Hardware flow control

[Guy Harris]

> Does the dz-11 support hardware flow control under 4.3BSD?

The term "hardware flow control", without further qualification, is
meaningless.  I know of at least *three* different schemes for using various
RS-232 lines for flow control:

	1) If CD drops, suspend output; when CD comes back up, resume output.
	   No reverse channel.

	2) Raise RTS.  If CTS drops, suspend output; when CTS comes back up,
	   resume output.  No reverse channel.

	3) Same as 2), only the host uses RTS as a reverse flow control
	   channel, instead of as "Request To Send"; i.e., RTS and CTS
	   are crossed, and the host drops RTS when it's not ready for
	   more input.

There may be a fourth scheme, which is to 1) as 3) is to 2).

According to "dzreg.h", at least one of the DZ11/DZ32 supports all of the
aforementioned modem control lines.  However, the DZ and tty drivers support
only the flow control mechanism described in 1) above.

> If not, do any of the serial board drivers?

I think the other serial board drivers do the same; they just hand CD up/down
indications to the tty driver, which uses that to perform flow control if
MDMBUF is set.