IC and connector pin numbering
When reading schematics or following repair guides there are often references to pin numbers on ic's or connectors.
How do you easily find the correct pin ? Or how do you correctly replace an ic or an eprom in a socket ?
Luckily the numbering follows a universal standard in the electronics world.
IC pins are numbered counter clockwise
Pin 1 (or the side where to start counting) is normally indicated on the IC.
How it is visually presented can have many forms - a physical indentation, or it can be a printed mark.
Find the marking / notch. Then follow counter clockwise.
When replacing chips (ICs or eproms), always check for the indentation to find pin 1. Never use a label or printed text as orientation of direction the chip needs to be inserted.
These different ICs are all oriented the same - pin one is always at the bottom left. This shows you many different ways it can be indicated and on what identations are important and which not.
The LM339N IC on the top right shows the most straightforward way - there is an indentation on a short side, and no other markings. Follow the rule, go counter clockwise starting at the indentation.
The two ICs on the left are similar - only the identation in the small edge count. You ignore any identations on top.
The IC at the bottom right is different, it has no indentations at its sides. Pin 1 identified with a printed white square. You ignore the round indentation next to it..
On boards, pin 1 of a connector is indicated
There isn't a strict rule, and it depends of who designed the board.. so if you want to be safe then you always search what is printed on the pcb.
If you want a rule of thumb : connector pins often go clockwise. For instance on Bally / Williams WPC games and Data East games, this rule is followed by the board designers. Even the connector numbers themselves usually follow this (J1 will be at the top, J2, J3, .. follow clockwise). Unfortunately exceptions are always possible, sometimes connectors get added later and thus the numbering doesn't follow, or board designers do their own thing..
Let's see in detail how it is on Bally / Williams machines.
Connector pins are numbered clockwise. That's easy to remember, right ?
Look at the picture to understand this, as explaining it in words can become confusing.
Follow the edge of the board clockwise, starting from the top.
At the top edge, go clockwise, so count pins from left to right. At the right side, follow the pins up to down. At the bottom and left side of the board, the pin numbering is opposite : right to left and bottom to top.
Especially at the bottom people often get confused, as you're used from reading left to right. Counting pins from right to left is not intuitive. I've seen a few errors because of this, sometimes people rebuild a connector and put the wires in the opposite order.
Note how pin 1 and 7 are printed for connector J114 ? The J113 flatcable connector also has pins 1 and 2 and the last pins marked.