The AS-210 Frequency Calibration System is comprised of a series of modular plug-ins housed in a mainframe. This mainframecontains the motherboard to interface with all plug-in modules, all the necessary power supplies, and a small, rugged Rubidium Frequency Standard. The output of the Rubidium cell is distributed to all possible plug-in positions so that the accuracy of each plug-in is related directly to the Rubidium Frequency Standard. An external frequency source may also be used through a rear panel input connector. The AS-210 system is controlled by an internal Intel 8085 microprocessor via an IEEE-488 bus. Operating control of the system is either through the AS-210 controller keyboard or through an external IEEE-488 interconnect on the rear panel. The AS-210 system has been designed so that it can be used as part of automated measurement and test systems through this external interconnect. The system accepts functionally related ASCII character strings from a GPIB Controller. This sets controller functions, provides status reports, and retrieves data. The GPIB controller may be any commonly used programmable controller such as Fluke 1720, HP-85 HP-9825, or Interface Technology 488. The available Portable Mainframe is constructed so that when its front and rear covers are in place, a light weight, dust-proof and water-resistant package results. The system is then easily transportable, requiring only removal of the front and rear covers and connection to an ac power source to put the system on-line. Battery plug-ins are also available to provide continuous operation during transportation or power outages.