The figure showing Maximum current available based on cable network length in Volume I, Chapter 10 in the DeviceNet Specifications is based on an 8 amp power supply. Shouldn^t there be a similar figure based on a 4 amp power supply because of the recommended 4 amp power limitations in the U.S. and Canada? Can I use the existing figure if I have a 4 amp power supply? Is the graph accurate for all distances beyond 70 m? Can I have 4 amps up to 70 m?
Answered by Bob Lounsbury, Physical Layer SIG, e-mail: relounsbury@ra.rockwell.com
A76) These tables are based on Common mode offset and maximum current allowed in the cable due to I/R drop and heating. For a 4 amp system, the graph would simply limit at 4 amps instead of 8 amps. All you need to do is draw a line from the 4 amp graduation out to the graph at about 70 m. The graph assumes that all the nodes are placed at the end of the network (farthest from the power supply). This is usually not the case in network designs.
I have heard that using a 100VA DeviceNet power supply will inhibit the DeviceNet system. (According to my colleagues, NEC Article 725 states that a 100VA supply should be used to power DeviceNet systems.)
Answered by Matt Kuzel, Physical Layer SIG Chairman, e-mail: kuzel@voyager.net and Bob Lounsbury, Physical Layer SIG, e-mail: relounsbury@ra.rockwell.com
A79) 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. It is important to remember that DeviceNet only needs to follow NEC guidelines when installed as building wiring. When installed as Machine Tool wiring, consult NFPA^79 for wiring requirements.