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Revision to CAN connection

David Crocker

+[title|CAN connection basics]
+
+
+Duet 3 uses a CAN-FD bus to connect expansion and tool boards to the Duet 3 main board (currently Duet 3 MB6HC or Duet 3 Mini). We chose CAN-FD because it is highly tolerant of noise (in particular, the ground noise generated by stepper motors), well-supported by modern microcontrollers, and the FD variant has enough bandwidth to support the high movement command rates needed in 3D printing and some other motion control applications.
+
+== Wiring scheme ==
+
+The CAN-FD bus is a two-wire bus with 120 ohm nominal impedance. Unshielded twisted pair cable is normally used; however over the short cable lengths typical of desktop 3D printers and CNC machines, the cable type is not critical. The bus should be terminated by 120 ohm resistors at each end.
+
+The Duet 3 MB6HC main board has a RJ11 CAN connector and permanent termination resistor, so it must be at one end of the CAN bus. Most Duet 3 expansion boards have two RJ11 connectors and two jumpers which can be fitted to provide termination. If the board is used at the other end of the CAN bus, only one of the RJ11 connectors will be used and the termination jumpers should be fitted. if the board is used at an intermediate position then both RJ11 connectors will be used and the termination jumpers should '''not''' be fitted. Although the two RJ11 connectors are labelled CAN_IN and CAN_OUT, they are connected in parallel and it doesn't matter if the cables to them are swapped.
+
+The Duet 3 Tool Board is an exception, because it is too small to accommodate RJ11 connectors. Instead it has a single 4-pin JST ZH connector which is intended to be used for CAN_IN and CAN_OUT functionality. The Tool Distribution Board provides four similar JST ZH connectors. Preferably, connect each Tool Board to the Tool Distribution Board using two twisted pair cables, and remove the two jumpers on the Tool Distribution Board that are provided to bypass that connector. However, unless the cables are very long, you may get away with using a single twisted pair between the Tool Board and the Tool Distribution Board and leaving the jumpers in place.
+
+If a Tool Distribution Board s that the end of the CAN bus, then leave the CAN_OUT RJ11 connector not connected, and fit the termination jumper.
+
+== Cables ==
+
+To be completed.
+
+== CAN addresses ==
+
+To be completed.