Patent ID: 8250242

Claim:
A field bus system for downloading, from a host computer to a controller, software for controlling operation of a field device, comprising: a host computer; a controller that is connected through a communication line to the host computer; and a field device controlled by the controller, wherein the controller comprises: first and second memory areas provided as memory areas for software installation; an active/inactive switch configured to receive an activate command from the host computer to switch either the first or the second memory area to be an active memory area and to switch the other to be an inactive memory area; a field device operation controller configured to control the operation of the field device using the software that is stored in the active memory area; a downloading device configured to download, to the inactive memory area, updated software sent from the host computer; and a simulator configured to perform a simulation of control of operation of the field device using the updated software that is downloaded into the inactive memory area, the host computer comprises: an updated software sender configured to send updated software to the controller; a monitor configured to monitor a state of execution of the simulation using the updated software in the controller; and an activate command sender configured to send an activate command to the controller, the field device operation controller calculates an actual value for control output to the field device from an input value that is read in from the field device, using software that is stored in the active memory area, and the simulator calculates a simulation value for the control output to the field device from an input value that is read in from the field device, using updated software that is downloaded to the inactive memory area.