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annotate ueda/doc/uschemlang.txt @ 96:82d22cdaa74c

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SML-510MW.fp: created based on E-Sample and FCDEV3B versions
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 26 Sep 2019 05:30:41 +0000
parents cd92449fdb51
children
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1 uschem language for schematic sheets
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2
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3 uschem is the component of uEDA that deals with the graphical or non-graphical
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4 schematic sheets as opposed to the MCL. These schematic sheets are ASCII text
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5 files (conventional suffix is .usch, although it isn't hard-coded into any uEDA
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6 tools) in a special language defined in this document.
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7
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8 At the high level each schematic sheet may be either graphical or non-graphical
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9 and consists of a set of objects that embody electrical and/or graphical
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10 information. Component objects (component instances) provide a link between the
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11 schematic sheet and the MCL, Net and GraphNet objects declare electrical
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12 interconnections (see netlisting.txt), GraphBlock objects serve as containers
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13 for purely graphical elements, and a few other object types will be explained in
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14 detail later. Most object types accept decorations, which are the uschem
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15 syntactic term for information elements that are not objects in themselves, but
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16 are attached to a particular object.
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17
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18 At the low level the uschem syntax is based on the same principle as C and many
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19 other languages used in the UNIX world: the source file is treated as consisting
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20 of tokens which are either self-delimiting or delimited by white space; with a
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21 few exceptions all white space (spaces, tabs and newlines) is treated the same,
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22 i.e., line boundaries have no special significance to the parser. The tokens
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23 which make up the language consist of:
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24
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25 * ASCII characters (),;={} are self-delimiting tokens.
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26
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27 * A quoted string (enclosed in double quotes '"') is a quoted-string token.
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28 To include a '"' character in a quoted string, escape it as '\"'; to include
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29 a backslash '\', escape it as '\\'.
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30
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31 * The following tokens are delimited by white space, by any of the self-
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32 delimiting tokens listed above, or by '%' comments (see below):
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33
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34 + Keywords
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35 + Numbers (decimal integers)
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36 + Any other strings (unquoted-string tokens)
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37
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38 Wherever the syntax calls for a string giving some user data, either a quoted or
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39 an unquoted string may be given; quoted strings are required only if they
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40 contain special characters. Keywords and numbers must not be quoted.
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41
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42 Comments
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43
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44 The uschem language supports two kinds of comments. The first kind are
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45 PostScript-style comments. These comments are introduced by a '%' character and
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46 continue to the end of the line (one of the few places in the uschem language
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47 where newlines differ from other white space). This comment style is very
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48 convenient, but they suffer from one serious problem: if a schematic sheet is
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49 processed by a program that parses the original, does some transformations to
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50 it, and writes out a new .usch file (uschem-rewrite(1) or the future graphical
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51 schematic editor), PostScript-style comments will be lost because they are not
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52 structured and thus cannot survive the parsing process.
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53
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54 The other kind of comments are structured comments. These take the form of
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55 Comment objects or Comment decorations on other objects; the body of the comment
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56 takes the form of a string token (usually quoted). Because they take their
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57 proper place in the uschem language of objects and decorations, such structured
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58 comments can survive processing by tools that manipulate schematic sheets at
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59 that level.
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60
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61 Graphics model
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62
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63 The graphical aspects of uschem schematics have been borrowed heavily from
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64 gschem, the schematic capture tool from the gEDA project - the latter is the
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65 closest thing to a mainstream EDA package that has ever been used by the author
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66 of uEDA. All geometric dimensions for graphical elements on schematic sheets as
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67 well as the dimensions of the sheets themselves are given in gschem units, and
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68 most of the commonly used graphics primitives have also been copied from gschem.
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69 Users wanting to make use of the graphical features of uschem need to be
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70 familiar with gschem as these gschem-derived graphical features won't be
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71 described here in detail.
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72
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73 There is, however, one major difference between the graphics models of gschem
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74 and uschem. gschem is a GUI application and its graphics primitives are
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75 intended primarily for on-screen display and editing; gschem supports PostScript
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76 output as an afterthought. uschem on the other hand is designed with the
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77 philosophy that PostScript is the canonical and authoritative representation of
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78 a graphical schematic sheet; whatever form is used to represent graphical
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79 elements in the source code, the assumption is that they are all ultimately
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80 destined to become PostScript.
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81
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82 Graphical elements emitted into the PostScript output by uschem-print(1) come
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83 from the following sources:
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84
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85 * GraphBlock structures (objects or decorations) which are containers for purely
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86 graphical elements in schematic sheets;
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87
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88 * Graphical symbols pulled from the library to represent component instances as
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89 well as for other purposes - see graphsyms.txt;
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90
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91 * Text emitted by DisplayAttr and DisplayNetName decorations;
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92
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93 * Lines drawn for GraphNet, NetLine and BusSeg objects.
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94
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95 GraphBlock structures may be of two forms: GraphBlockG and GraphBlockPS. The
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96 former contain gschem code, the latter contain PostScript code. PostScript is
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97 the native form; gschem code is printed by turning it into PostScript, hence
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98 GraphBlockPS can do everything that GraphBlockG can plus much more. GraphBlockG
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99 should be considered a backward compatibility feature. A similar situation
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100 exists with graphical symbols as described more fully in graphsyms.txt.
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101
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102 See psfeatures.txt for the information you need to know in order to write
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103 PostScript code for inclusion into GraphBlockPS structures or symbols.
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104
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105 Coordinate system
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106
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107 As already noted earlier, all graphical coordinates are given in gschem units.
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108 Analogously to both gschem and PostScript default the origin of the coordinate
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109 system is in the lower left corner; x coordinates increase to the right and y
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110 coordinates increase upward. There is, however, one subtle difference between
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111 gschem and uschem coordinate systems. In gschem schematic sheets the absolute
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112 coordinates are essentially arbitrary and one needs to use a special titleblock
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113 "component" to set the overall dimensions of the drawing sheet; the coordinates
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114 that really matter then need to be calculated relative to the arbitrary ones of
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115 that titleblock component. uschem has done away with such silliness and every
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116 graphical schematic sheet in the uschem language begins with an explicit size
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117 declaration.
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118
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119 The sheet size declaration given at the beginning of a uschem sheet sets the
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120 effective limits for the coordinate system, but it does not cause any border or
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
121 title block to be drawn. If such features are desired (they normally are), they
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
122 need to be embodied in a GraphBlock object.
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
123
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
124 One gschem misfeature that has unfortunately been retained in uschem is the
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
125 fictitious nature of the drawing sizes. Using gschem and following the size
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
126 conventions set for its standard symbol library, one has to declare the drawing
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
127 as having size D (use title-D.sym) in order to make it reasonable for printing
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
128 in size B; this situation unfortunately remains in uschem. The sheet size
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
129 declaration merely establishes the size of the in-sheet coordinate system in the
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
130 fake gschem units, whereas the actual print size is given on the command line to
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
131 uschem-print(1). uschem's PostScript prolog code calculates and effects the
cd92449fdb51 initial import of ueda and ifctf-part-lib from ifctfvax CVS
Space Falcon <falcon@ivan.Harhan.ORG>
parents:
diff changeset
132 necessary scaling.