thank you for your answer, or better, for your questions.
a) it is exactly one servo and it is now powered up by a separate power supply (one of these small switching power supplies, which are put in a socket. It is adjusted now at 9V.) The Dynamixel does flash its LED, when powering up.
My setup is probably not the most beautiful, but it should work:
The cables are from dynamixel, the pcb just connects pin 1 with pin 1 and pin 3 with pin 3.
pin 2 (the middle one) is not connected.
Red is 9V, blue is GND. red is connected to pin 2 of the dynamixel, blue is connected with GDN of the dynamixel/U2D2 (the lowest pin at the pcb)
the green annotated wires should be data lines.
b) its exactly one, yes. But “brand new” is a good point! At first, I connected it with RS232 (12V), because I was not aware of the difference RS232/TTL. Tomorrow, I have access to a second dynamixel, which was definitely only connected to TTL. I will try it with that device again.
At least, it should have still its defaults, since I can not communicate with it.
c) device is /dev/ttyUSB0. By the way, I had to recompile the kernel with options for TTL-serial-modules enabled (gentoo). After booting into this kernel and plugging in the U2D2, the modules usbserial and ftdi_sio were loaded, and /dev/ttyUSB0 appeared. Additionally, if I start the ping executable, the green LED at the U2D2 flashes. So I am quite sure that there is everything o.k. But tomorrow I have access to a oscilloscope and will check the data wire, for being sure.
Well, and maybe there is another problem. Can I change the baud rate permanently at the device? because my SPS is not able to do 1000000, and it is intended to connect the dynamixel with the SPS…