A method of reducing the `downtime` of the printing line involves the inclusion of at least one spare printing module, and a method of transferring digital representations of page images between modules. As soon as a fault is detected, a digital image data describing the sheet printed by the faulty module is transferred to the adjacent printing module downstream from the faulty module. The data in this module is simultaneously sent to the module downstream of it. This process repeats until a page image is sent to the spare printing module, which is best located as the most downstream of all the printing modules. Once all of the data has been transferred, printing can recommence. If this data transfer can occur in less than the time required to print a sheet, the line can continue printing without stopping, and without any wastage of printed copies. If more than one spare printing module is included at the end of the printing `assembly line`, then more than one simultaneous fault can be accommodated without productivity loss.