I incorrectly stated the purpose of the column 2 character in the 3178 Operator Information Area (status line). The A with underline or B with underline are not associated with the A or B row of connectors on the 3174.
THE STORY - HOW IT KNOWS
Instead, the B is shown when SNA connects to the terminal. The A will appear for either BSC line remote 3174 or channel attached 3174 systems once the first command is sent to the terminal since power-on or a terminal reset. These include Write, Erase/Write, Copy, and Read Modified commands. In other words, the first time that data streams are sent to the terminal, it illuminates the A character in column 2.
The word TEST in columns 3 to 6 is switched on by the 3174 when the TEST key is pressed, and switched off by another press of that key. When the terminal is put in TEST mode, the A in column 2 goes away.
When the terminal is accessed in A mode, the symbol in column 3 becomes the solid filled rectangle. It does away simultaneous with A. SNA connections to the terminal set the B in column 2 and set column 3 to one of the three rectangles - solid, with ? and with stick figure of a person - depending on what kind of SNA activation was implemented.
Thus, the 3174 control unit that is configured for SNA access will use the B in column 2 and the SNA symbols in column 3. It knows it is SNA and it knows what kind of connection it has.
The 3174 that is either remote with BSC or channel attached will use A because it knows it is NOT SNA and it will use only the solid rectangle in column 3 (or TEST).
The question that remains, and probably can't be answered, is why the heck the designers of those terminals thought it was essential to put that info on a dedicated line and keep it there all the time? I used 3270s for software development for several years and I can't think of a time those values were of any use whatever. Except to indicate maybe that the mainframe was down, which would be obvious anyway.
ReplyDeleteHi David
DeleteI, like you, used these all the time for systems programming and operations tasks but never paid any attention to the status line.
On the other hand, I think I can understand two broad users of this information for which it is worthwhile -
1) maintainers and diagnosticians handling a report of a terminal problem, and 2) those who use these every day for data entry or other application interface
The symbols can report various issues, such as detected errors inside the terminal, comm line errors, SNA errors, even whether the terminal and control unit can talk.
If you don't see the symbol in column 1, you can binary search by moving the coax to a different controller port, swapping in a different terminal, or replacing the coax cable since the flaw is localized to a few spots. If you don't see the characters in columns 2-6 then you know that nothing was ever sent to the terminal. It could be a problem in software, operational procedures, or in controller hardware. Ident string can be requested to identify the logic line that this terminal is associated with, for further diagnosis.
For the terminal power user, it tells the type of error they caused by typing in a protected area or alphas in a numeric field. It reminds you when you are in caps lock, when you have a numeric only field, and when you are in insert mode. Sure, you could infer that from what displays after a keypress but this is more certain feedback.
If you requested a print of the screen, it can tell you if you have a problem with the printer or confirm that it has completed.
I can see reasons that the designers may have wanted that information on the bottom line. And, the most compelling reason of all - the memory was 'free'. A 24 line by 80 column display needs 1920 bytes of display memory, but RAM doesn't come in any unit that has mod N = 0 so there is extra memory sitting there. Enough for a full 25th line, but not quite a 26th. Now, if you ask why they didn't just define the displays at 25 lines x 80 columns, I have run out of after-the-fact justifications.
Ken Shirriff did some research into why 24 versus 25 and the answer is both historical and technological - 480 bit shift registers used for display storage which conveniently provided 1920 positions using 32 of them. This set 24 lines and when the switch occurred to RAM with 2048 byte capacity, we had the 25th line available 'free'
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