wmartin@brl-tgr.ARPA (Will Martin ) (11/02/84)
I had posted this to net.periphs instead of to net.info-terms, so sorry for the duplication if you are seeing it again. I normally use a Lear-Siegler ADM42 terminal, dialled up through a 1200-bps Vadic 3434. I've noticed an odd effect on my main computer, a VAX 11/750 running the BRL version of 4.2 BSD UNIX: I'll lose a character out of every screenfull (or so) of text displayed. For example, I can compose a message, and, when I review it, if it is more than 22 or so lines long, there will be a character missing near the end of the displayed text. When I go into an editor and look at that particular line by itself, it is intact. The character is in the data, just is skipped in the display. This seems to be positional (like every 1nnn characters), because, if I change the text BEFORE that missing character, and redisplay, the missing character position has shifted, forward or back depending on if I had deleted or added characters in front of it. I only see this on the ADM42, never on another variety of terminal. What is odd is that I do NOT see this effect over here on this host, which also runs BRL 4.2 BSD, nor do I see the effect on a TOPS-20 host I also use. All of these connections are through a MILNET TAC, but I see the effect on my home host (ALMSA-1) if I dial it directly, too, so I figure it must be due to something host-related. We have very few ADM42's here, so I haven't pressed our Operating Systems people to investigate the problem. I was hoping that someone might recognize this as "the old xyz bug" or something and know of the cause and cure. This ring any bells out there? (ctrl-g, ctrl-g :-) Will Martin USENET: seismo!brl-bmd!wmartin or ARPA/MILNET: wmartin@almsa-1.ARPA
jerry@oliveb.UUCP (Jerry Aguirre) (11/07/84)
I saw a similar problem when I hooked up a Racal Vadic modem thru our
Micom port selector. The symptoms were not so precise but when a large
burst of characters was sent to the terminal some of the characters would
be lost or garbled.
I finally resolved that the problem was that the Micom was not talking
exactly 1200 baud. As others have pointed out the 1200 baud modems are
really synchronous and use internal buffering to appear asynchronous.
Thus they have a limited ability to handle data faster than 1200 baud.
If the data is sent at 1204 baud the modem's internal buffer will
gradually fill up and when full a character will be lost.
The Racal Vadic modem that I was using has a "allow 1205 baud" switch
in it. After I set this everything worked fine.
Your loss of data seems to be more repeatable than I would expect for
this kind of problem. Also I can't understand why the problem only
occurs on a particular terminal. It may be that other terminals require
delays that allow the modem to catch up.
Jerry Aguirre @ Olivetti ATC
{hplabs|fortune|idi|ihnp4|ios|tolerant|allegra|tymix}!oliveb!jerry