Asynchronous coupling of data between a word-oriented I/O module and the memory of a programmable controller

A programmable controller having a memory which stores command words and an input/output scanner circuit which periodically reads selected command words out of said memory and couples them through a data bus to an associated input/output module which is enabled by an address generated by said input/output scanner circuit and received on an address bus, said input/output module being suitable for driving a stepping motor and including:

• a storage register coupled to said data bus to receive a multi-digit number contained within said command words and being operable to store a multi-digit number which indicates pulse rate;
• a presettable counter coupled to said data bus to receive a multi-digit number contained within said command words and being operable to store a multi-digit number which indicates the number of pulses to be generated;
• a pulse generator connected to said storage register and said presettable counter and being operable in response to the multi-digit numbers stored therein to generate the indicated number of pulses at the indicated pulse rate;
• and
• a decoder circuit connected to said data bus to receive an identification code contained within each command word, and in response thereto, selectively enable said storage register or said presettable counter to receive and store the multi-digit number associated with the identification code.

A programmable controller, the combination comprising:

• a memory which stores on its lines a set of command words, an output image table and a control program;
• an input/output module responsive to a received command word to perform an operation;
• an input/output scanner circuit coupled to said memory and said input/output module and being operable to periodically and repeatedly read a word on a selected line in said output image table out of said memory and couple it to said input/putput module;
• a processor connected to said memory to execute said control program at a rate which is independent of the rate at which said input/output scanner circuit reads said word from said memory and including means responsive to the control program for sequentially transferring command words to said selected line in said output image table and timer means for establishing a time period which is greater than the time period which elapses between successive readouts of the word at said selected line by said input/output scanner circuit, said timer means being coupled to said means for sequentially transferring command words for establishing the maximum rate at which said command words are transferred to said selected line of said output image table by said transfer means.