In one of your "Dr. DeviceNet" answers, you state: "The requirement in NEC-725 is that a Class 2 circuit must be supplied by a Class 2 "listed" power source. This means that the power source must have a Class 2 "Listed" name plate rating of 100VA. It does not say that this is the system limit. This is the limit for a Class 2 "circuit." A system such as DeviceNet can be made of multiple circuits. This means that if the power bus is segmented, there can be a supply on each segment." Allen-Bradley^s publication on installing and configuring a DeviceNet system shows examples of networks with multiple power supplies. These examples only break the V+ wire. They then calculate current. For those networks with a total current over 4A (Round cable system) they state that this exceeds the NEC limit. 1) When you talk about "segmenting" the power bus, does that mean a physical break in both the V- and V+ wires? 2) If the V- on the two segments are isolated, how do we ground and shield the second segment, and will this cause instabilities on the network?
Answered by Matt Kuzel, Chairman of the Physical Layer SIG, e-mail: kuzel@voyager.net
A32) Segmented power means a break only in the V+ wire. The V- wire should stay connected to make sure that all nodes have a common reference. Breaking the V- is possible, but presents many issues (like the ones you mention). One solution may be isolated physical layer repeaters (not currently in the spec). Breaking the V+ also has some potential issues. The voltage drop on the V- line will be an accumulation of the voltage drops from all the power segments. It is important that the current direction alternates on the segments. For example, with two segments they should both be powered from the center or both from the ends so that the current in the two segments flows in opposite directions. With more than two segments it gets more complicated.
I have heard that the maximum number of power supplies on a single DeviceNet network is four. Is that correct?
Answered by Matt Kuzel, Chairman of the Physical Layer SIG, e-mail: kuzel@voyager.net
A36) There is no such limit in the Specification. It is important that no two points on the network V- line (black wire) exceed a 5-volt difference. This is the reason for the power limits on DeviceNet. If segmented power busses (separate V+) are used, it is important to make sure that this limit is not exceeded. It is possible that you are thinking of a limit of 4 Amps. This is a limit imposed by the National Electric Code (NEC) in the United States. This is imposed for Class 2 Power limited circuits, which is how the NEC views DeviceNet thick and thin cable.