RTS/CTS flow control

Guy Harris guy at auspex.auspex.com
Wed Feb 6 06:49:19 AEST 1991

>I'm running SunOS 4.0.3 on a 3/50.  I want to run a modem of ttya
>using RTS/CTS for flow control, as the networking software I want to
>use wants an 8-bit data path.  The modem is doing 12,000 bps, so I
>want to run the port at 19,200.  However, when I try this, I start
>getting silo overflows.  Does this not work as documented,

It does work as documented; from the 4.0.3 ZS(4S):

     If the CRTSCTS flag in the c_cflag is set,  output  will  be
     generated only if CTS is high; if CTS is low, output will be
     frozen.   If the CRTSCTS flag is clear, the state of CTS has
     no effect.

but it may not work the way you *want* it to.  Note that the above says
nothing whatsoever about the Sun using RTS to tell whoever is on the
*other* end of the wire to stop sending stuff *to* it; the only RTS/CTS
flow control supported by 4.0.3 is unidirectional - i.e., using CTS to
control the flow of data *from* the Sun.

(Of course, this also assumes that either 1) your networking software has
an option to set the CRTSCTS flag or 2) you've set it somewhere else
yourself, but since you want bi-directional RTS/CTS flow control, you're
out of luck regardless of whether the networking software can set it or

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