2.2 The IBM 7171 ASCII Device Attachment Control Unit - Index of

Keyboard Assignment ConsiderationsX' 18' Typeahead Purge, CTRL-XX' 11 ' Pacing Start (XON), CTRL-QX' 13' Pacing Stop (XOFF), CTRL-SX' 17' Maintenance Facility Console Toggle, CTRL-W.There are over 60 unique keyboard functions supported. Since most terminals donot have 60 function keys, special techniques must be used to define any additionalkeys. One approach is to use a "shift" key (one that is held down while pressinganother key). Such a key is the CTRL key (marked ALTon the IBM 3101), whichtypically permits the generation of all 33 ASCII control characters from the keyboard.On the IBM 3101 with the AL T key depressed and pressing graphic character"A" produces the ASCII control character SOH. Another approach is to usea special "introducer" function key which sends a two character sequence. In thecase of the IBM 3101, eight Program Function (PF) keys provide such a capability.Finally, some terminals have a special function shift key which embeds the charactergenerated by the next normal graphic key pressed in the middle of a specialthree character sequence (e.g. SOH char CR is sent).The first task is to segregate the useable keys. Some keys have alternate functions(shifted and unshifted, for example). It is important to distinguish the codes whichcan easily be generated (unshifted) from those which require two fingers. Forexample, on the IBM 3101, the best keys are Backspace, Return, and Tab (becausethey are large and are closest to the center of the keyboard). Backtab and SENDwould be very good, but they do not transmit anything. The next best keys are thecursor movement keys and DEL (they are on the right side and most people areright-handed). Then come ERASE EOF, ERASE EOS, and ESC. Finally, we havethe AL T key (which requires that one hand be used to hold down AL T while theother presses the key), HOME, CLEAR, ERASE INPUT, and the PF keys. Allother keys on the IBM 3101 do not transmit. Some terminals have a pad of twelveor more keys arranged in a 3x4 layout similar to a real 3270. Often it is temptingto use this pad for 12 PF keys, no matter how they are named, because much of theIBM documentation and assignment of meaning to PF keys is based on a particularlayout of the PF keypad.The next step is to evaluate the relative importance of the various functions availablein the application environment under consideration. Usually ENTER, ERASEEOF, NEWLINE, TAB, DELETE, INSERT, cursor movement, and the first 12 PFkeys are most important. Sometimes CLEAR is critical (as in CMS). OftenERASE INPUT is a dangerous key to have. In the distributed tables there is noERASE INPUT key assignment for most terminal types. Specific applications mayhave a need for CURSOR SELECT or TEST REQUEST while most systems donot use them. The user must balance the keys available on a specific terminal withthe needs of the application.It is usually a good idea to select keyboard assignments based on the name of thekey itself, but key location can be important. If "ERASE EOL" is written on a keyit may be a good idea to make it emulate the ERASE EOF function because thissimplifies the documentation problem and reduces the amount a user mustremember. These are not the only considerations. Also to be considered are keysize, right-handedness, the proximity of "bad" keys (like a Master Microcode Resetkey), and typamatic behavior to consider. Typamatic is critical for cursor movement,useful for TAB, and useless for ERASE EOF.4-10 IBM 7171 Reference Manual and Programming Guide