Schematic Library:
Suggested Orthography Rules

Tim Williams

The following file is an HTML conversion of the license notice and instructions for my schematic library. As mentioned earlier, I have a library of circuit symbols and have released it publically. This file represents two things: one, an update to the library, including many more symbols; and two, to set down a formal description of their use.

Introduction

This document goes with the library of symbols I created. It serves as a reference for rules on drawing and annotating schematics drawn using my font, and also as a reference for drawing legible schematics in general. Part of my motivation in writing this is to improve usage; the tube symbols are especially easy to make `ugly' drawings with. If there is any confusion in reading my own schematics, it will also bring understanding. These rules are only my suggestion, of course, and following them exactly will essentially lead to my drawing style. Schematics, as one of the engineer's arts, should be a little distinctive, so not all of these rules should be followed, but clarity and readability of the drawing is paramount, and those rules must always be followed.

Because these symbols were created by, and are used in, Microsoft Windows Paint, they are very accessible to anyone who wishes to draw schematics but doesn't have the software. It is a very tedious process, requiring attention to detail and per-pixel placement and adjustment. Personally, I complete an average sized drawing in about an hour.

These symbols are licensed under the GPL public license. You are welcome to make any modifications and distributions as you wish, so long as you include this file and give credit to the creator (myself). You don't need to credit the font in individual schematics (I know it when I see it), but I would suggest you give credit somewhere, like on your website where schematics drawn with this font are available, or where you're talking about drawings of yours which use this font.

History

The first of these symbols were created in December 2000, out of the need to share schematics online. Photographs of drawings never come out well, and without a scanner, I had no better choice than to draw schematics by computer. Besides, it looks better - not that I have a particularly bad hand, just that everything is neater on the computer. But not having a schematic drawing package, I went about it the same way I do everything, which is, make do with what I have. So, I used Microsoft Windows Paint, creating these pictures as monochrome bitmaps, pixel for pixel. These symbols are more or less the same as the symbols I am used to drawing, neatened up a bit. They may differ from industry standards (e.g., Internation Rectifier's IGBT symbol) or ANSI standards (e.g., shift register), but they are generally simpler and easy enough to understand.

As time goes on, more symbols have been added. The symbols are created with some style in mind; discrete semiconductors and vacuum tubes, for instance, always start with about the same diameter circle. Most active devices are circled. Tubes are usually drawn with two opposed semicircles with straight sides inbetween, forming a flat-sided oval to accommodate extra grids or sections. Integrated circuit symbols range from amplifiers and gates to anonymous rectangles. ICs are represented in several ways: as individual components (e.g., halves of an LM358, the triangle amplifier symbol), as nondescript rectangles with pins called out in order and equally spaced, or as rectangles containing a descriptive representation of their functions, pins arranged logically rather than physically. Time will tell what new symbols are added to this set.

Rules: Parts Placement and Orientation

General

• Conventional current flow places the highest (most positive) voltages at the top of the schematic, and the lowest (ground or negative) voltages at the bottom. This leads naturally to components (such as pull-ups and transistors) "hanging" down from the rails in vertical fashion, while signals travel horizontally, left to right. Rarely are signals travelling right to left (most often, a feedback loop), and positive-voltage sources should only be placed on the bottom under special circumstances.
• Fully balanced circuits, like oscilloscope deflection amplifiers, are often drawn with the bottom half upside-down. This emphasizes the symmetry of the circuit. This style may or may not be preferred. Be careful reading circuits laid out this way. Wherever possible, all two-terminal components involved in carrying bias, load, filter or bypass currents to or from the supply rails should be aligned vertically. For example, all four resistors in the common-emitter transistor amplifier circuit (two base/bias resistors, one collector resistor, one emitter resistor) should be vertical. Likewise, components carrying signal voltages, currents, etc. should be laid out horizontally. The coupling capacitors used in the same amplifier circuit should be horizontal.
• Symbols are always rotated clockwise a minimum number of steps, so that, for example, all resistors (where possible) have their peaks pointing to the top or to the right, all electrolytic capacitors have the plus sign clockwise to the flat electrode, etc. When rotation works, a vertical flip is never used. Exceptions include variable resistors, where the wiper must point where needed, and transistors and other symbols, which may also need to be mirrored to get the correct orientation.
• All other components should be oriented naturally (as written). Transistors should have the base to the left, and collector (NPN) or emitter (PNP) on the top, which follows from conventional current flow. There are many variations and exceptions to this, such as complementary transistors (in the sense that their function or location in the schematic is asthetically complementary, not that they are electrically complementary): for instance, as in a differential amplifier pair.
• Exceptions to rules on placement and orientation are not made based on space: if you are running out of space, stretch out the diagram and make more room. Or split up the project over several sheets. Empty space is not wasted, it improves readability. The average screen resolution is higher today, so one can afford plenty of area in a drawing.

