RS485 is a serial communication standard that has been the backbone of industrial automation for over 40 years. PLCs, RTUs, flow meters, pressure transducers, energy meters and dozens of other field devices still use it as their primary communication interface. The protocol running over the top is almost always Modbus RTU.
How the Modbus Gateway Works Inside the Router
Industrial routers like the Milesight UR75 include an RS485 serial port and firmware-level Modbus gateway functionality. The conversion happens inside the router – no additional hardware required. The field device outputs Modbus RTU frames over RS485. The router converts these to Modbus TCP. The converted traffic travels over the cellular WAN via VPN tunnel to the SCADA host. The SCADA software polls the remote device exactly as it would a locally networked Modbus TCP device.
Wiring RS485 to the Milesight UR75
The UR75 has a terminal block for the RS485 interface. RS485 A+ connects to the A terminal, RS485 B- to the B terminal, signal ground to GND. RS485 supports multi-drop – up to 32 devices on a single bus, each with a unique Modbus address (1 to 247). For cable runs over 100 metres, fit a 120-ohm termination resistor at each end. The UR75 has a built-in switchable termination resistor for this purpose.
Configuration Steps
Set the RS485 serial parameters to match the field device: baud rate (typically 9600 or 19200), data bits (8), parity (none), stop bits (1 or 2). Enable the Modbus gateway function in RTU-to-TCP mode. Set the TCP port to 502 (standard Modbus TCP port). Configure the VPN client to tunnel traffic to the SCADA host. The SCADA software at the control centre points to the fixed IP of the UR75 cellular connection on port 502.
Latency Considerations
SCADA systems poll field devices on a cycle of 1 to 10 seconds for most monitoring applications. Cellular networks introduce 20 to 80ms round-trip latency on a well-configured 4G connection. For the vast majority of UK remote monitoring applications – water, energy, utilities, environmental monitoring – this is entirely acceptable. For fast control loops or time-critical protection functions, cellular is not the right primary network and this guide will tell you that clearly.
Recommended Hardware
The Milesight UR75 is the reference product for RS485 Modbus gateway applications. It includes RS485, RS232, digital I/O, dual SIM, 5G NR with 4G LTE fallback, and DeviceHub management. The UR35 is the 4G-only equivalent for sites where 5G coverage is not a current requirement.