We are working on a design with a DeviceNet slave interface. We would like to combine this interface with other network interfaces which isolated driver section is powered by an internal isolated DC/DC-converter. Is it possible to do this also for DeviceNet and thus not using the external power connection, except the V- reference? Answered by Matt Kuzel, Chairman of the Physical Layer SIG, e-mail: kuzel@voyager.net . A222) Yes, you can design as you describe. In Errata2 the requirement to power the transceiver from the bus was removed from the spec. Be aware that some older transceivers had impedance characteristics which were supply dependent. The nodes need to meet the impedance requirements if powered or not. Are we failing to meet generally accepted wiring practices if we use 1 to 12 DeviceNet (thin round) drop cables rated at 2.4 Amps (Lumberg # RSRK 50-614/1.5 ft) and a single (thick round) trunk cable rated at 8 Amps with a 24 VDC power supply rated at 5, 10, or 20 AMPs (depending on the # of devices). Each of the drop cables will connect to a device requiring just under 1 Amp. It seems that if there was a short in a drop cable, the power supply could supply much more current through the drop cable, resulting in an unsafe condition. Is this a problem, and if so what is the best solution? Thank you! Answered by Matt Kuzel, Chairman of the Physical Layer SIG, e-mail: kuzel@voyager.net . A228) Yes, you may be heading for trouble. First, you should not be designing a system with a source that can out drive the wiring. So, a 20 A supply on thin cable is not a good idea. The NEC specifies current capacity for different wire sizes. Second, if you are dealing with the NEC, you should be familiar with the power limited circuits. The typical DNet cable restricts the circuits they are in to Class2. This restricts the source (supply) to 100 VA. There are some rules that relax this based upon if you are machine wiring rather than building wiring, but still, the power is rather limited. This means that your supplies should not be rated the way you describe. This also means that you may need more supplies, although smaller, to handle the loads. With the class2 limit, you are looking at four nodes per supply. There are some new cables coming out, for example flat cable, which are rated such that the circuits they are in can be Class1 power limited. I think the common description is tray cable rated or TC rated. Be careful moving this way, because you still need to make sure that the devices can be used on other than Class2 circuits. Spacing in devices need to be bigger to handle the less limited classification. This is because voltages can be higher. Voltage rating, by the way, is the reason that the cables are limited to Class 2 circuits. The typical Dnet signal wires are 300V rated. The power bus has to have the same rating since it is in the same jacket. Flat cable & perhaps some others in the future will have higher voltage rated insulation (600V) which will allow the cables to move to the less restricted Class1. Third, realize that connectors have their own rating, so, you can end up with cable, supplies, and nodes which all are capable of much higher currents, but the connector will be the weak link. I believe that micro connectors are the only ones that will limit things more than the cables. I would think that if you are heading for lots of 1A devices, that you should try to use all thick (or equivalent size) cable, and no micro connectors. I am not sure what the 1A device are, but another alternative is to find devices which can use and share a separate auxiliary supply. Since the bus voltage can drop to 11 volts, device which use bus power often draw more current than ones that use a separate 24V supply. Then you can size the wires as needed where needed.