Communication functions carry out communications through the system's serial and parallel I/O ports. The following list briefly describes each communication function:
| Function | Description | |
| BuildCommDCB | Fills a device control block with control codes. | |
| ClearCommBreak | Clears the communication break state from a communication device. | |
| Function | Description | |
| CloseComm | Closes a communication device after transmitting the current buffer. | |
| EscapeCommFunction | Directs a device to carry out an extended function. | |
| FlushComm | Flushes characters from a communication device. | |
| GetCommError | Fills a buffer with the communication status. | |
| GetCommEventMask | Retrieves, then clears, an event mask. | |
| GetCommState | Fills a buffer with a device control block. | |
| OpenComm | Opens a communication device. | |
| ReadComm | Reads the bytes from a communication device into a buffer. | |
| SetCommBreak | Sets a break state on the communication device. | |
| SetCommEventMask | Retrieves and then sets an event mask on the communication device. | |
| SetCommState | Sets a communication device to the state specified by the device control block. | |
| TransmitCommChar | Places a character at the head of the transmit queue. | |
| UngetCommChar | Specifies which character will be the next character to be read. | |
| WriteComm | Writes the bytes from a buffer to a communication device. |