Variable module memory

In a programmable controller for selectively connecting and disconnecting a power signal to each of a plurality of output devices in response to computed respective on/off states generated by execution of a stored control program on a plurality of binary parameters, wherein the value of each of said parameters corresponds to a state of a respective input device, a variable module memory for storing said control program comprising:

• a. a set of up-to-N memory modules, each for storing a plurality of program instructions of n binary bits each;
• b. N receptacle means for receiving said set of up-to-N memory modules, and for coupling each of the received memory modules to a common serial output bus;
• c. memory address control means coupled via said receptacle means to said up-to-N memory modules for providing address signals for sequentially addressing each of ten memory modules and sequentially addressing the program instruction words of the addressed memory module to provide the contents stored therein in series to said common serial output bus;
• and
• d. means for detecting the absence of memory modules in said receptables and for forcing said common serial data output bus to a predetermined binary state for all bits of all program instruction words of vacant ones of said receptable means.