RS-232 Handshaking

In order that data can be exchanged on an RS 232 link, the control signals must indicate that the equipment at either end of the link is ready to send the data and ready to receive the data. This can be achieved in a number of ways, but one of the more common is to use the RTS, CTS, and DTR lines.

These lines are found in the Data Terminal Equipment, DTE and Data Communications Equipment, DCE as follows:

LINE ABBREVIATION	LINE NAME	EQUIPMENT
RTS	Request to Send	DTE
CTS	Clear to Send	DCE
DTR	Data Terminal Ready	DTE

The handshaking exchange to start the data flow is quite straightforward and can be seen as a number of distinct stages: 

1. RTS is put in the ON state by the DTE. 
2. The DCE then put the CTS line into the ON state. 
3. The DTE then responds by placing the DTR line into the ON state. 
4. The DTR line remains on while data is being transmitted. 

At the end of the transmission, DTR and RTS are pulled to the OFF state and then the DCE pulls the CTS line to the OFF state. This series of handshake controls was devised to allow the DTE to request control of the communications link from the related modem, and then to let the modem inform the terminal equipment that the control has been acquired. In this way the communications will only take place when both ends of the link are ready.

The RS 232 data communications standard is a reliable for of data communications which has been used for many years and shows every sign of being used for many years to come. In order that it is able to communicate satisfactorily the RS 232 signals and voltage levels must be able to ensure that the line receivers are able to decode the data with no errors and that the communications protocols are adhered to. Once these are all established, data can be exchanged reliably and efficiently.