What is the difference between DTR DSR and RTS CTS flow control

Question

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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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

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DTR - Data Terminal Ready DSR - Data Set Ready RTS - Request To Send CTS - Clear To Send   There are multiple ways of doing things because there were never any protocols built into the standards  You use whatever ad-hoc  standard  your equipment implements   Just based on the names  RTS CTS would seem to be a natural fit  However  it s backwards from the needs that developed over time  These signals were created at a time when a terminal would batch-send a screen full of data  but the receiver might not be ready  thus the need for flow control  Later the problem would be reversed  as the terminal couldn t keep up with data coming from the host  but the RTS CTS signals go the wrong direction - the interface isn t orthogonal  and there s no corresponding signals going the other way  Equipment makers adapted as best they could  including using the DTR and DSR signals   EDIT  To add a bit more detail  its a two level hierarchy so  officially  both must happen for communication to take place  The behavior is defined in the original CCITT  now ITU-T  standard V 28     The DCE is a modem connecting between the terminal and telephone network   In the telephone network was another piece of equipment which split off to the data network  eg  X 25   The modem has three states  Powered off  Ready  Data Set Ready is true   and connected  Data Carrier Detect   The terminal can t do anything until the modem is connected   When the modem wants to send data  it raises RTS and the modem grants the request with CTS   The modem lowers CTS when its internal buffer is full   So nostalgic

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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

[serial-port] What is the difference between DTR/DSR and RTS/CTS flow control?

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