Consider the situation where someone is helping you picking apples from a tree. Your helper climbs in the tree and throws all the apples to you. You have to put them in buckets. In the normal situation, you can easily catch all apples, but when one bucket is full and it has to be replaced by an empty one, this action costs more time than is available between two apples thrown by your helper.

Two different things can occur. Your helper stops until the new bucket is in position, or some apples are damaged because they fall on the ground in the small period you are not able to catch them.

You would probably prefer the first method where your helper stops for a small period. To achieve this, there will be some communication, eye-contact, a yell, or something like that to stop him from throwing new apples. How simple, but is it always this simple? Consider the situation where one computer device sends information to another using a serial connection. Now and then, the receiver needs to do some actions, to write the contents of its buffers to disk for example. In this period of time no new information can be received. Some communication back to the sender is needed to stop the flow of bytes on the line. A method must be present to tell the sender to pause. To do this, both software and hardware protocols have been defined.

Both software and hardware flow control need software to perform the handshaking task. This makes the term software flow control somewhat misleading. What is ment is that with hardware flow control, additional lines are present in the communication cable which signal handshaking conditions. With software flow control, which is also known under the name XON-XOFF flow control, bytes are sent to the sender using the standard communication lines.

Using hardware flow control implies, that more lines must be present between the sender and the receiver, leading to a thicker and more expensive cable. Therefore, software flow control is a good alternative if it is not needed to gain maximum performance in communications. Software flow control makes use of the datachannel between the two devices which reduces the bandwidth. The reduce of bandwidth is in most cases however not so astonishing that it is a reason to not use it.

Two bytes have been predefined in the ASCII character set to be used with software flow control. These bytes are named XOFF and XON, because they can stop and restart transmitting. The bytevalue of XOFF is 19, it can be simulated by pressing Ctrl-S on the keyboard. XON has the value 17 assigned which is equivalent to Ctrl-Q.

Using software flow control is easy. If sending of characters must be postponed, the character XOFF is sent on the line, to restart the communication again XON is used. Sending the XOFF character only stops the communication in the direction of the device which issued the XOFF.

This method has a few disadvantages. One is already discussed: using bytes on the communication channel takes up some bandwidth. One other reason is more severe. Handshaking is mostly used to prevent an overrun of the receiver buffer, the buffer in memory used to store the recently received bytes. If an overrun occurs, this affects the way newcoming characters on the communication channel are handled. In the worst case where software has been designed badly, these characters are thrown away without checking them. If such a character is XOFF or XON, the flow of communication can be severely damaged. The sender will continuously supply new information if the XOFF is lost, or never send new information if no XON was received.

This also holds for communication lines where signal quality is bad. What happens if the XOFF or XON message is not received clearly because of noise on the line? Special precaution is also necessary that the information sent does not contain the XON or XOFF characters as information bytes.

Therefore, serial communication using software flow control is only acceptable when communication speeds are not to high, and the probability that buffer overruns or data damage occur are minimal.

Hardware flow control is superior compared to software flow control using the XON and XOFF characters. The main problem is, that an extra investment is needed. Extra lines are necessary in the communication cable to carry the handshaking information.

Hardware flow control is sometimes referred to as RTS / CTS flow control. This term mentions the extra input and outputs used on the serial device to perform this type of handshaking. RTS / CTS in its original outlook is used for handshaking between a computer and a device connected to it such as a modem.

First, the computer sets its RTS line to signal the device that some information is present. The device checks if there is room to receive the information and if so, it sets the CTS line to start the transfer. When using a null modem connection, this is somewhat different. There are two ways to handle this type of handshaking in that sitiuation.

One is, where the RTS of each side is connected with the CTS side of the other. In that way, the communication protocol differs somewhat from the original one. The RTS output of computer A signals computer B that A is capable of receiving information, rather than a request for sending information as in the original configuration. This type of communication can be performed with a null modem cable for full handshaking. Although using this cable is not completely compatible with the original way hardware flow control was designed, if software is properly designed for it it can achieve the highest possible speed because no overhead is present for requesting on the RTS line and answering on the CTS line.

In the second situation of null modem communication with hardware flow control, the software side looks quite similar to the original use of the handshaking lines. The CTS and RTS lines of one device are connected directly to each other. This means, that the request to send query answers itself. As soon as the RTS output is set, the CTS input will detect a high logical value indicating that sending of information is allowed. This implies, that information will always be sent as soon as sending is requested by a device if no further checking is present. To prevent this from happening, two other pins on the connector are used, the data set ready DSR and the data terminal ready DTR. These two lines indicate if the device attached is working properly and willing to accept data. When these lines are cross-connected (as in most null modem cables) flow control can be performed using these lines. A DTR output is set, if that computer accepts incomming characters.