# Protocol details

Each message sent or received begins with a `$` (hex 24).  Messages end with a carrage return (`\r` / hex 0D).

### Message types

| Msg char | Hex value | Description |
| -------- | --------- | ----------- |
| r        |  0x72  | Get meter reading |
| j        |  0x6a  | Join the meter |
| m        |  0x6d  | Get mac address (of the MGM111)|
| i        |  0x69  | Get install code |
| f        |  0x66  | Get firmware version |
| d        |  0x64  | Reset - Sent from the ESP32 after holding the reset button for 5 seconds |
| e        |  0x65  | Unknown - Occasionally sent from the MGM111 with the V8 firmware |

## Sending messages

Messages from the ESP to the MGM111 have just the previously mentioned `$` starting delimiter, then a message type character and the 
ending `\r` delimiter.  Each message is therefore exactly 3 bytes long.  For example, to get the a mac address, send the hex bytes
`24 6D 0D` (`$m\r`)

## Receiving responses

Response format bytes:

|  0 |  1 |  2 |  3 | ... | x |
| -- | -- | -- | -- | --- | - |
| `$` | 0x01 | *\<msg type\>* | *\<payload length\>* | *\<payload\>...* | `\r` |

Byte 1, 0x01 indicates that this is a response
Byte 2 is the same messages type that was used to trigger this response
Byte 3 is the length of the payload that follows
Bytes 4+ are the payload of the response
The final byte is the terminator

Ex:  
`24 01 6D 08 11 22 33 44 55 66 77 88 0D`

#### Mac address response payload
The mac address reponse bytes are in reverse order, if the device responds with `11 22 33 44 55 66 77 88` then the mac address is
`88:77:66:55:44:33:22:11`

#### Install code response payload
The install code bytes are not swapped like the mac address payload.

#### Meter join response payload
Seems to return 0x01 even if the Vue is not in range of the meter and seems to return 0x00 before ever being provisioned with the meter. At the least, `1` is good and `0` is bad.

#### Meter reading response payload
The meter response payload has its own page [protocol-meter-reading.md](protocol-meter-reading.md)

#### Unknown 0x65 type
Occasionally sent by the MGM111 with a 1 byte payload that has a value from 0 to 2. No discernible pattern.