phil@amd70.UUCP (Phil Ngai) (05/13/84)
Why does my terminal at home sometimes start printing out UUUs when I am
dialed up to my company's computer with a 212 modem? I haven't found any way
to stop it except by breaking the connection. I have a theory it has something
to do with loopback testing but was wondering if anyone out there had more
information. Specifically, how can I prevent it or at least stop it without
breaking the connection? In general, what's going on?
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Phil Ngai (408) 749-5286 {ucbvax,decwrl,ihnp4,allegra,intelca}!amd70!philmel@pegasus.UUCP (Mel Haas) (05/14/84)
The UUUUU's are the test pattern coming out of the 212A at the other end. Some sorts of "line noise" cause the automatic loopback test mode to be entered. If you see the UUUU's, it is not your modem doing it. If your modem seems to go dead, and/or has a different from normal pattern of lights on, you may be spitting UUUUUU's at the other end. You can disable the automatic loopback test by pressing switch 7 of switchblock 2 (option YL instead of the default YK). Easy to do on your end, but usually not too easy to get the computer center at the other end to do for you. (strictly the personal opinions of) Mel Haas , houxe!mel
mcferrin@inuxc.UUCP (P McFerrin) (05/15/84)
The above string is a pattern of alternate 1's and 0's. When a 212 dataset is in 'self test' mode, it will constantly send this pattern to the telco line side of the modem. Another anoying problem is that a dataset will all of a sudden go into loopback mode. If this occurrs on the host end, the symptom is echoeing of your characters without processing. (e.g. a carriage return will be echoed but the line feed usually xmited by Unix will be missing) This usually is a result of some noise causing a remote dataset to go into remote loopback. Switch #7 on switch pack #2 can disable this feature thus forcing the dataset to go into loopback mode ONLY when the DL button is depressed. I beleive (which means I am uncertain) that this switch will probably not have any effect on the dataset going into self test mode. Therefore the solution is to notify the computer center of this condition and have the dataset replaced. Paul McFerrin
faunt@saturn.UUCP (05/15/84)
The UUUUUU's are the loopback test. Racal-Vadic 34xx series modems seem to be put into this mode by a very short interruption of carrier from the other end, (if enabled). I suspect VA212's do the same thing. I had a problem with one of my users usually leaving the dial-in's in loopback mode after disconnect, since he lived in an area with flaky telephone service that also affected his voice service. Since the modem "busied out" on test, the next time he dialed in, he got the next modem, and did the same thing to it. He could eat all seven modems on one machine by Saturday afternoon.
rib@edsel.UUCP (RI Block) (05/17/84)
The UUUUU's are the test pattern coming out of the 212A at the other end. Some sorts of "line noise" cause the automatic loopback test mode to be entered. If you see the UUUU's, it is not your modem doing it. Not always so. Host systems which echo what is received (like UNIX) will cheerfully feed you back a stream of UUUUUU's that your modem might be sending. I recall (but I don't know from where) seeing a report of a "crash" analysis showing all the clists filled with U's.
kenner@acf4.UUCP (05/22/84)
There is a specific case of this problem when using an ADM3A terminal and a Racal-Vadic 3451 modem connected by a cable which has ALL 25 PINS. In this case, if you are sent a bell (^G), somehow the terminal triggers the modem to enter self-test mode. In this case, the problem is also fixed by simply not running all 25 pins in the cable. Another note about U's: For the VA3400 protocol, the encoding format is a phase shift of 0, 90, 180, or 270 degrees for each pair of bits. It turns out that a string of U's is a continuous 010101 (or 101010, I forget which) which is a string of 0 degree phase shifts. But the receiver will interpret this as something being wrong. So if you ever try to type out a file containing a large number of U's, you will see some rather interesting effects when using 3400 protocol!