## 1 Introduction  
With the move towards Industry 4.0, manufacturing and production in the industrial field are evolving towards digitalization and intelligence [1]. As an important cornerstone of intelligent factories, industrial control networks (ICNs) play a key role in industrial control systems and are an important research area. Generally, ICNs must meet the strict requirements of strong real-time, high reliability, and low jitter [2]. Currently, ICNs can be classified into three categories: Fieldbus, industrial Ethernet, and industrial wireless network (IWN). As the most traditional communication modes in the industrial field, it is known that Fieldbus and industrial Ethernet protocols have perfect real-time and reliability. It has been found that the maximum transmission rate of the PROFIBUS protocol is 12 Mbps, and the real-time is about 10 ms. Furthermore, for the EtherCAT protocol and PROFINET protocol with a transmission rate of 100 Mbps, the real-time of the former is lower than 10 us, while the latter has variations in real-time from 100 ms to 31.25 us in different application scenarios [3]. Furthermore, there are also advantages for wired ICN protocols, such as complicated wiring, poor flexibility, and high maintenance costs. In order to avoid the inconvenience caused by the wired ICN protocol, IWN has been proposed. Currently, IWN protocols mainly include WirelessHART, WIA-PA and ISA100.11a, which are mainly oriented to industrial process automation with about a 250 Kbps transmission rate. Furthermore, the WIA-FA protocol has been developed for factory automation, and can achieve a transmission rate of 54 Mbps and end-to-end delay of less than 10 ms [2]. Generally, existing IWN protocols can satisfy the communication requirements of process automation, but there is still a big gap to satisfy the emerging strict requirements of factory automation. Along with the rapid development of 5G technology, the international telecommunication union (ITU) and the third-generation partnership project (3 GPP) are working towards ultra-reliable and low-latency communication (URLLC) for industrial control applications [4]. The 5G URLLC technology promises an advanced index of 1 ms air interface access delay and 99.999% reliability. More recently, based on the air-interface of 5G URLLC, WIA-NR has been proposed to achieve 1 ms delay and 99.999% reliability over the unlicensed bands [5]. Therefore, we introduce 5G technology into industrial automation production. It is expected to break the \"pyramid\" mode of the existing stacked heterogeneous industrial control system and realize the flattening of the network. Nevertheless, we know that 5G technology still lacks the knowledge precipitation of industrial application and cannot completely replace the existing industrial control protocols. Hence, we strive to study the protocol adaptation scheme and realize the interconnection between 5G and the existing industrial protocols. For this reason, through the investigation of the relevant literature, we select the most widely used industrial control protocol, Modbus, to investigate the protocol adaptation, prototype development and experimental evaluation of a 5G-based industrial wireless controller. Specifically, there are different versions of the Modbus protocol, such as Modbus remote terminal unit (RTU) and Modbus transmission control protocol (TCP) [6; 7; 8]. Thus, we first analyze the frame structure characteristics of Modbus RTU, Modbus TCP and 5G. Then, we propose a protocol adaptation scheme with protocol resolution and conversion between Modbus RTU/Modbus TCP and 5G. To evaluate the effectiveness of the proposed scheme, we further design a prototype system for a wireless programmable logic controller (PLC) based on 5G. Both hardware architecture and software design are introduced in detail. In order to illustrate the feasibility of the protocol adaptation scheme and developed prototype, we carry out extensive experiments in the system environment.