RS-485 Multidrop Over Ethernet

Multidrop Applications

The RS-485 serial protocol is one of the most popular serial protocols in the industrial world. Not only does it break the RS-232 distance boundary and extend it up to 1200 meters (4000 feet), it also supports multi-drop communication up to 32 nodes over the same bus. Only the device with the corresponding ID number will respond the command or request.

Although the RS-485 is designed for long distance serial communication, it can be extended even further by using the Ethernet network. Engineers can communicate the device over the Internet with no more distance limitation. Here are two different methods to convert the RS-485 multi-drop to the Ethernet network.

1. Within Distance Limitation

Within the RS-485 distance limitation: with existing RS-485 daisy chain, the serial bus can be connected to the serial port on the device server. A virtual com port can be created on the host PC or HMI by using the utility from the vendor, and all the settings can remain the same on the PC or HMI.

2. Beyond Distance Limitation

Beyond the RS-485 distance limitation: if the RS-485 devices are spread out, beyond the distance limitation, the RS-485 device can be connected to a device server which is set as UDP Server/Client mode.

Every RS-485 device on the multi-drop chain should have its own unique ID number. When the host PC sends the command, the ID number will be packetized in the data stream. The device server will broadcast the command via UDP protocol to all the device servers within the assigned IP range (up to 4 different IP ranges). All the RS-485 devices will receive the same command along with the ID number; however, only the device with the matched ID will respond to the command. Then the device server will broadcast the message back to the host PC based on the IP address assigned on the device server.

Case Study

On the diagram below, a command was sent from the HMI to the serial device with ID 2. The command and ID is packetized by the device server ( attached to the HMI, and broadcasted to other device servers in the assigned IP range. In this example, the IP range is to All the device servers within the IP in this range will receive the same command. The device servers un-packetize the data and forward it to the serial devices. Since the command is for the device with ID 2, only the ID matched device will respond. The response will be sent back to the device server attached, and broadcast it back to the assigned IP (which is the device server attached to the HMI). With this method, engineers can easily send commands to any serial devices around the world and have no worries about this distance limitation.

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