I^m confused about what the common mode voltage is and what could be its maximum value on the V+ and V- wires. I^ve read that it is important to ground the V- conductor, shield and drain wire at only place. Can I connect the drain wire and shield to the V-conductor at any place and only ground the V-conductor? Answered by Matt Kuzel, Chairman of the Physical Layer SIG, e-mail: kuzel@voyager.net . A247) Common mode means that the signal (or noise) is present on both CAN_H and CAN_L. Since the receivers are differential, meaning that the desired signal is obtained from the difference between CAN_H and CAN_L, the common signal is almost exactly subtracted from itself. The "almost" is what defines the common mode rejection ration in receivers. On DeviceNet, the transceivers are all referenced to the black (V-) wire. Voltage differences can develop between one point on the V-line and another due to the current flowing and the resistance of the wire. Thicker wires have lower resistance and therefore lower voltage differences or voltage drops along the wire. On very long trunks with a lot of current, significant voltages (several volts) can develop. If a transmitting node is at one end of the trunk and the receiving node is at the other, this voltage difference is present and "common" to both CAN_H and CAN_L. Obviously too much common mode voltage is not good. DeviceNet limits the amount of current that can be used on the trunk line to limit this voltage drop to 5 volts. The transceivers will bias the bus to about 2.5 volts. So the shift of + or -5 volts, due to the V-wire can cause the common voltage to be roughly anywhere from -2.5 volts (2.5-5) up to 7.5 volts (2.5+5). Regarding grounding. The reason for one point of ground is to prevent large loops which can become antennas for magnetic fields. While the connections that you describe will accomplish this, they may not be the best method. The shield can also act as a transmitting antenna for high frequency noise even if there are no loops. If the V-wire is noisy and the shield is attached it can become a good radiator of noise. A better choice would be to connect the shield to a ground which has no noise.