On Wed, 6 Sep 2006, Edd Barrett wrote:
> If it works with the sun box, I assumed it's the correct cable?
Not necessarily the case, said the grey old admin, sighing and
wincing with the facial tic he thought he had lost in the mid 1990's.
Getting a serial terminal to work is one of the bitchier bits of
olde-tyme sysadmin work. (It is not as bad as getting a serial
printer to work.) Google for "null modem pinout" and you should
get a sample of the sort of witchcraft to which desperate admins
have had recourse over the decades.
There are different kinds of null modem cable, varying by mfg and
"philosophy". Null modems were at one time a seductive sink for
the sort of creativity that now is reserved for quirky software
license schemes and object-oriented programming. An RS-232 cable
can have, alas, 25 lines. Since only 7 (2 data, 1 gnd, 4 control
(2*(I'm busy, I'm alive))) are useful in any *practical* sense, the
other 18 (as well as the 4 "useful" control lines) can be combined
in a variety of intriguing ways to achieve or suppress some obscure
feature. This includes crossing-over, looping back, joining together,
and hard-wiring to ground or to certain voltages. Sometimes a tiny
resistor or capacitor might find its way under the shield of the
cable, cunningly soldered across the backs of two pins.
DEC cables usually work between DEC terminals and DEC computers.
Sometimes DEC cables work between genuine DEC terminals and PeeCees.
Hooking up an IBM terminal (even emulating (often this means
"mocking") a vt100 or vt220 -- this emulation may not extend to the
finicky hardware parts) to a Sun with a Sun (?) cable, experiencing
the rare joy of "working terminal" does not completely predict
behavior of that same terminal and cable with a PeeCee. Some sly
manufacturers may have stooped so low as to create "console" ports that
require a "straight" cable; this is where "philosophy" can rear
its head. PeeCees don't do that, at least the "standard" (har-har)
ports don't.
Try setting "local" in /etc/ttys. How you make that IBM terminal
give up trying to set/read various control lines I don't know.
Before you throw in the towel, try a simple 3-conductor null modem
cable, simply carrying through signal ground and swapping the two
data lines, in conjunction with "local" in /etc/ttys. After that
barely works, you will have to figure out how to set XON-XOFF flow
control with stty maybe. Some of this junk can be set with tset,
too.
Dave "My VT-101 works and I'm not touching it again."
--
Experience runs an expensive school, but fools will learn in no other.
-- Benjamin Franklin
