The XUARTs are ttl serial ports implemented in the FPGA. These communicate with the userspace driver xuartctl. Each XUART core in the FPGA can handle up to 8 XUARTs, though the default TS-4710 FPGA contains 6. The XUART serial ports have a single shared 4kByte receive FIFO which makes real time interrupt latency response less of a concern and in actual implementation, the serial ports are simply polled at 100Hz and don't even use an IRQ. Even with all 8 ports running at 230400 baud, it is not possible to overflow the receive FIFO in 1/100th of a second. The "xuartctl --server" daemon is started by default in the init scripts which sets up listening TCP/IP ports for all XUART channels on ports 7350-7357. An application may simply connect to these ports via localhost (or via the network) and use the serial ports as if they were network services.
The typical method for accessing xuarts is using the pts layer. For example:
eval $(xuartctl --server --port 3 --mode=8n1 --speed 9600 2>&1); ln -s $ttyname /dev/ttyxuart3
This will set up XUART port 3 to 9600 baud, 8n1, and symlink it to /dev/ttyxuart3. In your application you can open the /dev/ttyxuart3 and for most part you can access this just like any other uart. When using the PTS layer, there are several operations that are not supported. The mode and baud rate must be set up with xuartctl, and cannot be programatically changed with the standard ioctl.