Concept:

Schematic diagram:

• A  schematic diagram is a fundamental two-dimensional circuit representation showing the functionality and connectivity between different electrical components.
• It is vital for a PCB designer to get familiarized with the schematic symbols that represent the components on a schematic diagram.

The following points should be considered while drawing schematic diagram:

Name:

• Use manufacturer's part name (some exceptions to this rule, such as for common symbols such as passives where the generic name is used, like resistors, capacitors.)

Pin Configuration:

• ​Inputs on left
• Outputs on right
• VCC & power pins are placed on the upper right for ICs
• GND pins on the lower right for ICs
• I/O pins on the middle-left (occasionally middle-right too if there are many)
• Control pins at the upper left
• Use one symbol pin per function even if it maps to various package pins
• Pin type must reflect input type (Ex. io, in, out, pas etc.). for proper ERC/DRC checks

Special Pins:

• No Connect pins must be named as NC on the symbol
• Active-low pins should be designated with a bar above the symbol name
• Clock pins designated with a triangle

 Additional data: 

• Add MPN(manufacturing part number) above symbol
• Add reference designator and value placeholder below symbol
• Reference designator prefix should be as per component eg. R for resistor, J-jumper, L-inductor etc.

Physical and electrical Rules:

• Always add block diagram of the circuit so as to get the quick view of the various interfaces(most important)
• Add title block which includes organization name, page size, date, sheet number,version, title of the schematic name etc.
• Use A4 size sheets.
• Keep the grid spacing at least 2mm so that while creating pdf of schematic you can easily see the signal name.
• There should be no identical part reference.
• Make sure you are always using right reference designator for the symbols eg. R-resistor,C-capacitor etc.
• Every pin must be connected to a net if there is NC pin add no connect property to it so that no DRC error pop-up while generating DRC report.
• If there is any heterogeneous symbol make sure every part of the symbol has to added on the schematic sheet .
• Minimum font size should be 12 (I prefer while drawing schematic, don't know about industry standards regarding this.)
• Every symbol have a footprint attach to it.
• Make sure pin-type is correct. Add power pin type to the pin to which power signal has to be connected
• No duplicate net name.
• The movement of switches, push buttons and contacts should be from left to right.
• Add Power and Voltage rating to each resistor used in the schematic.
• Off-page connector used to connect the signal on two or more pages must have same net name.
• If there is any critical net/high speed signal like address or data signal of DDR,PCIe, HDMI,USB, or any high speed interface always mention the routing guideline/precautions while routing the signals.

Explanation:

Switches, push buttons and contacts should be drawn in actuated position must NOT be considered while drawing a schematic diagram.