# Connecting BeagleBone Black using serial port - voltage

I want to connect PC to BeagleBone Black using serial port. There is a note on the BeagleBone website that I need a 3.3 serial adapter.
I do have a converter USB-rs232 using FT232RL, that gives me at the output 5v (I think so).

Can I make a voltage divider for output of my converter to make it 3.3V or do I need another converter 5V-3.3V?

EDIT:
This converter produces 0-5V voltages, I was using it earlier for communication with Atmega16 and it worked fine.

You need to have a 3.3 V serial adapter. A voltage divider is ill advised, because you need to maintain low impedance and you don't want to draw that much current on either side. If you insist on using the 5 V, then an active level shifter is recommended.

It is not surprising that it worked on the Atmega. Most microcontrollers will have pins that will tolerate 5 V.

No, if you have a USB-to-RS232 converter that truly produces RS232 voltages it will not be compatible with a 3.3 V logic-level connection. RS232 voltages are $\le -3$V for a logic 1 and $\ge +3$V for a logic 0.

• I don't know the naming conventions, so maybe I named it wrong. This converter produces 0-5V voltages, I was using it earlier for communication with Atmega16 and it worked fine. – Piotr Curie May 1 '14 at 18:49
• It sounds like the part in question is actually a USB to UART converter. Not sure why RS232 is even in the name. This answer is correct if it were actually a USB to RS232 converter. – dustincarr May 1 '14 at 19:28

The ft232rl is a usb to ttl serial adaptor. It can work at multiple voltages based on its vccio pin. If your adaptor doesn't have a selection jumper, or you can't/won't rewire it, then you have two options. Build a level shifter, or buy one.

A voltage divider will work fine for the ft232 tx to bbb rx. Inputs are low impedence signals. You arnt using it to drive a load. For bbb tx to ft232 rx, its a bit more difficult. It depends on what the ft232rl's minimum for a Vin logic high is. If its 0.7VCC (0.7 * 5v = 3.5v) the BBB tx will most likely not work. If its 0.6VCC (3v) or TTL level high 1.8~2.2V, the BBB tx will work as is. Otherwise you need a level shifter.

There are some transistor non inverting level shifter designs as well. Or use a dedicated level shifting ic. You only need 2 single direction lines after all for basic tx/rx shifting.

This site has a bunch of questions on 5v to 3v or 3v to 5v level shifting that can apply to your needs.