Resistors For Serial Communication

I am trying to learn more about Arduino. I heard in almost all cases I should use a pull up or pull down resistor for serial communication. It helps with serial communication. Is this true?

By pull up resistor, I mean placing a resistor (typically 10 ohm) between the power and serial communication. By pull down resistor, I mean placing a resistor, typically 10 ohm, between ground the serial communication. When should I use pull up or pull down; when would this work? Is it typically 10 ohms based on the number of volts arduino can handle?

Not sure why this comes under programming ;)

For serial communications using UART it is not usually necessary to use pull-up/down resistors. The reason for pull-up resistors is that when a line is not being driven specifically high or low the line is floating, just picking up the electrical noise in the environment. In these cases you would get spurious signals from your connection, so a pull-up/down resistor is added. Someone might put a pull-up resistor on a UART line as this causes the line state to default to HIGH (which is what UART defines for an idle line). This is unnecessary for most cases though, if the transmit is on and initialised the device should be holding the Tx line HIGH. If you are designing a system where you are unlikely to have your UART bus connected at start-up then it might be a good idea to put on a pull up resistor on the Rx line just in case.

Pull-up/down resistors are usually in the range of 10 kOhm not 10 Ohm.

Thanks. Sorry if I posted this in the wrong forum- not sure how to move it. Would it hurt anything to put on a 10k pull up resistor or pull down resistor? Let's say that the system was communicating fine- could it cause an issue?

jjrr007: Let's say that the system was communicating fine- could it cause an issue?

Why fix it when it's not broken?

The higher the resistance the slower any capacitive charge in your wire will discharge through the resistor. At the same time the lower the resistance the faster the discharge, but also the more current gets drawn, wasting power. Overall it's a balance between these two factors, but as codlink says, Why fix it when it's not broken? If you don't have problems then you don't need to add more complexity to your circuit. I doubt you'll find many UART systems that have pull up resistors on, there's just no need.

Thanks for the replies. Good question. I would add a resistor on a working serial connection to increase reliability is my thought. Based on the earlier post, a pull up resistor can help ensure that the line is high like the "default high" . I have noticed that some hall effect sensors need it (i.e.http://www.seeedstudio.com/wiki/index.php?title=G1/2_Water_Flow_sensor) and some equipment could be hard to service in remote locations.

You are saying there is only a battery impact to adding it and it can help improve things. Is that right? Would it make a working serial connection more reliable to use a 10 K ohm pull up resistor? Which is better for making something reliable- pull up or down for reliability?