If you have only one X carriage and therefore only one print cooling fan, the simplest option is to connect it to Fan0 and put whatever you currently have connected to Fan0 on Fan3.
The alternative is to use fan mapping in the tool definition. Add parameter F3 to your M563 tool definition command. It's supported in firmware 1.16 and later.
Edited 2 time(s). Last edit at 01/21/2017 02:17AM by dc42.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].