or both. Serial ports on PCs typically have 16-byte hardware buffers built into the UARTs. In the receive direction, the UART can store up to 16 bytes before software needs to read them. In the transmit direction, the UART can store up to 16 bytes and transmits the bytes using the selected protocol. Some UARTs, including those in many Virtual COM-port devices, have larger hardware buffers. 27 Chapter 2 COM-port drivers in PCs maintain software buffers that are programmable in size and can be as large as system memory permits. The driver transfers data between the software and hardware buffers as needed. Buffers in microcontrollers tend to be small. Some UARTs have no hardware buffers at all. A computer with small or non-existent buffers may need to use other methods to prevent lost data. Event-driven Programming and Polling Events that can occur at a serial port include sending and receiving data, changes in flow-control signals, errors in received data, and timeouts when attempting to send or receive data. There are two ways for program code to detect these events. One way is to jump to a routine when an event occurs. The code responds quickly to port activity without having to waste time checking only to learn that no activity has occurred. This type of programming is called event driven because an external event can break in at any time and cause the program’s execution to branch to a routine to handle the event. The .NET Framework’s SerialPort class includes the DataReceived, Pin-Changed, and ErrorReceived events. Handler routines can execute when a software buffer’s count reaches a trigger value, when the state of a flow-control or status pin changes, and when an error or timeout occurs. Many microcontrollers have hardware interrupts that can perform similar functions. The other approach to detecting events is to poll the port by periodically reading properties, signal states, or registers to find out if an event has occurred. This type of programming doesn’t u prom-electric.ru