Handshaking Mechanism

Handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins. It follows the physical establishment of the channel and precedes normal information transfer.
It is usually a process that takes place when a computer is about to communicate with a foreign device to establish rules for communication. When a computer communicates with another device like a modem, printer, or network server, it needs to handshake with it to establish a connection.
With handshaking signals, a transmitter can indicate when it has data to send, and a receiver can indicate when it is ready to receive data. The exact protocols that signals follow may vary, though many RS-232 and RS-485 links follow standard or conventional protocols.

In hardware handshaking, the receiver brings a line high when it is ready to receive data, and the transmitter waits for this signal before sending data. The receiver may bring the line low any time and the transmitter must detect this, stop sending, and wait for the line to return high before finishing the transmission.
Other links accomplish the same thing with software handshaking, by having the receiver send one code to indicate that it is ready to receive, and another to signal the transmitter to stop sending.