Hardware flow control under 4.1 and higher

[Guy Harris]

>I would like to run my MNP5 modem efficiently.  This means running the tty
>line at a higher speed than the link speed.  Obviously I need to have
>hardware flow control.  I have noticed that under SunOS 4.1 and 4.1.1 when
>I set crtscts, characters received are thrown away.

Yup.  That's what you should expect; "crtscts" implements *hardware* flow
control, i.e. flow control implemented by the serial port *hardware*, and
the serial port *hardware* only lets the device receiving data *from* the
Sun control the flow of data *from* the Sun - it doesn't let the Sun
control the flow of data *into* it.

(If I ever find the person who first dubbed flow control done - perhaps in
software - by using various modem control lines "hardware flow control",
I'm going to shoot them; unfortunately, everybody else seems to have
picked up that usage from them....)

The only *software* flow control SunOS implements is XON/XOFF flow
control; it doesn't implement bi-directional RTS/CTS flow control (wherein
the Sun can drop RTS to ask the device to stop sending data) - that
wouldn't be hardware flow control, 'cuz the hardware won't do it
automatically.