Preforming impedance termination for UART Communication (TX and RX) pins

Question

I need two Arduino devices (Arduino Mega and Nano) to communicate with each other via UART. The two Arduino devices are long distance apart from each other. The distance is 1.5 meters.

• The Arduino 1 TX pin is connected to Arduino 2 RX pin.
• The Arduino 1 RX pin is connected to Arduino 2 TX pin.

Now, I have to use resistor to preform impedance termination so that the long distance does not cause transmitting and receiving issues between the two devices. I did some research and found an image that shows the signal differences when impedance termination is preformed and when it is not.

I have two questions:

1. Since the distance between both Arduino devices is 1.5 meters, what resistor value should I use? How is that calculated?

2. Where do you put the resistors? It's not really clear on where to connect the resistors. Between ground and the RX pins on each device?

I don't know the gauge ratings of my wire but if that information is needed, I am using this wire.

Answer 1

The relevant parameter is not the length of the link, but its characteristic impedance. For a twisted pair connection, this is typically in the order of 100 to 120 Ω. It should be specified in the datasheet. Your cable is not a twisted pair, but its geometry is quite close. Just make sure that there is a ground or Vcc line between the two data wires in order to minimize cross-talk. If you want to compute that impedance yourself, here is an online calculator.

Connect the resistor in series with the TX pin, i.e. between the pin and the data wire. Note that the pin already has an internal output resistance of about 25 Ω, so you would add no more than 100 Ω in series.

If you search online about termination resistors, you may learn tat they are often put between the receive pin and ground, or even at both the sending and the receiving end. In high frequency applications, the latter is often the preferred configuration, as it prevents the signal to be reflected from either end of the link. However, for logic-level signals, that configuration would attenuate the signal too much. On the other hand, terminating only at the receiver would put excessive load on the TX pin. Thus, sender-side termination is your best option. A resistor in series with TX does not prevent a reflection at the RX end, but it prevents the reflected signal to be reflected again and cause oscillations.