_qty);, I receive the equivalent character not number.

For example :

msg from MQTT >>> messageTemp = 2 
 cups_qty = messageTemp; 
send over serial port >>> Serial.write(cups_qty);

What I see on serial RealTerm terminal

(Display As Ascii) >>> 2
(Display As Hex) >>> 32
(Display As int8) >>> 50 

What I want to see on the terminal:

(Display As Ascii) >>> something unknown
(Display As Hex) >>> 02 
(Display As int8) >>> 2 

The confusing part is when I write something like Serial.write(2), it works just as expected.

• Edgar Bonet: An hex number is, almost by definition, a series of ASCII characters.
• jsotola: The confusing part is when I write something like Serial.write(2), it works just as expected. ... you are doing Serial.write('2') .... the two are not the same ... your question. is not about Arduino ... it is about C++ language
• the busybee: You might want to check by Serial.print(cups_qty, HEX); what data you actually received.

Answer 1

You wrote:

if I receive messageTemp = 2 and send it to variable cups_qty and then send cups_qty to serial via Serial.write(cups_qty);, I receive the equivalent character not number.

This is incorrect. If you Serial.write(2) you will receive, at the other end, a byte with the numeric value 2. Try it!

The function Serial.write() transmits binary numbers as-is, without attempting any conversion. The following lines:

Serial.write('2');
Serial.write(0x32);
Serial.write(50);
Serial.print(2);

all do the same thing: transmit a byte with the numeric value 50. The three expressions '2', 0x32 and 50 are just three ways of writing the same number. Note that, unlike Serial.write(), Serial.print() formats the numbers in ASCII before transmitting.

It looks like your MQTT server sent you a number in ASCII, which you probably did not expect. If you know for sure that this is a single ASCII digit, you can convert it to binary by subtracting the ASCII code for '0', namely 48:

Serial.write(messageTemp - '0'); // send messageTemp in binary

• Eric Matevosian: thank you Edgar for your answer. I did what you have suggested >>> Serial.write(messageTemp - '0'); and it works but only when the received data is just one digit. for example the MQTT server sends 4 payloads>> 2, 80, 4, 5 and after conversion I receive the following over serial port in HEX format >> 0x02 , 0X38, 0x04, 0X05 the data 80 converted to 0x38 which is ASCII format of digit 8! what do you suggest? the incoming data could be 1 , 2 or 3 digits
• Eric Matevosian: for example I want to receive the data 80 as >> 0X50 or the data 90 as >> 0X5A
• Edgar Bonet: @EricMatevosian: You want to read a number that is formatted as a text string. This is called parsing. Search the Web for "Arduino parse number".