MODBUS CRC Generation

I've been trying to understand the implementation of CRC on a datasream that I send to and from an ESP32 via standard Bluetooth. Without having the technical knowledge to fully understand the CRC process, I've been able to implement (read:uncover) code that would appear to be common between the ESP32 and App Inventor. Namely, this is the link that has bee referred to by many:

I have managed to get this working on the ESP32 for a string i'm sending:

#define UInt16 uint16_t
// CRC should be DD18
char *t = (char *)"151.45671118|33.5678|0|123.4|A|0000000|12.3|16%";
// Compute the MODBUS RTU CRC
UInt16 ModRTU_CRC(char * buf, int len)
  UInt16 crc = 0xFFFF;
  for (int pos = 0; pos < len; pos++) {
    crc ^= (UInt16)buf[pos];          // XOR byte into least sig. byte of crc
    for (int i = 8; i != 0; i--) {    // Loop over each bit
      if ((crc & 0x0001) != 0) {      // If the LSB is set
        crc >>= 1;                    // Shift right and XOR 0xA001
        crc ^= 0xA001;
      else                            // Else LSB is not set
        crc >>= 1;                    // Just shift right
  // Note, this number has low and high bytes swapped, so use it accordingly (or swap bytes)
  return crc;  

void setup()

@ABG has created an untested block set that i'll base my evolving understanding on here:

BUT. I don't just want to get the answer need without understanding how I get to the result I need. I'm still trying to understand the char *t = (char *) component at the start of the ESP32 code. Any help here or pointers to help me best understand this process would be appreciated.

Just as another note to this. I feel like I need to get a better grip on the basics of variables in all their different forms before I start playing with this too much.

I'm currently working with strings and arrays in arduino environment and I haven't quite grasped that either. So to try and bring that half-baked knowledge into this question right now might be a leap too far.

You are wise to treat cautiously.

I would start with

When learning data types, it is important to know

  • how many bytes are used?
  • how are negative values stored?
  • are the values big-endian or little-endian?
  • Is the data type composed of more than one smaller data types?
  • can the data type expand dynamically to cover more storage?
  • does the data type contain pointers to other data in memory?
  • is the encoding binary or decimal, if it is a number?
  • is the encoding of text simple ASCII, or is it extended to a multi-byte scheme like UTF-8 ?

AI2 tries to gloss over these questions, but primordial cave languages like C expose all the innards.

Thanks @ABG,

The guys over on the Arduino forums have helped me better understand how I was poorly constructing my Strings and i've now better understand how i'm actually combining doubles, floats, const chars, and ints into a string. I can now parse that string well on the ESP32, and while I still don't fully follow the maths on the CRC code (or the CRC process in general), I am getting repeatable and expected results.

Moving back over to AI2, I now understand the code (or the flow of code) you'd prepared in some older posts i've noted above. I've been able to modify this to interpret a broader range of messages than comma separated messages. I also realised the code relied on ASCII decimal entries, so to resolve this i've used the Xoma ASCII to decimal extension. This is the aia:

MODBUS_CRC16.aia (8.6 KB)

My next challenge is to resolve how to split the last value from a delimited text string. eg:


Noting the last value in the text string can be of various lengths.

Slightly more complicated than I thought. Here is my second attempt (after my first attempt failed)!

Uses the Uno Double Reverse to get rid of the last "|" delimiter.
Then it deletes the inverted )( brackets by splitting out the middle between those first and last characters.
Then breaks the split text into a list at the " ".
And finally picks the first item of that list.

I'm sure there's a simpler, more elegant way of doing it, but for now it's working.

Sorry, this isn't really the intent of this original post title.

Thank you. Nice procedures.

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