O'Riley Book (was Re: more on the HFC saga)

Greg Andrews gandrews at netcom.COM
Sat May 25 16:20:33 AEST 1991

In article <1991May24.054753.28804 at colnet.uucp> res at colnet.uucp (Rob Stampfli) writes:
>  [quoting from the book "Managing UUCP and Usenet"]
>  "In the half-duplex standard, the DTE [ computer ] asserts RTS when it
>  wants to send data.  The DCE [ modem ] replies with CTS when it is ready,
>  and the DTE begins sending data.  Unless RTS and CTS are both asserted,
>  only the DCE can send data.
>  "However, in the full-duplex variations, RTS/CTS is used as a kind of
>  throttle.  The signals have the opposite meanings than they do for
>             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>  half-duplex communications.
>  ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Unfortunately, this last statement is completely untrue with respect to
the CTS signal.  In both forms of hardware flow control, the CTS signal
indicates when the modem is ready to receive data from the computer.
Compare the description of half-duplex flow control in the first paragraph
quoted above, with the one quoted below:

>  "When a DTE device is able to accept data, it asserts pin 4, Request to
>  Send.  If the DCE is ready to accept data, it asserts pin 5, Clear to
>  Send.  If the voltage on RTS or CTS drops at any time, this tells the
>  sending system that the receiver is not ready for more data...

You can see that the actions performed by RTS are very different between
the two methods, but the actions of CTS are the same.  CTS does *not* "have
the opposite meaning" in full duplex flow control than in half duplex flow

>PS:  The short excerpt from "Managing UUCP and Usenet" is typical of the
>calibre of information contained in this publication.  I would recommend it
>highly to anyone owning a Unix-PC.  It is indeed the UUCP "bible".

I've seen the book flamed because it completely overlooks the need to use
the ",M" code in the Devices file, and the "\M" and "\m" codes in the Dialers
file when a modem is properly configured for dial-in/dial-out operation.
These are needed for almost any AT&T system, and not just the Unix PC.
(but many vendors of 386-based implementations have re-written cu and uucp
to bypass the need for these extra codes)

>Rob Stampfli, res at kd8wk.uucp (osu-cis!kd8wk!res), kd8wk at n8jyv.oh

 |  Greg Andrews   |       UUCP: {apple,amdahl,claris}!netcom!gandrews    |
 |                 |   Internet: gandrews at netcom.COM                      |

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