# Communication Settings

V6 USB serial and SPI protocols use CAN, CAN FD, and RS485 settings stored by the firmware. Linux SocketCAN uses settings configured by Linux. These are separate configuration paths.

| Host method | Where to configure | After a power cycle |
| --- | --- | --- |
| USB serial / SPI | Send `AT+` commands through USB serial | Saved settings remain active |
| SocketCAN | Linux `ip link` | Configure each `canX` interface again |

Send `AT+` commands as text with a `CRLF` line ending. Do not send the characters `\r\n` as ordinary text.

## Factory Defaults

| Item | Default |
| --- | --- |
| Classic CAN | 1 Mbps |
| CAN FD arbitration phase | 1 Mbps |
| CAN FD data phase | 5 Mbps |
| RS485 | 4 Mbps, one stop bit, no parity, eight data bits |
| SPI | SHORT, 23 bytes |
| RS485 RAW replies | Disabled |
| RS485 RAW target | `0`, both RS485 channels on the current MCU |

## Configure CAN and CAN FD

```text
AT+SETFDCAN=<arbitration-bitrate>,<data-bitrate>
```

The first parameter is used for classic CAN and the CAN FD arbitration phase. The second parameter is used only for the CAN FD data phase.

| Parameter | Supported values |
| --- | --- |
| Arbitration phase | `1000000`, `500000`, `250000`, `125000`, `100000` |
| Data phase | `2000000`, `3000000`, `4000000`, `5000000` |

All four CAN FD data rates use exact bit timing with 0% frequency error. Prefer the device's factory default, normally 5 Mbps, and change it only when every device on the bus uses the same setting.

Set classic CAN/CAN FD arbitration to 1 Mbps and CAN FD data to 5 Mbps:

```text
AT+SETFDCAN=1000000,5000000
```

Success returns:

```text
OK
```

Query the current values:

```text
AT+SETFDCAN?
```

Example response:

```text
FDCAN:1000000,5000000
```

The setting is applied to both CAN/CAN FD interfaces on the current MCU and saved to Flash.

## Configure RS485

```text
AT+SETRS485=<bitrate>,<stop-bits>,<parity>,<data-bits>
```

| Parameter | Supported values |
| --- | --- |
| Bitrate | `1200` to `4000000` |
| Stop bits | `1` or `2` |
| Parity | `0`: none; `1`: odd; `2`: even |
| Data bits | `8` or `9` |

Example for Unitree's common 4 Mbps, 8-N-1 setting:

```text
AT+SETRS485=4000000,1,0,8
```

Query the current value:

```text
AT+SETRS485?
```

Example response:

```text
RS485:4000000,1,0,8
```

The setting is applied to both RS485 interfaces on the current MCU and saved to Flash.

## Configure SPI Length

```text
AT+SPIMODE?
AT+SPIMODE=SHORT
AT+SPIMODE=LONG
```

| Mode | SPI transfer | Purpose |
| --- | ---: | --- |
| SHORT | 23 bytes | Classic CAN; up to 18 bytes of RS485 data |
| LONG | 73 bytes | Classic CAN, CAN FD; up to 68 bytes of RS485 data |

Success returns `OK`, and the setting is saved. The SPI host must stop current transfers, wait at least 10 ms, and then use the new fixed length.

## SocketCAN Settings

Linux configures SocketCAN. Classic CAN example:

```bash
sudo ip link set can0 down
sudo ip link set can0 type can bitrate 1000000
sudo ip link set can0 up
```

CAN FD example:

```bash
sudo ip link set can0 down
sudo ip link set can0 type can bitrate 1000000 dbitrate 5000000 fd on
sudo ip link set can0 up
```

Check the result:

```bash
ip -details link show can0
```

SocketCAN, USB serial, and SPI ultimately use the same physical CAN controller. If several host paths reconfigure the same interface, the last setting becomes active. In production, use one host entry point to control a given CAN bus.

## Restore Communication Defaults

```text
AT+CFGERASE
```

Success returns `OK` and restores the defaults on this page. The command clears saved CAN/CAN FD, RS485, SPI mode, RAW target, and power-on batch settings. It does not delete offline batch contents or factory identification.

If repeated configuration changes return `ERR,5`, use this command to clear the communication configuration area and then set the required values again.

## Troubleshooting

| Symptom | Action |
| --- | --- |
| Changing the USB terminal bitrate does not change the bus speed | This is normal for a virtual USB serial port; use the `AT+` commands on this page |
| A configuration command returns nothing | Send it as text with a `CRLF` line ending |
| Command returns `ERR,1` or `ERR,3` | Check the parameter count and allowed values |
| Command returns `ERR,5` | The configuration area is full; run `AT+CFGERASE` and configure it again |
| SocketCAN stops working after reconnecting | Reapply the Linux `ip link` configuration |