Dear Ian, excuse me if I am bold, but I worked 19 years at IBM and 14 at
teleprocessing dealing with all kinds of telecommunication systems and
modems in several different communication protocols, I also developed
some communication devices to IBM.  I am not challenging Horowitz, but,
DTE asserts RTS (Request to SEND) when it wants to "SEND", not a single
moment before, except if it is a full-duplex system, when the
transmission and reception can happens simultaneously, then RTS still
active all the time (can be programmed to be).  DTR means Data Terminal
Ready to the DCE (data communication equipment, modem), informing that
the DTE is ready, doesn't mean only ready to receive, just "ready", able
to work.  In any case, RTS is not used (never) to means DTE is ready to
receive.

By the other side, the DCE (modem or whatever) also needs to tell the
DTE that it is "ready", so it activate the "DSR" circuit (Data Set
Ready).  If the DCE (modem) is set to work in "half-duplex" mode,
(receive OR transmit), the RTS from DTE will only be answered with CTS
(Clear To Send) from DCE, IF the DCE is not receiving something, or when
the reception ends.  In the full-duplex case, RTS from DTE will always
be answered with CTS from DCE immediately after an internal delay
RTS-CTS defined at the modem. This delay is necessary in half-duplex
mode, to allow the phone line to get quiet without echoes from the
reception, so the modem can transmit without interference.

There are much more details about RS232, including "carrier" (DCD),
"ring" (RI), internal or external clock for synchronous communication,
secondary signals and so on, not relevant at this post.

If someone wants to get this in time chart just ask me via private
email.

I really don't know if Horowitz wrote the way you said, if yes, he is
wrong. Sometimes this happens.

I am just clarifying this point to avoid people to learn it wrong.

Wagner Lipnharski.
http://www.ustr.net

Ian Cull wrote:
> You are challenging me now!
> OK. From "Horowitz & Hill" (a great electronics bible; p724-725) ...
> A DTE asserts RTS and DTR when it is ready to receive, and a DCE asserts CTS
> and DSR when it is ready to receive.
> Note that the signal names make sense only as viewed by the DTE (eg: pin 2 is
> called "TD" even though DTE asserts it and DCE receives it). Thus the name of
> a pin isn't enough to tell you if its an input or output ... you also need to
> know whether the device thinks its a DTE or DCE.