Use of VHDL DID (Ref. 2) and Mil-HDBK-454 (Ref. 1). Digital electronics are only part of most military systems. Most modern weapons platforms use sensors and actuators that are tightly coupled with the digital electronic systems; however, the modeling of these sensors and actuators is outside the scope of this handbook. Many military electronic systems have both digital and analog components. The modeling of only the digital components is discussed in this handbook. Researchers are currently exploring the use of VHDI, for analog components and considering changes to the language to allow VHDI, to better support modeling analog and hybrid components and subsystems. Although VHDI, models are frequently used to provide test beds for testing software before the hardware is fabricated, this handbook does not discuss the issues of developing tests for software.