rts/cts - a tutorial on flow control

[Phil Ngai]

In article <879 at sdcsvax.UUCP> brian at sdcsvax.UUCP (Brian Kantor) writes:
>Phil Ngai suggests that hardware flow control is gross, and that
>XON/XOFF is the proper way to go.  As this attitude is widespread among
>those who don't really understand why hardware flow control exists:

No, I don't think hardware flow control is gross, what's gross is implementing
bi-directional hardware flow control on a 25 pin connector and calling it
RS-232. It's not RS-232 after you have done that and you shouldn't call it
RS-232. If you want to use RS-232 then use in-band flow control (XOFF/XON).
But those signals called DTR and CTS, etc, have defined meanings and if you
change their behavior they aren't the same signals anymore.

On a more global sense, you're really out to change the world if you want
to use bi-directional hardware flow control with the most of the equipment
that you will find out there.

I work with networks for a living and do understand why flow control exists.
I also understand RS-232 better than most people. And do you know which vendor
seems to do RS-232 best? Believe it or not, IBM. Their 7171 product follows
the spec in all the little details that other vendors seem to ignore. And
I bet you didn't know IBM did ASCII or RS-232. Well, they made some mistakes
when they first started. Their 3101 terminal has a female 25 pin connector,
which is wrong. But they learn from their mistakes and listen to their
customers. The IBM-PC and the 7171 are both DTE and have male connectors,
just like they're supposed to. The 7171 understands the Ring signal,
Data Terminal Ready, Clear To Send, Data Set Ready, Data Carrier Detect,
and Request To Send. How many pieces of equipment do you know that look
at or do anything meaningful with DSR?

Argh. I cring at the idea of using DTR for flow control. I am sure people
will say, printers do it all the time, or "it works for me". I still cring.
I'd like you to try doing it through a 212 modem some time.
-- 
 What do you do the day after a peak experience?

 Phil Ngai (408) 749-5720
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