integrated-circuit
I have a SN75176 chip. The datasheet shows it with a semicircle at one end for orientation however my chip looks like this:
Is it safe to assume the pin by the circle is pin 1?
Why would the datasheet show a semicircle and then a circle at the other end?
The physical pin-outs of all "555" devices that I know of are the same.
In general, schematic symbols are generated to match the standards and/or prefernces of the organization or engineer that is using the component. Sometimes, a PCB designer creates the schematiic symbol in any way that seems fit. Many times a designer will use whatever schematic symbol they can find in a manufacturer or third party library.
Some like to make the symbols match the manufacturer data sheet. Others like to make the symbols match the physical pinout (so they can visualize a layout while looking at the schematic or, when debugging, figure out what pin to probe more quickly). Some will arrange symbols to have inputs on the left and outputs on the right. Some organizations allow pins on the top and bottom of symbols, some don't, and some only allow power and ground pins. Some symbols (for digital devices and op-amps etc.) are drawn without power or ground pins; these pins are "globally" connected to the power rails.
In my opinion, many of these standards do not make sense for many chips. In my mind, a schematic should show the "scheme" of the circuit; accurately capturing design intent and allowing easy design analysis.
The bottom line is that you may see as many schematic symbols for a part as there are engineers to create the 'perfect" symbol.
Check the diagram in the datasheet, this chip is 'just' a bunch of 5..7V zener diodes.
If you use a low voltage (<5V) you can test the diodes. If your multimeter uses a low voltage you could even use it to measure the resistance: pins 1-2 should have a low resistance in at least one direction, much lower than 4-5.
But the fact that the dot has disappeared makes me wonder whether the chip should still be trusted. Note that this is a protection chip, so soldering it in and checking that the device works is no guarantee.
Best Answer
There are a number of ways of denoting which pin on an integrated circuit is pin 1. The most common is probably a small dot, either in the plastic molding or printed on it, but you also frequently see ICs with a notch cut out of one end.
Here are a few demonstrations: On a DIP, SOIC, SSOP, or similar package, you usually see either the notch or the dot:
or even both:
On a QFP, it's usually the dot:
or the dot plus notched corner:
and on a large BGA or PGA, and maybe also LGA (though I'm not too familiar with LGA) you might see a notch cut out of one corner:
SIP, TO-220, TO-92, DDPAK, and other packages just have their pins numbered from left to right as seen with the part number facing you. These sometimes have a dot as well:
In general, if you aren't sure, check the datasheet. Any nonstandard pin numbering will be explained in it. (and note that some pin numberings will skip numbers! This is especially true for DIP modules that have 'missing' pins.)
As a further note, some packages (such as the TO-220 in the last image above) have an additional 'pin'; in the case of the TO-220, it's a large tab with a hole in it designed to be mounted to a heatsink.
In other cases, you might see a package labelled, for example, SOIC-EP, standing for Exposed Pad, which has a large metal pad on the bottom of the chip, intended for thermal dissipation.
What number these tabs or exposed pads are given depends on the device; sometimes, they share a number with one of the regular pins, which they're also internally connected to, but sometimes they have their own number. They're usually intended more for thermal purposes than electrical purposes, but they are almost always electrically connected to something internally.
Most images obtained from http://www.amiga-stuff.com/hardware/pinnumbering.html.