Pololu USB-Serial Adaptor Problem

Hi to all:

Please, Can someone help me with this problem? I have an ATmega16 working as Master (I2C) of 6 Salves for control of temperature. The communication between Master and PC was working with the MAX232 and Serial Port on a Desktop PC. In the last days, I had to change the application to a Notebook PC. As this laptop does not have a serial port, I bought a pololu usb-serial adaptor for a direct communication with the Master without the MAX232.

The problem is that when I connect the pololu module to the PC, this is recognized as COM1 and Red LED is on; but when I turn on all the system (Master and Slaves) the Red LED is turned off and any comunication is possible until I disconnect and connect the usb cable to the PC. Also, when the module USB is working together at the system, I dont know why the USB module stops to work (Red LED is off) even if the Pololu USB-Serial as COM1 is listed in the Device Manager.

I just connect three cables: Pololu Tx -> AVR Rx; Pololu Rx -> AVR Tx; GND -> GND.

Looking for a explanation of this problem, I use the HyperTerminal to check the connectivity and when the Red LED is on there is comunication, and when the Red LED is off, the comunication is lost and the windows notification says that it is impossible to access to COM1. Also I measured the voltage in the AVR Master when the USB-Serial adapter is connected to the laptop and the system (Master and Slaves) is turned off, I found that there is a voltage of 1.95V coming from the Pololu Module.

Does anyone have an idea about this problem? I so sorry if I can’t explain it so good. Thanks.


Could you post pictures of your setup, including all of your connections? Also, could you try doing a loopback test? You can do a loopback test by connecting the USB-to-serial adapter’s RX pin to its own TX pin, disconnecting the rest of your system and sending something with a terminal program to see if it gets echoed back.


Thank by your interest BrandonM.

I will show you a picture of the physical connections. But here, There is an short schematic of the connections. Well, The problem is that the Pololu module is working very good a couple of hours, but without a reason, the module and system stop to work. As I told before, the system adquisition works very good with a MAX232 an serial COM; but I need to change the old desktop PC by an Notebook and the convectional USB-Serial converters do not work as the Pololu module in the laptop.

Also, as I told before, At the beginning the resistors inside of the red box (schematic) were not present. Measuring with the oscilloscope and trying to solve the stopping problem I did a voltage divisor and the level maximum in the Rx of Pololu is around 3.6 V; I did this by a recomendation of a collegue because We were thinking that maybe the diference of Voltage (AVR at 5V and Pololu at 3,3V) could be a problem in time and the Laptop or Pololu desactivate the comunication for auto-protection. I left the system to work but today I found that the problem was not solved.

Do you know something about this issue? Thanks in advance.

PD.- Today I will try the system with the old computer and Pololu to check of there is a problem with the laptop because it is very curious that the conventional USB-Serial converters work well with the old computer and with the laptop they have a problem. In any way, Thank you very much by your help.

Our USB-to-serial adapter is 5V tolerant, so you should be able to remove the voltage divider between the two modules. Could you double check that you have a common ground between the adapter and microcontroller?

Have you had a chance to test your setup with the desktop PC? I am also still interested in seeing pictures of your setup and connections.