Modbus is a communications protocol for data transfer. In the petrochemical industry, Modbus is widely used for connecting measuring and control devices to process control systems (PLCs). Most PLCs can be fitted with interfaces for Modbus communication. Modbus interfacing enables the reliable transmission of information over just one data line. It can be used both to read information from analyzers and to control certain functions. A distinction is drawn between Modbus RTU, Modbus ASCII (both based on serial field bus communication) and Modbus TCP (Ethernet-based).

Modbus is supported by MAXUM edition II, SITRANS SL and SIPROCESS GA700


PROFIBUS (Process Field Bus) is a standard for fieldbus communication. The two main versions are:

  • PROFIBUS DP (distributed I/O), designed for the efficient exchange of data at the field level. This is where the central automation devices such as the PLC/PC communicate with distributed field devices over a high-speed serial connection. Data exchange with the distributed devices is primarily cyclical.

  • PROFIBUS PA (Process Automation) is used for communication between measuring and process devices, actuators and process control systems or PLC/DCS in process engineering. PROFIBUS PA is not a separate bus: it is a version of PROFIBUS with a physical layer designed for process automation (transmission layer). PROFIBUS PA is also suitable for hazardous atmospheres thanks to power limitation.

PROFIBUS DP and PROFIBUS PA are supported by Series 6 and ULTRAMAT 23 gas analyzers.


OLE for Process Control (OPC) is a vendor-neutral software interface. It enables standardized access from Windows applications to field devices, for example. OPC has a typical client/server architecture. Standard OPC DA (Data Access) is available for Maxum edition II und MicroSam data transfer.

NAU - Network Access Unit

The Network Access Unit (NAU) is an input/output station for Siemens process chromatographs. It can be used to centrally access, process and relay data. The NAU is used if the electronics system cannot be connected close to the analyzers, there are not enough inputs/outputs in the Maxum edition II/MicroSam, or installation in a central control room is required. This significantly reduces the wiring required to the control room. The data can also be provided via Modbus.