Vacuum Tubes

• Most tube symbols are drawn with extra connections, most of which should never be used. This was done to align grid connections properly from either side, without flipping the symbol. All unused connections should be erased. Where multiple connections are made to the same lead across several tubes, using the extra line as a lead-through is discouraged, since it implies the tube has two connections on that grid, which isn't always true, and may not even be what you meant. (For aesthetics, you may prefer to mirror a symbol anyway, such as two triodes (not in the same tube) used in a differential pair. Dual tubes, with both sections used adjecent, should be shown with a dual symbol.)
• Dual symbols should never be broken up. Tubes are always drawn with solid circles, no open arcs or dashed lines implying the pairing of sections. If the tube is a section of a multisection tube, specify in the label ("V2b").
• The symbols typically include a heater, which is convienient for simple connections (like a 6X4 rectifier, where the heater supply is probably near where the tube is used), but where illustrating heater connections is superfluous, the heater should be erased. Seperate heater symbols are provided to illustrate their connections, usually placed in the power supply section.
• Tubes should be drawn upright, but as with transistors, there are occasions where different is best. Balanced circuits are often drawn with dual triodes rotated 90°, and pass regulators at 270°.

Labeling

• Values should be applied near all components, centered along one side where possible, and spaced off about three pixels. Multi-line labels are usually left-justified, but center or right justify may look better at times. Part numbers should not be line-wrapped.
• The typeface should be Microsoft Sans Serif, 8pt, for all numbers, types, labels, etc. Pin numbers are given in Small Fonts, 7pt. Headings should use Microsoft Sans Serif 14pt or so, depending on how large the heading needs to be.
• Labels should be applied to all components in large or formal circuits. When components are labeled, they must still be given values. Never supply values only in a seperate parts list. The component's number ("Q5") should appear on the first line, alone. Part number or value goes on the next line. Numbers are assigned sequentially starting from the first (C1, C2, etc.), or if the drawing is sectioned up (in one file or over multiple files), starting from hundreds (R101, R201, etc.) for each section.
• Sectional information should be given for any multisection device. Some examples are:

The `fraction' goes up sequentially, specifying which section is used.

• Pin numbers are called out on all integrated circuit rectangles, but are optional on components, including amplifier sections, vacuum tubes, etc. On ICs, pin functions are always labeled in Small Fonts, 7pt, placed inside the rectangle, centered beside the pin, two pixels away from the rectangle.

Wires, Routing and Placement

• Adjecent wires and components are spaced reasonably evenly, typically 8, 12, 16 or 24 pixels apart. Resistors, transistors, ground symbols, etc. are aligned in rows or columns where possible, yielding pleasant symmetry.
• Wires are always laid perfectly vertical or horizontal, except in certain cases where a diagonal line is more elegant. Examples include R/S flip-flop, star grounding, etc.
• Intersections: nodes of three or more wires joining are always indicated with a 3 ×3 pixel square dot centered on the crossing. Crossovers are always drawn by removing 3 pixels from the vertical line, centered on the horizontal line. The horizontal line continues unbroken, while the vertical line seems to disappear behind the horizontal's shadow. (Although producing a mild illusion of depth, this rule is not intended to produce a consistent depthwise aspect. When many lines cross over each other, all vertical segments always yield to horizontal lines.) Where diagonal lines cross, one or two pixels are removed from the upper-left-to-lower-right angled line, enough to give a visible break.
• External connection points and signal/power sources (but not loads) are denoted by an open circle, 5 pixels across. Flags can also be used, in which case the angled end indicates general forward signal flow. A circle is also used to indicate temporary connections, such as a device under test. Modular connections are indicated with the double-chevron symbol (an arrow and a mating inverted arrow, with no direct overlap).
• Wires are routed in a clear manner, spaced as mentioned and arranged to produce as little confusion as possible. Bundles of wires may be labeled at each end for perfect clarity, and bundles of many similar wires (e.g., address or data bus) should be gathered together using the bus symbols. Buses are represented with a five-pixel-wide line. Where wires enter or leave a bus, which lines they are should be labeled.

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