When a PC wants to send data it sets the data terminal ready line. This DTR signal goes into the DTR line of the DCE. The DCE recognizes that the DTE is requesting a connection. if an open phone line exist for the DCE, it sets the DSR and data carried detected. When the PC sees the 2 signals on its DCD and DSR input lines, it sets the request to send line - which says that the PC has data to send to the DCE. if the DCE is clear to accept data, it sets the clear to send line - which tells the PC that the DCE is free to receive, and the PC begins transmitting data over the TxD line where it is received on the corresponding line on the DCE.

The problem with the standard is that it assumes that once the communication link was established, it would not be broken and that there was no speed mismatch with the two partners. It is true when working with terminals - but not with computers. Since a printer is much slower that the PC, we have a problem here and handshake would solve it. Since it is not standard - it won't succeed.

DTE connected to DTE

Here we have a PC connected to Qume Sprint serial printer. The printer is much slower than the PC and the printer manufacturer implemented a hardware handshake. When the printer's buffer becomes nearly full, its lowers the DTR line - tells that it is not ready. When the PC recognizes this, it stops sending until the buffer is empty and the DTR is set again.

At first we match the send lines with the receive lines on the other device by connecting the RxD on one device with the TxD wires on the other. (This is the minimum request for DTE-DTE).

The PC and the printer come up with their DTR lines set. Consider the signals as seen by the printers. The PC's DTR line satisfies the printer's DCD and DSR input line requirements. The printer responds by setting RTS, which in turn satisfies the PC's need for a DCD and DSR signal. The PC also requires its CTS send line to be set before it can transmit, but notice that the printer's DTR is wired to the PC's CTS, so this requirement is satisfied. The PC can now transmit and receive and so is the printer. All the standard's rules are satisfied except the order in which the PC's CTS input line was set, but this makes no difference in its function.

The following diagram illustrate the above :