Abstract: A data handling module connects to the memory of a programmable controller and operates to periodically steal memory cycles from the controller processor to read data out of the memory and write data into it. The data handling module includes a microprocessor which is programmed to perform a number of functions. The function to be performed is indicated by a control status register which is set by the control program and which is periodically examined by the data handling module. The status of the data handling module is also indicated by a register which is examined by programmable controller instructions. In response to directions indicated by the control status register, data files may be transferred between a data file storage and an active data file storage area and messages may be coupled to a TTY which connects to the data handling module.

Abstract: A program protection module for a programmable controller checks the critical, or protected, portion of the control program each time the controller is switched to the run mode. This is accomplished by reading each control program instruction out of the controller memory and comparing it with a corresponding control instruction stored in a nonaccessible read-only memory. If all the compared control instructions are identical, the module sets a status bit in the programmable controller which enables the protected portion of the control program to be executed. If identity is not found, an alarm is generated and the protected program is not executed.