let the noob questions continue:
is this a pull-down resistor?
And if so:
-Is it just to keep a pin "low"?
-would i use it on any out pins as well?
If no voltage is going to
If no voltage is going to the serial in pin, then the voltage is kept low. If that is not there, the pin could register either high or low depending on the environment, and that can upset how your chip works.
and the other way around
If the resistor would connect the serial in to V+ it would be called a pull up resistor.
A piece of wire would do, but the current to the port (or from the port to GND) would get too big and you would fry something. You don’t need any real current, so that’s why a resistor is the preferred choice.
You know about V=I*R right. Or in this case I=V/R. In a 5 V system and a 10 kilo Ohm resistor, the (max) current would be 0.5 milli Amp. You could even go for a bigger R and the (max) current would be limited to 0.05 mA. That’s fifty micro Amp. And would still keep the pin high, or low.
A pull resistor like that is not just for protecting your chip anymore, but also for keeping any current from leaking away from your precious battery charge when your bot is just sleeping.
Well, some sort of
Well, some sort of resistance needs to be there. Electricity takes the path of least resistance, and with just a wire (0 ohm of course!) it might take that path instead of the serial in pin when voltage is applied. Also a reason why the resistor is so high, to make sure that the voltage applied goes to the pin, but the pin is secured in an "off" position when no intentional voltage is applied.
so the tearms you need to
so the tearms you need to know are
pull up - pulls the logic high
pull down - pulls the logic low
floating - logic is not pulled up or down. This, as zanthes mentioned, can cause unpredictable behevior in some cases.
in the case of the circuit you are showing, that 10 res is critical to the operation of the chip and if you build your own circuit board and use one of the picaxe programming adapters. you will need to pull the serin pin low.
And of course wikipedia is a great source http://en.wikipedia.org/wiki/Pull-up_resistor
on your second question, yes it’s possible, say if you are connecting to a logic chip that needs to be pulled down by default. I think like a AND gate.