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

14. megal ASCII Character is a character or string which is written to the terminalto represent an unprintable character. The TDTs supplied with the distributedsystem use a colon (:) for this purpose. However, on certain terminals it maybe desirable to choose a less ambiguous character. For instance, if the user hasonly Model 2x versions of the IBM 3101 terminal, a suitable alternate choicemight be:ESC,E (X I 1B45 I)which writes an unusual graphic character (normally called the "SENDMARK") that produces a unique display. On the other hand, on the IBM3101 Model Ix it is possible to display a square blot on the screen with themore complicatedDLE,STX,DEL,DLE,ETX (X I 10027F1003 I)Although this string contains five control characters, it produces the requiredresult to be the illegal character token: it displays something unusual in thescreen position, occupies only one space on the screen, has no effect on thecharacters which follow or precede it, and when done it has advanced thecursor one position to the right. Screen wrap at end of line is also handledproperly.15. megal APL Character is a special string for displaying illegal characters in APLmode. The normal setting for this string is:n,BS,z (X I 6E087 A I )The lower case here is misleading. The control sequence strings are nevertranslated by the APL mode setting, so lower case prints as upper case. "N"overstruck with "Z" produces a fairly solid blot on most APL terminals.16. APL ON provides a string which may exist on terminals to turn APL mode onto permit display of both APL and normal ASCII characters on the samescreen image.17. APL OFF turns APL mode off again.APL ON/OFF provides two strings which may exist on terminals to turn APLmode on and off dynamically to permit display of both APL and normal ASCIIcharacters on the same screen image. On most terminals with dynamic APLcapability, the control character SO (Shift Out) turns APL mode on and SI(Shift In) turns it off. However, it is possible that some other terminals woulduse another control sequence for this purpose.18. Display Mode is a string that the IBM 7171 uses to support highlighting andbasic color (see "Graphic Rendition Strings" on page 4-16 for details).19. Alternate Clear Screen is the control sequence that clears the display screen andselects the alternate screen size.Chapter 4. Customizing IBM 7171 Tables 4-15