Can 1756 I/Os be connected on a Devicenet network?

If I have some ControlLogix I/Os on a 1756 rack and a 1756 DNB module in the same rack , Can i be able to read and write to these I/Os from a controlgix sitting in another rack which has one more 1756-DNB? If yes, can anybody please explain how to configure.. If no, is there any other adaptors available for these I/Os on devicenet.

Also I always noticed that I will not be able to browse the backplane of a ControlLogix rack while coming from a Devicenet network and DNB selected.. Where as Ethernet and Controlnet allows that. In fact i am not able to download my program from PC-> serial port -> KFD-> Devicenet -> DNB -> Backplane -> ControlLogix .. I understand that all these networks are on CIP protocol and should allows bridging from one network to another . Then why this is not happening for DeviceNet alone. Is anybody there to clear my doubts?

Although the 1756-DNB does use CIP messaging, I think Allen-Bradley has chosen not to implement the messaging services that allow you to go online with a controller.

Although you can get DeviceNet to run pretty fast, the limitation of 255 bytes in an explicit message and the high overhead of the fragmentation protocol make it unsuitable for carrying the kind of bandwidth you'd need to go online with a ControlLogix, especially if you also want to maintain good I/O throughput at the same time.

There's a DeviceNet specification that lets it transport the older
"PCCC" protocols encapsulated in Explicit Messages; this is what lets you go online with a MicroLogix connected to a 1761-NET-DNI.

But you're correct in your observations that you can't upload or download to a ControlLogix, nor go online, through a DeviceNet link.

I guess it would help to answer *both* questions !

You cannot use the 1756-DNB in "Adapter Mode", in which it would control a rack full of I/O and behave as a slave device to a 1756-DNB in ordinary "Scanner Mode".

The 1756-DNB can be a slave device to another scanner (even at the same time as it is a master to I/O on the DeviceNet), but it needs to be in a chassis with a ControlLogix controller.

The reason is similar to the reason you can't program a ControlLogix over DeviceNet; the I/O connection size on DeviceNet is limited to 255 bytes. This is fine for small modular I/O form factors like Compact I/O and Flex I/O, but ControlLogix 1756 modules have considerably more data and features and only a high-bandwidth network like ControlNet or EtherNet/IP is suitable for carrying that much data rapidly.