Digital interface for gas measuring devices

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The digital interface for gas measuring devices (DSfG) is a technical guideline that was created by a working group of users and manufacturers from the gas sector under the umbrella of the DVGW and is regularly maintained to this day.

When it was first published as "Technical Rule G485" (TR G485) in 1991, a platform was created on the basis of which measuring devices from different manufacturers and with different tasks in a large gas measuring system can exchange data, especially under the aspect that these Data originate from calibrated devices and the procedure for data exchange meets the requirements of the calibration law ("DSfG bus")

Three revisions have been released since then, the last in 2004.

Interfaces

The TR G485 describes in two parts how the interface should be designed.

  • In the first part (worksheet, also called guidelines), specifications have been made that are of a more general nature: Regulations on the seven layers of the ISO / OSI reference model for communication between open systems ( DIN ISO 7498), description of the instance model and references to others Regulations, guidelines and standards. The following entities are currently described: Volume corrector, recording devices, monitoring and control devices , gas quality measuring devices , data transmission module including remote control center, revision instance .
  • In the second part (technical specification for DSfG implementations), in addition to further specifications for layers 1–6 of the ISO-OSI standard, specifications are made for each individual instance up to layer 7 (the actual user layer). The data element lists (DEL) can be found there, which precisely describe each data element (DE) of DSfG interest that occurs in an instance with regard to its content, its form and its DSfG address.

Classes

Two classes of DSfG devices have been described since the third edition of 2004:

  • Class A: Worksheet G 485 complied with across all seven layers of the ISO / OSI reference model
  • Class B: Worksheet G 485 complied with across layers three to seven of the ISO / OSI reference model

Technically, the electrical properties of a DSfG bus are based on the EIA RS-485 standard (differential voltage signals via a twisted pair of cables). The length of the bus (bus trunk line) can be up to 500 m. The coupling of the individual DSfG participants to the bus takes place via bus stub lines. Up to 31 participants can be connected to the bus, one of which must act as the bus master.