Don't forget Ring Indicate

[Chris Torek]

In article <571 at virginia.acc.virginia.edu> scl at virginia.acc.virginia.edu (Steve Losen) writes:
>We have a lot of our hosts connected to an Ungermann/Bass LAN
>(essentially a port switch).  We noticed that the UBLAN had the nasty
>habit of raising CD whenever it saw DTR.

The U/B is doing it wrong, then.  Of course, that is no help for
those who have U/B equipment.  It is even arguably correct, since
the thing is in command mode, but they should have thought `modem',
not `weird network device', and at least made this configurable.

>With a little more investigation I found that the UBLAN and most
>(if not all) modems make use of ring indicate (RI pin 22).

Unfortunately, as you mention, most multiplexors (or at least those
for the Vax) pay no attention to RI.  You could wire a flip-flop
to be set by RI and cleared by ~DTR, and tie that to the multiplexor
board's CD, solving the U/B problem, but that would take extra hardware.

>If someone calls into a modem and the host has DTR off, the modem
>raises RI instead of answering the phone.  When the host sees RI,
>it raises DTR, the modem answers the phone, and the modem raises
>CD, allowing the open(2) on the tty to succeed.

In fact, when we were running 4.1BSD, we had two Vaxen connected
to each other by a hardwired line, and decided to use *it*
bidirectionally.  For this we came up with what we called `passive'
mode: open on the passive device would wait for CD without asserting
DTR, and would assert DTR only when it saw CD.  Opening the dial
device would assert DTR without waiting for CD.  The normal device
would raise DTR and wait for CD.  Hence a passive device would
speak to a dial device `properly', while two `normal' devices would
wage wearying war.  Summary:

	normal:	 raise DTR, then wait for CD.
	dial:	 raise DTR.
	passive: wait for CD, then raise DTR.

The interlocking required for dial/normal/passive is identical
to that for dial/normal, since one uses only one of normal/passive.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris