Hi, I have some confusion related to the RTS and CTS hardware flow control in RS232 serial communication. RTS is request to send.Is it send by device A if it wants to send data to another device B or when it is ready to receive data from another device? How about the CTS signal. How can sender be stopped sending using these signals? Please clarify my doubt.
Thanks in advance,
-ABLR
Commonly RTS (Request to send) is asserted by the device which wants to send the data. This line would connect to the CTS (Clear to send) line of the unit that is to receive the data.
The unit to receive the data would then, if ready, assert it's own RTS line, which would be connected to the CTS line of of the sending device. The sending unit would then send it's data.
At the end of sending the sending unit would clear its RTS line and the system would revert to it's original state.
Alternatively the receiving unit could use it's RTS line to instruct the sending unit when to send or not. This is often used when the receiving unit has a small(ish) buffer which cannot be emptied fast enough (X-0n/X-off can also be used to control sending).
HTH Alan
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This description glosses over the intrinsic asymmetry in the definiton of RS-232 signals. The RS-232 pin functions are defined between DTE (data terminal equipment, terminals, printers, computers) and DCE (data communications equipment, modems, mice).
TxD. data from DTE to DCE. RxD. data from DCE to DTE. RTS. from DTE to DCE. Sometimes enables RCVR on the DCE. says DTE is on. CTS. DCE to DTE. Usually enables XMTR on DTE. says DCE can receive. DSR. DTE to DCE. Sometimes enables XMTR on DTE. says DCE is on. DCD. DCE to DTE. Sometimes enables RCVR on DTE. says modem detected carrier. DTR. DTE to DCE. Sometimes enables XMTR on DCE. says DTE RCVR buffer not full.
As you can see from the "sometimes" and "usually" qualifiers, the use of these control signals is not very standard. You really have to look closely at individual devices to see how they treat these signals. As for hardware flow control, the most usual way to stop a DTE transmitter is by lowering CTS. The most usual way to stop a transmitter on a DCE is by lowering DTR. When connecting a DCE to a DCE, you need to look at the wiring of the null-model cable that connects them to see which signals are defined as what.
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