# CH32-16-CH-EXT

A simple CH32V003-based extension board providing 16 outputs (2x 74HC595), primarily designed for controlling [Chinese 16-relay modules](https://www.uctronics.com/download/Amazon/U604302_print.pdf) ([archive](https://web.archive.org/web/20220709045418/https://www.uctronics.com/download/Amazon/U604302_print.pdf)) over RS485. Uses 64-byte internal flash page for state persistence.

KiCad hardware design files are in the `hw` directory.

![photo](img/ch32_extension_photo.jpg)
![render](img/ch32_extension.png)

## comm protocol

uses Modbus RTU protocol over RS485

### Config

```
Baud Rate: 9600
Data Bits: 8
Stop Bits: 1
Parity: None
```

### 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

```
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
```