1Reference Module: Programmable 4 channel voltage calibration module, with built in charging management, loading and unloading protection, ensuring safe and reliable operation. 2Multimeter Calibration: Use for HP3458A 8 and a half digit multimeter for calibration (use after sufficient preheating), with a calibration value of 6 and a half digit counting, accurate and reliable. 3With Indicator Light: The voltage calibration board is equipped with indicator light. Press and hold for 2 seconds before turning on. After turning on, all 4 indicators will light up simultaneously. 4Output Voltage Setting: With precise reference voltage source, this module can set 4 commonly used output voltages, namely 10.0V, 7.50V, 5.00V, and 2.50V, with a wide range of applications. 5Micro USB Interface: This module has Micro USB charging and discharging interface, with 2mm copper test socket, easy to insert into a multimeter to measure or calibrate other instruments. 1 x Reference Voltage Module