What's the difference between DTR/DSR and RTS/CTS hardware flow control? When is each one used? Why do we need more than one kind of hardware flow control? :)
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serial-port
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The difference between them is that they use different pins. Seriously, that's it. The reason they both exist is that RTS/CTS wasn't supposed to ever be a flow control mechanism, originally; it was for half-duplex modems to coordinate who was sending and who was receiving. RTS and CTS got misused for flow control so often that it became standard.
An important difference is that some UARTs (16550 notably) will stop receiving characters immediately if their host instructs them to set DSR to be inactive. In contrast, characters will still be received if CTS is inactive. I believe that the intention here is that DSR indicates that the device is no longer listening and so sending any further characters is pointless, while CTS indicates that a buffer is getting full; the latter allows for a certain amount of 'skid' where the flow control line changed state between the DTE sampling it and the next character being transmitted. In (relatively) later devices that support a hardware FIFO it's possible that a number of characters could be transmitted after the DCE has set CTS to be inactive.
Source: Stackoverflow.com