A method and apparatus to modify control devices residing on a Fieldbus communications network, without interrupting the operation of the control devices. The control device updating may further be controlled and monitored by a remotely located host that also communicates on the Fieldbus network. The control device may comprise at least two distinct memory areas, wherein at least one memory area must be active, and at least one memory area must be inactive. Active memory areas provide the control device microprocessor operating system with executable instructions or data. The host downloads new executable instructions or data to inactive memory areas, with associated data entry points, during unscheduled communications periods wherein data input/output is not being performed between the control device and the host or another control device. Upon a full data transfer and proper verification of the new data, the host may issue an activation command that causes a selector device to activate the previously inactive memory area by directing the microprocessor to the entry points of the newly downloaded executable instructions or data. The memory activation must occur while the microprocessor is not performing application execution, application input/output, or application communications. By timing the memory activation in this manner, the microprocessor may be redirected to the newly downloaded executable instructions or data without microprocessor interruption.