MODBUS Serial to MODBUS TCP Convert

Building Bridges: A Guide to Converting Modbus Serial to Modbus TCP

In the industrial world, one of the biggest challenges is making "legacy" and "modern" technologies work together. The key to making a 1970s-era power analyzer with a serial port (RS-485) talk to a modern Ethernet-based SCADA system lies in converting Modbus Serial to Modbus TCP.

1. Why Do We Need Conversion?

Although Modbus RTU (Serial) and Modbus TCP speak the same language, they use different "vehicles" for transportation.

Limited Distance: RS-485 cabling is limited to approximately 1200 meters; however, with Ethernet and fiber optics, data can be carried over kilometers or even across continents.
Device Access: In a Modbus RTU network, only one "Master" can make a request at a time. In Modbus TCP, multiple clients (Clients) can access the same data simultaneously.
Modernization: Next-generation PLCs and cloud systems generally only feature RJ45 (Ethernet) inputs.

2. How Does the Conversion Work? (Gateway Logic)

A Modbus Gateway is used for this process. The gateway understands the rules of both protocols and acts as a translator.

Operational Steps:

Request (TCP Side): A SCADA system sends a Modbus TCP packet to the Gateway's IP address. This packet contains information on which Slave ID is being queried and what data is requested.
Translation: The Gateway strips the unnecessary headers from the TCP packet and converts the data into Modbus RTU (binary) format.
Request (Serial Side): The Gateway queries the relevant device over the RS-485 line.
Response: The serial response from the device is repackaged into a TCP packet by the Gateway and sent back to the SCADA system.

3. Critical Points in Configuration

To avoid errors during the conversion process, the following parameters must match exactly:

Serial Settings: Baud Rate, Parity, Stop Bit, and Data Bits must be identical for all devices on the RS-485 line.
Slave ID Routing: Modbus TCP is normally IP-based, but when transitioning to a serial line, the "Unit ID" (Slave ID) becomes vital. The Gateway uses this ID to know which physical device to forward the TCP request to.
Timeout: Serial lines are slower than Ethernet. The "Response Timeout" on the Gateway must be set high enough to tolerate the latency of the serial line.

4. Modbus Gateway Solutions with Hubbox

Hubbox Connect devices are intelligent Modbus Gateways that offer much more than a passive converter.

Transparent Mode: Hubbox acts as a simple bridge, forwarding incoming TCP packets directly to the serial port.
Data Caching: Hubbox can continuously read data from the serial line and store it in its own memory. This allows the SCADA system to receive an instant response without waiting for the serial line.
Multi-Master Support: It enables a device on the serial line to be read by both a local HMI and a remote cloud system at the same time (Traffic Arbitration).
Logging and Analysis: You can log all traffic passing through the line during conversion and detect communication interruptions.

5. Summary

The transition from Modbus Serial to TCP allows the data in your factory to be "set free." By bringing isolated data from the RS-485 line into the Ethernet network, you take your business one step closer to the Industry 4.0 and IoT era.