The EDC Model 522 is a microprocessor controlled enhanced version of the field proven Model 521. An industry standard 6500 series microprocessor has been incorporated to improve the reliability, and versatility of the instrument. The Model 522 Programmable DC Voltage Standard is a highly versatile reference source, designed to meet the needs of computer systems, production line testing, automated calibration, and standards laboratories. The instrument has a specified accuracies, that are traceable through a bank of saturated standard cells to the U. S. National Institute of Standards and Technology. Resolution of each range, in each function, is 1 part per million. The instrument is a highly accurate reference which can be used for calibration of digital voltmeters, analog meters, semiconductor analyzing systems, analog references for computers, analog-to-digital converters, telemetry and data acquisition systems, and wherever a stable source is required. The variable, constant current mode is designed for use in calibration and simulation of strain gages and other transducers. There are no adjustments made during normal operation; the trims are made during calibration and are described in the calibration procedure. The circuitry is completely solid state made of discrete, hybrid and/or integrated circuits packaged on etched glass circuit boards. These are proven circuits, using derated components to insure long life and maximum reliability. The instrument is overload and short-circuit proof, and is fully operational in normal environmental conditions. The Standard Source will drive a short circuit indefinitely without damage to the instrument, and will recover to rated specifications in less than 100μs.