Variable module memory

Abstract

A variable module memory, particularly suitable for use in a programmable controller, includes a set of up-to-N memory modules each for storing a plurality of words of n bits. N receptacle means are provided for receiving the set of memory modules and for coupling the received memory modules to a common serial bus. Control means is provided, coupled via said receptacle means, to said up-to-N memory modules for providing address signals for selectively addressing the words of a selected memory module to provide the contents stored therein serially to said common serial bus. A pull-up resistor connecting a predetermined voltage to a common serial bus detects the absence of memory modules in the receptacles and forces the serial bus to a predetermined binary state for all bits or all words for a vacant receptacle means.