Legal use of Y-Cables?

[jgraber]

Originally Posted by Disciple855

Also, I wonder what would happen if the sensors being Y-cabled were different. Like say a Gyro and Potentiometer hooked up together.

Y of two potentiometers would likely provide an average of the two,
since there are no active circuits.
A Gyro has an active output buffer, so it would likely fight another active output.

Originally Posted by magicode

Y-cabling works for boolean sensors as an OR gate: the output is 1 if either inputs are 1, and 0 if both inputs are 0. I suppose you could use it to average analog sensors, but you would have to non linearly map the output (Ohms law). Using y cables to combine other digital sensors, or different sensors altogether will give you no meaningful data.

Almost right. Y cable on bumper switches will be an active-low logic wired-OR.
Default input value is 1, provided by an internal (to the Cortex) pullup resistor.
Pressing A OR B switch (which shorts signal line to Ground) will cause input to read 0 instead.
This type of wired-OR active low is common in electronic circuits.
For example, several daughter-cards on a mother-board can share an interrupt line with wired-OR, so any one of them can signal "hey, there is a problem".
The MB doesn't know who yanked its chain, so it has to go ask each board individually (polling).

[bobsalive]

Originally Posted by jgraber

Y of two potentiometers would likely provide an average of the two,
since there are no active circuits.
A Gyro has an active output buffer, so it would likely fight another active output.

Wouldn't a Y of two potentiometers basically put the two pots in parallel, causing you to receive a value that is related to (R1xR2)/(R1+R2) ? I guess it wouldn't be the same as (potValue1 x potValue2)/(potValue1 + potValue2) though, so might not be very useful anyway.

[Quazar]

Originally Posted by bobsalive

Wouldn't a Y of two potentiometers basically put the two pots in parallel, causing you to receive a value that is related to (R1xR2)/(R1+R2) ? I guess it wouldn't be the same as (potValue1 x potValue2)/(potValue1 + potValue2) though, so might not be very useful anyway.

You would have to be careful with putting two pots on a Y cable. If they are turned to opposite extremes you could have significant power flowing through them (in the neighborhood of 5W) which could damage them.

Cheers,
- Dean

[magicode]

Originally Posted by jgraber

Almost right. Y cable on bumper switches will be an active-low logic wired-OR. Default input value is 1, provided by an internal (to the Cortex) pullup resistor. Pressing A OR B switch (which shorts signal line to Ground) will cause input to read 0 instead.

Yes, I am familiar with how the digital inputs work, I was referring to how they appear on a programming platform. ROBOTC at least registers 1 when the button is pressed and 0 when it isn't (if the senosr configured as a button, which the OP said he was using). I'm don't know if EasyC does this as well.

[jpearman]

Originally Posted by bobsalive

Wouldn't a Y of two potentiometers basically put the two pots in parallel, causing you to receive a value that is related to (R1xR2)/(R1+R2) ? I guess it wouldn't be the same as (potValue1 x potValue2)/(potValue1 + potValue2) though, so might not be very useful anyway.

To make a log pot a fixed resistor is often put between the wiper and ground, this is the graph obtained using one pot midway and moving the other, a nice non linear curve.



As quazar said, it's generally a bad idea as you can short power and ground with the pots at opposite ends. So the general rule here is don't do it.