[Abstract] This paper mainly introduces the electrical control system of the active wire-laying winch of a field optical cable production line, implemented using S7-300 PLC and DANFOSS VLT 5000 series frequency converters based on PROFIBUS-DP fieldbus technology. It includes the method of controlling VLT 5000 with PROFIBUS-DP fieldbus technology, parameter settings in the control system, data types, and the process of configuring the PROFIBUS-DP fieldbus control system using STEP 7 and the STEP 7 software programming design. [Keywords] PROFIBUS-DP fieldbus; VLT 5000 series frequency converter; PLC; active wire-laying winch 1 Introduction With the development of the communications industry, optical fiber cables have entered various fields of wired communication, including postal and telecommunications, broadcasting, power communication, and military communication. The structure of optical cables always evolves with the development of optical networks and the requirements of the usage environment. Different occasions and different requirements have led to the development trend of multi-structure optical cables. Due to the special field environment, the requirements for field optical cables are very different from those for outdoor optical cables. They should not only have a certain tensile strength, but also be small in diameter, flexible, easy to bend, and flame-retardant. When there are many tightly-buried optical fibers or the outer diameter is large, the tightly-buried fibers must be twisted in a single spiral to ensure good bending performance of the optical cable. To accommodate this special twisting method, the production of field optical cables requires a dedicated production line. One of the key processes of the entire production line is how to release the optical fiber with constant tension while simultaneously untwisting the tightly-buried fiber cage and the tightly-buried fiber release unit. This is because the tension of the tightly-buried fiber affects its transmission performance. Too little tension will cause the tightly-buried fiber to leak; too much tension may damage it. Therefore, the design of constant tension control for the active release cage is the focus of the entire production line design. To this end, we designed a system where the voltage signal from the tension sensor is sent to the frequency converter, and then transmitted to the PLC via the PROFIBUS-DP fieldbus. The PLC's internal PID process control module is used for adjustment, and then the control signal is sent to the frequency converter. The advantages of this approach are: 1) The PID process control module inside the PLC has superior performance and can better control the tension. 2) Even in the event of a malfunction (communication failure, PLC system failure, etc.), production can continue using the PID process control module built into the DANFOSS frequency converter. [b][align=center]For details, please click: Fieldbus Technology in Field Optical Cable Production Line[/align][/b]