Steve, First, its always "fun" to tackle some of the questions that you throw at us when you are checking to see whether or not we are awake. So my "fun" answer follows, but should not be taken to seriously. I'm actually going to pull a quick one on you here and say that "none of the above" (or in this case "below") are correct. First and foremost, the schematic symbol in both cases is that of a regular signal diode, as opposed to the correct symbol for a LED (Light Emitting Diode), which universally would have some kind of little "squiggly" ray, or at least a little arrow or two, beside the diode symbol. Secondly, while it is almost universal that the Anode is the longer of the two leads, I have personally experienced LEDs with both leads cut to the same length, and additionally LEDs that were in "tape" where you cannot tell anything by the length because the ends are obscured. The problem here is that by industry standard, almost universally, diodes of every kind, including LEDs, are actually identified by a band or some other marking on the Cathode lead of the diode. Specifically, when it comes to LEDs, I think that you will find that in spite of the Anode lead generally being longer, that the actual preferred industry standard is to have a "flat" on the side of the LED which identifies the Cathode lead, and that it is this "flat" that is actually what has been standardized as the orientation identifier. There are of course, just as with everything else in the world of PCB Design, exceptions to the rule. One example would be a LED which did not have a flanged base which could have the "flat", but it is hard to tell from your picture whether or not the LED in your case is round or square, and whether or not this would apply in your case. Another would be in the case of different SMT packages, which is obviously not the case in your instance. Respecting the case where an LED did not have a flange, I have actually seen LEDs that didn't have the flange, but which still had an indented "flat" on the circular body. Actually, the real answer to your question is dependant upon the actual physical part that you specify in the Design, and whether or not it has any specific distinguishing orientation marks or characteristics, and also importantly whether or not there any approved substitutions which may differ from the primary source part. In any event, your drawing does not look at all like your part, which again could lead to ambiguity, and more importantly does not indicate the industry standard "flat" (which again may not be present in all cases), but more importantly I would be afraid that with all of the information that you have given them in your pictorial explanation, that some would except it all as the "gospel truth", and save a copy of your little pictorial explanation for future reference, and possibly make improper references somewhere down the line, such as using the shape of the internal part of the leads for identification, when they should be looking first for a "flat", and secondly I would give you the lead length for most cases, but I would not bet my life on all LEDs having the "die" attached to the Cathode (although I would probably give you five 9's on that one). Point here being that you can give someone too much information, such as the picturing the internal structure, when they should really be looking at other things. Remember who we are dealing with here, and that it is your responsibility to make it clear on the drawing so that there isn't any ambiguity. Sometimes the problems arise when there is not enough information to work with, but conversly, sometimes the problems really can arise because of too much information. Respecting an interesting example that I just grabbed at random from Digikey, look at page two of the following PDF: http://rocky.digikey.com/WebLib/Lite-on/Web%20Data/LTL-2F3VxKNT,H3VxKNT,P3VxKNT,R3Vx KNT.pdf . . . and then answer this question for me: Which is the proper interpretation respecting a LED which has a long lead on the Anode, and a short lead on the Cathode (please select only one answer): O The longer lead identifies the Anode. O The shorter lead identifies the Cathode Six of one, half a dozen of the other. I would have to fall back on the industry standard convention of identifying the Cathode with a "flat", and in the lack of a "flat" I would have to fall back on the manufacturers datasheet, which oddly enough in this example identifies the shorter lead in addition to the "flat". Now as to which lead should be "Pin 1". You will never resolve this issue to everyone's liking, but the fact is that the industry standard is and always has been to identify the Cathode end of the diode for orientation. Getting back to the primary issue, which is how will we resolve this issue in the real world of PCB Design and Manufacturing: IPC-7351 has resolved the issue by stating that from a Component Library of the PCB Design perspective, all two terminal components shall be oriented horizontally, with "Pin 1" on the left (at "zero degrees of rotation" (before being placed in the Design, which may require rotation)), and that further, that when two terminal components are "polarized" (require specific orientation to operate properly), that the Polarity Indicator shall always be "Pin 1", which again is always to the left, and that for Capacitors the Polarity Indicator shall be the Positive Terminal, and that for Diodes the Polarity Indicator shall be the Cathode Terminal (as has always been the industry standard). "Pin 1" is always to the left at "zero rotation" in the Component Library, and it is always the Cathode. So will this make everybody happy? Obviously not, but it will resolve the issue, and it's related problems, "IF" it is properly applied and adhered to. Now respecting the where the "square" pad should be, if one is used at all, in view of the above, and in view of the fact that the industry always identifies the Cathode of a diode (yes I know that someone will show me a conflicting example), I would say that if it was used at all, that it should be on the Cathode lead, but preferably, in most cases, a standard T 1 3/4 LED should be identified on the silk screen with a circle that has a "flat" at the Cathode lead, and beyond that, do whatever it takes to get your shop to put the LED in your Board correctly. Now, respecting those who would cry that the leads should be identified as "A" for Anode and "C" for Cathode, I would throw the original versions of ANSI Y32.2, and even possibly back as far as MIL-STD-15, at them, and say that they would have to make that "A" for the Anode and "K" for the Cathode. Boy, talk about a screw up, the Military really blew it on that one, and messed up the industry for a number of years on that one, but what do you expect, SNAFU. That one was almost as bad as the Navy changing the abbreviation for Ground from "GND" to "GRD", and then back again to "GND", but not quite. AAAARRRRRRRRRGGGGGGGGGHHHHHHH!!!!!!!!!!!!!! ( As he lets out the occasional Primal Scream ! ) Hope it's been "fun". JaMi * * * * * * * * * * ----- Original Message ----- From: "Steve Gregory" <[log in to unmask]> To: <[log in to unmask]> Sent: Tuesday, October 26, 2004 7:14 AM Subject: [TN] Diode Pin Numbering Question... > Mornin' All! > > There's been a pretty interesting thread going on in the Designers Council about what is supposed to called pin 1 of a diode...the Anode or the Cathode. Believe it or not, there's been conflicting answers. So I'd like to do a little test. > > Look at: > > http://www.stevezeva.homestead.com/files/LEDPin1.jpg > > and tell me if we installed this LED correctly. > > Then look at: > > http://www.stevezeva.homestead.com/files/LEDPolarity.jpg > > and tell me if this this illustration guide I made for our operators is correct. > > (This ought to be good) > > -Steve Gregory- > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 1.8e > To unsubscribe, send a message to [log in to unmask] with following text in > the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) > To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest > Search the archives of previous posts at: http://listserv.ipc.org/archives > Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 > ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------