Handshaking; Hardware Handshaking; Software Handshaking - HEIDENHAIN TNC 415 Technical Manual

1.2 Handshaking

A "handshake" procedure is often used in connection with interfaces. This means that two devices
are, as it were, working "hand in hand" in order to control data transfer. A distinction is drawn
between "software handshaking" and "hardware handshaking".
Either hardware or software handshaking can be chosen for communication between two units.

1.2.1 Hardware handshaking

With this procedure, control of data transfer is executed by electrical signals. Important information,
such as "Clear To send" (CTS), "Clear to receive", "Start transmission" and "Stop transmission", is
signalled by the hardware.
For example, when a computer character is to be transmitted, the CTS signal line (see Section 2.2
"RS-232-C/V.24 interface") is checked to see whether it is active (ON). If it is, the character is
transmitted. Otherwise the computer will delay transmission until the CTS line is switched to active.
Hardware handshaking requires, as a minimum, two data lines - TxD and RxD, the RTS control line
and the CTS signal line and a ground connection.

1.2.2 Software handshaking

With software handshake, control of data transfer is achieved by appropriate control characters
transmitted via the data line. One such handshake is the XON/XOFF method, which is in widespread
use on the RS-232-C/V.24 interface. The meaning "XON" is assigned to an ASCII code control
character (DC1) and the meaning "XOFF" to another (DC3). Before transmitting a character, the
computer checks whether the receiving unit is transmitting the XOFF character. If it is, it delays
transmission until it receives the character XON, indicating that the connected unit is ready to
receive further characters.
Apart from the data lines (TxD, RxD), and ground, no other lines are needed for software
handshaking.
01.98 TNC 407/TNC 415/TNC 425 1 Introduction 8-9