For first time users of asio it is not easy to get things started. I found a demo application (minicom_client) on the web, but it uses a raw io_service.

In the reference docs I found serial_port_service.

What is the difference? Can I use serial_port_service as a drop in replacement where an io_service normally is used?

It's a default template parameter of basic_serial_port, and IIUC it's created and used internally by the serial_port.