EPICS simulation tools for control system development

When developing control system software there are many times when the ability to simulate the response of the instrumentation can be very useful. Examples are: (i) when the operator interface is being designed and the users want an idea of what the finished system might took like; (ii) when the interface hardware is not yet available; (iii) when the reaction of the control system to an error condition must be tested, but the actual occurrence of such an error would cause undesirable side effects; (iv) when operators are being trained to use the system; (v) when an improvement or bug fix needs to be tested, but the running system cannot be shut down for long. The Experimental Physics and Industrial Control System (EPICS) provides tools for building simple simulations and interfacing to more complex simulations of accelerator hardware. At the lowest level an individual data channel can be switched to take its input from either a simulated data location or from the actual hardware. At a slightly higher level, sequences can be run on the real-time interface processor so that output to the hardware is intercepted and an appropriate substitute value is provided for the corresponding read-back records. At a still higher level a program can use the Channel Access software bus facility of EPICS to control some global aspect of an accelerator or can interface to an external accelerator simulation instead of the actual accelerator. The goal of testing control system software using simulated hardware is to minimize the changes required in shifting between the simulated system and the real system. The degree of success of the EPICS tools in meeting the minimum change goal will be addressed with suggestions for improvements. The implementation of simulated responses using EPICS tools will be discussed and examples of experience using the EPICS tools to create and interface to simulations will be given.