Arcane modem configuration (was Re: Modems on Ultrix)

[Phil Ngai]

In article <1149 at ncr-sd.UUCP> greg at ncr-sd.UUCP (Greg Noel) writes:
>A nit, but that should be DCD (Data Carrier Detect) instead of DTR (Data
>Terminal Ready).  The modem looks at DTR to see if the computer is active,
>while the computer looks at DCD to see if the modem has a data connection.

Hardly a nit, rather, that is an important point.

>The computer will provide DTR to the modem even if it is ignoring DCD.

In at least one version of 4.2 BSD, this is not true. When the system
is configured to ignore modem controls, it fails to turn on DTR.  I
have a friend who can testify to this. I verified it with a breakout
box. This was important because his terminals expected DCD and his
wiring system connected DTR from his VAX to DCD on his terminals.  It
didn't work and he couldn't understand why.

I agree with all the comments about the value of modem control. I would
like to add a few comments of my own.

1) If you require DTR from the terminal, then you won't get burned by
the following scenario: user disconnects vax cable to connect to local
computer. VAX cable floats and picks up noise on input lines. VAX
spends all its CPU cycles trying to login noise.

The loss of DTR allows the VAX to consider the cable disconnected and
it will ignore noise on the cable.

2) DTE does stand for Data Terminal Equipment but "Terminal" is not
strictly a thing you type on but simply something that you connect to
a modem. Real terminals (VT100) do fall into this class but so do
VAXen and statistic muxes. A number of computer manufacturers have
designed their serial ports to be DCE for convenience but this was not
the intent of the spec. Large companies which tend to be better about
conformance to specs like DEC and IBM usually make their computers
DTE.

3) We take the attitude that wire is cheap and use lots of it.  In
fact, we run a 25-pair cable for 6 terminals, or 8 wires per terminal.
This lets us use cheap Western Electric 258A outlet devices.

4) Why complain about Ultrix when Berkeley Unix told you no more about
modem control?
-- 
 Rain follows the plow.

 Phil Ngai +1 408 749 5720
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