I connected to Roboteq via USB-to-Serial and sent commands from PC. It was working. Then I connected Roboteq to Arduino via TTL-Serial adapter. The adapter got power from Roboteq. However, when I turned power on, Roboteq went into test mode (full speed, reverse speed, etc).
I checked that Arduino worked and sent commands to Serial port - I connected PC to TTL-Serial adapter and saw them. Let me remind you that the power was taken from Roboteq.
When I power the adapter and Arduino, wait 1 sec and then turn on Roboteq - everything works fine.