Abstract: A test facility is disclosed for error diagnosis in multicomputer systems, particularly in multi-microcomputer systems in which signal voltages on the system bus in the error diagnosis are made perceptible as optical signals by display devices directly controlled by the signal voltages. A decoupling device is provided for connecting an evaluation/control logic to the system bus and an address/data information display device, an access signal display device, a bus control signal display device and an interrupt signal display device are provided. At least one comparator module is provided whose first group of signal inputs receives a binary information corresponding to a selection address, adjustable by means of a selection switch, and whose second group of signal inputs receive information concerning the address line of the system bus.