Abstract: A data processing system is constructed from processing modules (e.g. LSI chips). Each module contains storage circuits which are connected together, for test and diagnostic purposes, to form a plurality of shift register paths in parallel between common input and output terminals. The common terminals on all the modules are connected to a diagnostic unit by way of common input and output lines. In operation, a first control message from the diagnostic unit is shifted into a master shift register, connected to the common input line, and this causes one of the modules to be selected. A second control message from the diagnostic unit is then shifted into a control shift register in the selected module, and this causes one of the shift register paths in that module to be selected. Test data can then be shifted into the selected path from the diagnostic unit.