RS232 related query

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   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,

Re: RS232 related query

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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).


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Manufacturers and Suppliers of HF Selcall
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Re: RS232 related query

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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
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
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.

-Robert Scott
 Ypsilanti, Michigan
(Reply through this forum, not by direct e-mail to me, as automatic reply
address is fake.)

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