> > This is a cmos logic device.. not a power driver. It will not burn > > out any resistor. You often see resistors inline to limit current > > (say, for LEDs or transients from off-board inputs), but these are > > NEVER zero ohm resistors. > > I will have to agree with that, a real small diode would be more > standard there, I've never seen a "0 ohm" resistor? > > > > A zero ohm resistor is a piece of wire. It is not a fuse. I would > > recommend that you re-read Charles Steinkuehler's analysis for the > > most likely function of this (most likely not installed) piece of > > wire. > > Yep, I did and what you are saying makes sense. Why would you > call a piece of wire (a jumper) a resistor? It seems that you would > call it "J8" or something or something along those lines. In any case, > what "R8" does, as Charles noted, takes the Wait# from the LD017_A0 > to pin 30 (ground) on the controller. Mike said the tech told him that: > "They use a hardware solution shunting R8 to ground", which indicates > to me that R8 was designed to be a component besides a piece of wire. > A shunt is typically a resistor or an inductor coil. A piece of wire > would work, but probably not what the design team had in mind. > Now, what the design(ers) had in mind, I won't necessarily guess at.
Don't let the reference designator "R8" fool you. Using zero ohm resistors (and that is exactly how they're referred to) is an old, old trick. There are typically two major uses for zero-ohm resistors: - The classic use is for "jumping over" other tracks on a circuit board. This is much more frequent with through-hole parts than modern surface-mount assembly techinques. If you've got a fairly simple PCB, using zero-ohm resistors rather than a PCB trace might allow you to use a single sided PCB instead of a double-sided, or a two-layer PCB instead of a 4 layer. Yes, the zero-ohm resistors cost money (about 0.0025 cents each...yes 1/4 cent), and can be auto-inserted and crimpted in place along with all the rest of the resistors on the board (normally, adding one more reisistor value to the parts list isn't that cost prohibitive, especially if you're saving a bunch on the PCB. - The other big use for these resistors (especially in the modern surface-mount era, where a resistor doesn't take up much space) is for build-time options. Basically, you're using the resistor as a switch...populate it and the switch is closed...leave it off the board, and the switch is open. This is almost certianly what the sst folks are doing with R8. It is definately what they're doing with R4. For those who haven't seen the schematic, R4 shorts across a 3.3V regulator for the "low voltage" version. If you purchase the 5V version of the ADM, you get a 3.3 V regulator, and no R4...purchase the low-voltage version, and you get a zero-ohm R4, but no 3.3V regulator. So why call them resistors when they're zero ohm? Well, because they are *NOT* shunts, or inductors, or even jumpers (although that description is fairly close). IMHO, the main reason they are still referred to as resistors is because the "look and feel" like a resistor. Zero-ohm reisistors come in the same size package as any other resistor, and are typically indicated on the schematic as a resistor symbol. Why? Because it's easy...the CAD tools already have a resistor symbol on the schematic, which is likely already configured to pull up the proper PCB footprint patterns, so just setting the resistor value to "0" is quite straight-forward. In the same vein, it's easiest to leave the reference designators as Rn, which further leads to the tendency to refer to the parts as resistors. NOTE: I personally also use the value "XXX" for parts that appear on the board, but should *NOT* be populated, but I don't call these parts "infinite ohm resistors", and AFAIK, such a part is not readily available, although I'm sure for the right price, someone would make some for you... Sorry if this is way too far OT, and I hope my ramblings haven't put anyone to sleep. ;-) Charles Steinkuehler http://lrp.steinkuehler.net http://c0wz.steinkuehler.net (lrp.c0wz.com mirror) _______________________________________________ Leaf-devel mailing list [EMAIL PROTECTED] https://lists.sourceforge.net/lists/listinfo/leaf-devel
