AN3123

UART protocol flow control

Doc ID 16862 Rev 2

13/30

5

UART protocol flow control

Flow control refers to the control of data flow between modems, or between the modem and

a computer. It handles the data in the FIFO buffer and starts and stops data flow between

the modems. Often, one modem may be sending data much faster than the other is able to

receive. Flow control allows the slower device to tell the faster device to pause and resume

data transmission.

There are two ways to handle flow control: hardware (RTS/CTS or DTR/DSR) and software

(Xon/Xoff or DC1/DC3):

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Hardware flow control is performed using the RTS and CTS signals. These signals may

be software controlled.

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Software flow control means sending an XOFF character to stop transmission, and

another character to start transmission.

The flow of data bytes in the cable between 2 serial ports is bi-directional so there are 2

different flows (and wires) to consider:

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Byte flow from the computer to the modem

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Byte flow from the modem to the computer.

HARDWARE FLOW CONTROL uses two dedicated "modem control" wires to send the

"stop" and "start" signals. Hardware flow control at the serial port works like this:

The two pins, RTS (Request to send) and CTS (Clear to send) are used. When the

computer is ready to receive data it asserts RTS by putting a positive voltage on the RTS pin

(meaning "Request To Send to me"). When the computer is not able to receive any more

bytes, it negates RTS by asserting negative voltage on the pin saying: "stop sending to me".

The RTS pin is connected by the serial cable to another pin on the modem, printer, terminal,

etc. This other pin's only function is to receive this signal. For a printer, another PC, or a

non-modem device, this “other” pin is usually a CTS pin so a "crossover" or "null modem"

cable is required. This cable connects the CTS pin at one end with the RTS pin at the other

end (two wires since each end of the cable has a CTS pin).

For the opposite direction of flow a similar scheme is used. For a non-modem, the RTS pin

sends the signal. Some non-modems may use other pins for flow control such as the DTR

pin instead of RTS.

SOFTWARE FLOW CONTROL uses the main receive and transmit data wires to send the

start and stop signals. It inserts the ASCII control characters DC1 (start) and DC3 (stop) into

the stream of data. Software flow control is slower than hardware flow control and it does not

allow the sending of binary data unless special precautions are taken. For example, you

need to be able to distinguish between an occurrence of a control code like DC3 when it

means a flow control stop and a DC3 that is part of the binary data payload.