The exact nature of the signals on these ports -- what values will appear and when, is called a protocol. Here are some of the things that a protocol specifies:

• the number of bits used for data accepted and data ready, and what their positions are in the control byte
• the exact sequence of values in the control byte
• who should set which bytes and when
• what character code will be used for the data (ASCII, EBCDIC, other)
• how the device tells the CPU when its done
• how the device tells the CPU about errors or abnormal conditions
• how the CPU gives commands to the device (start, stop, give me your status, wait for a while, etc.)
• how the CPU discovers when the device is out of data