chore: rewrite to modbus rtu

README.md

@@ -7,36 +7,42 @@ KiCad hardware design files are in the `hw` directory.
 ![photo](img/ch32_extension_photo.jpg)
 ![render](img/ch32_extension.png)
 
-## Communication Protocol
+## comm protocol
 
-### Message Format
+uses Modbus RTU protocol over RS485
 
-The RS485 communication uses a simple 4-byte message format:
+### Config
 
 ```
-Byte 0: Board Address (0x01-0xFE, 0xFF for broadcast)
-Byte 1: Command (0x01 for SET_OUTPUTS)
-Byte 2: Data High Byte
-Byte 3: Data Low Byte
+Baud Rate: 9600
+Data Bits: 8
+Stop Bits: 1
+Parity: None
 ```
 
-### Protocol Constants
+### Modbus
+
+- Function Code: 0x06 (Write Single Register)
+- Default Slave Address: 0x01
+- Register Map:
+  - Register 0x0000: Output Control Register (16 bits for controlling 16 relays)
+
+### Outputs
+
+Each bit in the control register corresponds to one output:
+- Bit 0 controls Output 1
+- Bit 1 controls Output 2
+- etc.
+
+### Msg
 
-```c
-#define BOARD_ADDRESS   0x01        // Default board address
-#define CMD_SET_OUTPUTS 0x01        // Command to set outputs
-#define BROADCAST_ADDR  0xFF        // Broadcast address
 ```
-
-### Example Message
-
-To control the outputs, send a 4-byte message:
-
-```rust
-let message = [
-    address,            // Board address (0xFF for broadcast)
-    CMD_SET_OUTPUTS,    // Command
-    (value >> 8) as u8, // Data high byte
-    value as u8,        // Data low byte
-];
+Byte 0: Slave Address (0x01)
+Byte 1: Function Code (0x06)
+Byte 2: Register Address High (0x00)
+Byte 3: Register Address Low (0x00)
+Byte 4: Data High Byte
+Byte 5: Data Low Byte
+Byte 6: CRC Low Byte
+Byte 7: CRC High Byte
 ```