Abstract: A tank side monitor includes two processor boards, a main/communication board, containing field communications interface circuitry and interface circuitry, and an optional IS module, containing HART interface circuitry. The two processor boards are link by an optically coupled serial communications bus. The HART circuitry is multiplexed and can be operated by either the Main/Communication board processor or a local processor on the HART IS board. The optional IS module, an extension of the HART IS board, provides options such as an IS 4-20 mA input or output or other IS I/O. The TSM employs a modular approach for hardware and software, whose implementation consists of a number of modules and programs, the first being the Main/Communications board software. Other programs are contained within the HART interface module. Due to the modular approach taken in the hardware design, the software is also modular and operates on two hardware modules: Main/Communications module software; and HART module software.