DAP_UART_Transfer (0x21):

Receive and Transmit data via target UART (only for transport via DAP).

DAP_UART_Transfer Request:

| BYTE | SHORT    | SHORT   | BYTE          |
> 0x21 | Rx Count | Tx Count| Transmit data |
|******|**********|*********|+++++++++++++++|

Rx Count: Maximum number of bytes to be read from UART receive buffer.
Tx Count: Number of bytes in Transmit data to be transmitted.

Note: DAP_UART_Transfer command can be used only if transport via DAP is enabled.; If only receive is required, Tx Count can be 0.
If only transmit is required, Rx Count can be 0.

DAP_UART_Transfer Response:

| BYTE | BYTE   | SHORT    | SHORT    | BYTE          |
< 0x21 | Status | Tx Count | Rx Count | Receive data  |
|******|********|**********|**********|+++++++++++++++|

Status:
- Bit 0: 1 = Receive enabled, 0 = Receive disabled
- Bit 1: 1 = Data lost detected during receive (automatically cleared)
- Bit 2: 1 = Framing error detected (automatically cleared)
- Bit 3: 1 = Parity error detected (automatically cleared)
- Bit 4: 1 = Transmit enabled, 0 = Transmit disabled
- Bit 7..5: reserved (0)
Tx Count: Number of bytes accepted from Transmit data and queued to UART transmit buffer.
Rx Count: Number of bytes in Receive data read from UART receive buffer.
Receive data: Bytes received from the target device via UART.