There are many types of DMMs being manufactured today. The ree most preva built on a are laboratory, bench systems, and There are card for insertion into a or industrial control system. Most of the laboratory and bench types have either an IEEE-488 bus or RS232 interface so that re mote computers using software such as LabVIEw Instruments can control them. Laboratory and bench units use internal microprocessors for performing com have mathematical calculations and calibration Handheld models typically such 3 and 4 digit displays There also special models designed for specific tasks are as automotive maintenance, process control measurements, and electron rcuit roubleshooting. Calibration of handheld units becomes more complicated as more functions are added The input to DMMs is eventually applied to an (AD) converter he most widely used are the dual-slope, remainder (R2), and the succes ve approximation converters. These were discussed earlier in this chapter. A block di agram of a DMM is shown in Fig. 5.16. Notice that there are two major sections in a MM. They are the measurement section and the control section The control section consists of a microprocessor and its support circuitry. The measurement section consists of the range and input variable selection, sample and hold (S/H), AlD, and reference subsections The DVM becomes an ohmmeter when a very accurate current source is includ ed within the meter. This source circulates the current through the resistance being measured, and the remainder of the DVM circuitry monitors the resulting voltage drop across the element. The current source is accurate only for voltages below the full scale voltage range of the DVM. If the resistance being measured is too large, the test cur rent from the current source will decrease. The accuracies of multipurpose DVMs which are used to measure resistance, can be as high as 2.2 ppm accuracy and 10 wn sensitivity. The input section of an average-responding, rms indicating, rectifier is shown in Fig. 5.17. It processes the input signal (ACmeas) via range amplifiers and then converts