Serial Ports, DTR and RTS/CTS

[Vernon Schryver]

In article <9104031623.AA18586 at AIVAX.RADC.AF.MIL>, marks at aivax.radc.af.mil (David Marks) writes:
> I want to connect 3 modems (actually NIU ports, but they LOOK and ACT like
> modems) to our 4D/70GT (soon to be a 4D/340VGX).  We have the 6 serial port 
> expansion interface, but those ports don't support DTR for detecting presence
> of a connection; an attribute we require....


On the contrary, the 6-port serial board supports a full 7-wire interface,
including pins 2,3,4,5,7,8, and 9, or RD,TD,RTS,CTS,SG,DCD, and DTR.
Please note that an IRIS is a DTE, and as such raises DTR just in case it
is prepared to talk.  DCD is the signal that standards say that a DCE uses
to tell a DTE that the modem (DCE) is interested in passing data, or what
might reasonably be called the "presence of a connection."

As described in Chapter 10 of "IRIX System Administrator's Guide",
doc. no. 007-0603-030, 
    - DCD/DTR hand shaking is available only when using the ttym* or
	ttyf* device name.
    - RTS/CTS non-standard but nearly universal "hardware flow control"
	is available only when using the ttyf* device name.
That manual describes the behavior of all 7 signals with the three device
names.

As described in /usr/sysgen/master.d/cdsio, there is an old error in the
cables used to connect the 6-port board to the connectors on the outside of
the machine.  That error confused DSR and DCD.  If you do not require
hardware flow control on any of the 6-ports, then the "old" cables can be
used.  Otherwise, it might be possible to get SGI Field Service to fix the
cables as described in the file.  Recently shipped 6-port boards for
machines with 9-U VME buses should come with fixed cables.  Obviously, one
could work around the error in the cable between the computer and the DCE.

The "integral" serial ports and the 6-port serial board ports act the same,
modulo bugs.  More to the point, there are no such bugs known concerning
DCD,DTR,RTS, or RTS, except for the cabling error mentioned above.

There is no programatic way to turn RTS/CTS flow control on or off, except
by choosing the correct minor device number, usually by choosing a suitable
name in /dev.

A "dumb console," as opposed to "the graphics console," must be on port 1.
That is where the PROM and a host of other things expect to find it.



Vernon Schryver,   vjs at sgi.com.