In the DeviceNet Cable System planning and installation manual, page 2-18, the V- and drain wires should be grounded to earth/building ground at one point. In our DeviceNet system, this was done. However, some of our instrumentation is experiencing extreme noise problems which clear when the V- to ground connection is lifted. Is this a correct installation? Can the V- connection to ground be eliminated without adversely affecting the DeviceNet system? Can or should the 24 VDC V- (common) be tied to the 28 VDC common?
Answered by Matt Kuzel, Chairman of the Physical Layer SIG, e-mail: kuzel@voyager.net .
A185) There should not be a problem with having the V- connected to earth/ground. I suspect that you have another connection. If you think that you only have one, disconnect it & put a DVM between V- and ground. It should not be low impedance if you really have only one ground. In the past, behavior like this was always attributed to having another connection to ground. Check to see if one of the other wires (blue or white) is shorted to ground too. This has happened before with very similar symptoms. If there is more than one ground, then find the other ground(s) & pick just one. If the DC/DC to convert 28 to 24 is not ground isolated, then ground only on the 28thside or the 24th side, but not both. I have seen systems run quite well with the bus floating (V- not connected). Having problems with a single point of ground suggests that there is a problem somewhere. Removing the ground may only be hiding it. 1. I develop a DeviceNet product which isn^t powered by the bus. When my power is off, my product doesn^t send or receive messages. Is this behavior correct for DeviceNet? Must I envisage a specific power so that my communication remains active (send and receive message Group 4)? 2. In the DeviceNet specification, the "Message router" object is mandatory. We thought that the "Message router" object was used to access by "Identity" object, "DeviceNet" object. In some device descriptions as AC drive, it appears that this object is optional so could you please explain this point?
Answered by Matt Kuzel, Chairman of the Physical Layer SIG, e-mail: kuzel@voyager.net .
A211) The DeviceNet Specification allows devices to be completely network powered, to make no use of network power, or to be partially network powered. If a device is not powered it will obviously not be able to communicate. The only requirement in the Specification for an unpowered device is that it must meet the impedance specifications so that it does not load the bus. The decision must be made by the device maker about how to power the communications on a device. 1)The group 4 messaging is used to change the node address of a device that has failed the duplicate Mac ID check process. It is not used for any other purpose. It requires that the node is able to transmit messages on the bus, so any power needs that your node has, either from the bus supply or from an internal supply have to be met. 2) The message router is required in all devices. The AC Drive profile is incorrect in stating it is optional. Because it doesn^t have any required attributes, it may not be directly addressable from the network, but it is in there none the less