It is possible to define a schematic symbol using a script. This method is used in some
of the internal scripts to create dynamic symbols. For example the transformer devices
allow the user to define the number of both primary and secondary windings. The
symbols for these are not stored in the symbol library but generated programmatically
using the commands described in this section.
Symbol scripts can also be useful to edit symbols using a text editor. Some operations
can be more rapidly performed by editing a text definition than by using a graphical
editor. To support this method, SIMetrix includes the MakeSymbolScript command that writes a script definition of a symbol in ASCII form.
The following sections describe how to define a symbol using a script.
Defining New Symbol
To define a new symbol (as opposed to modifying an existing one) proceed as follows:
Enter the text definition as described in Symbol Definition Format
into a text file (using NOTEPAD for example)
Load the new definition by simply typing the name of the file at the command
line
Test that your new symbol is as you expect. Use the menu Place|From Symbol
Library to place your symbol on a schematic
Note that as the schematic stores its own copy of each symbol, if you modify the
symbol after first defining it, the changes will not be reflected in any existing
schematics unless the "track" flag is set. This is done by providing the switch
/flags 1 on the CreateSym command line. This performs the same function as the
"All references to symbol automatically updated" check box in the symbol editor save
symbol dialog box.
To update the symbol on a schematic from the global library use the popup menu
Update Symbols.
Symbol Definition Format
The following commands are used to define schematic symbols:
To describe the symbol definition format consider the definition for the npn transistor
supplied in the standard symbol library. In text form this is:
* NPN BJT
CreateSym npn "NPN bipolar" analog
AddSeg 0 0 0 200
AddSeg 0 100 100 0
AddSeg 0 100 100 200
AddSeg 100 200 80 160
AddSeg 100 200 60 180
AddSeg 0 100 -100 100
AddPin C 1 100 0
AddPin B 2 -100 100
AddPin E 3 100 200
AddProp ref Q? 26
AddProp value NPN_MODEL 26
AddProp model Q 64
EndSym
Let's go through it line by line.
The first line:
* NPN BJT
is a comment. Any text may placed after a '*' as the first character will be ignored.
The next line:
CreateSym npn "NPN bipolar" analog
begins the symbol definition. The first argument - npn - is the symbol name. This must
be unique and cannot contain spaces. It is used to place the symbol on a schematic. The
second argument is the description and is optional. This is what will appear in the
choose symbol dialog box opened by the schematic popup Place|From Symbol
Library… menu item. If no description is given the symbol's name will appear in this
dialog box. The final parameter is the catalog name. This is used to categorise
symbols. Although the parameter is optional, it is strongly recommend that it is
included.
The following six lines:
AddSeg 0 0 0 200
AddSeg 0 100 100 0
AddSeg 0 100 100 200
AddSeg 100 200 80 160
AddSeg 100 200 60 180
AddSeg 0 100 -100 100
describe the symbol's six straight line segments. The four numbers on each line are the
x and y co-ordinates of the start and end of each segment. 100 units represents one grid
square (at X1 mag) on the schematic. The diagram shows the co-ordinates of each
segment end.
The next three lines:
AddPin C 1 100 0
AddPin B 2 -100 100
AddPin E 3 100 200
describe the location and attributes of the symbol's three pins. The first parameter on
each AddPin command is the pin's name. This must be the same as the pin name used
by the simulator for that type of device. If the name is different it will not be possible to
cross-probe currents for that type of device. See User's Manual/Further Information/Summary of Simulator Devices for details of pin names for devices supported by the
simulator. If the device is a subcircuit then any pinname may be used. The second
parameter is the pin's number. This affects the order in which the pin's connected nets
appear in the netlist. This must comply with the netlist format. Again refer to
"Summary of Simulator Devices" for full details of each device. The last two
parameters specify the co-ordinates of the pins on the schematic. They must be a
multiple of 100. If they are not it will not be possible to connect to them as wire ends
always snap to a grid point. See AddPin for more details.
The next three lines:
AddProp ref Q? 26
AddProp value NPN_MODEL 26
AddProp model Q 64
are the symbol's properties. A symbol's component reference, value (or model name)
and the type of device are all specified by properties. The first line above attaches a
"ref" property (aka component reference) and gives it an initial value of Q?. The final
parameter '26' specifies how it should be displayed on the schematic. The model
property in the third line specifies the type of device (e.g. resistor, capacitor, BJT etc.)
and is always a single letter. It is not compulsory. If it is omitted the first letter of the
ref property is used instead. See "Summary of Simulator Devices" for full list of
devices supported by the simulator and their required model properties. Full details on
properties are given in the User's manual. For more information, see AddProp command.
The final line:
EndSym
terminates the model definition. The symbol will not be recognised until this is
executed.
How Symbols are Stored
Symbol definitions are first stored in a .sxslb file which resides in the SymbolLibs
directory. These files are managed by the symbol library manager. When a symbol is
placed on a schematic, a copy of that symbol definition is stored in the schematic and
from then on the schematic will use that copy of it. This means that if you change a
symbol definition for a schematic that is saved, when you open that schematic, it may
still be using the old definition as it is saved with the schematic. Whether or not the
symbol is updated automatically depends on how it was saved. If /flags 1 was
included with the CreateSym command, then it will be automatically updated.
If you wish to force the schematic to use the new symbol, select the symbol or symbols
then select the popup menu Update Symbols. Note that all instances of the symbol will
be updated. It is not possible to have two versions of a symbol on the same schematic.
