Why Connect Legacy Equipment to a Network?

In many factories and workshops, some devices still communicate via RS232 or RS485 interfaces. GSK CNC systems, Siemens PLCs, temperature controllers on dyeing lines—these devices operate reliably, but their data remains locked in serial ports, requiring manual meter reading and entry, which is inefficient and error-prone.

The most common approach to get this dataonto a networkis to add aSerial to Ethernet Converter, which transforms RS232/RS485 signals into TCP/IP, enabling connection to a LAN or cloud platform.

This involves a core concept: aModbus TCP to MQTT protocol converter. Its function is to convert device-side Modbus RTU or other proprietary protocol data into Modbus TCP or directly into MQTT JSON format, allowing MES, SCADA, or cloud platforms to read data directly.

Three Typical Scenarios

1. CNC / Machine Tool Connectivity

In an automotive parts workshop, the serial parameters of a GSK or Siemens CNC system are typically: 9600 baud, even parity, 7 data bits, 1 stop bit. To report machining data to MES via Ethernet:

• Connect the serial server's RS485 port to the CNC's RS485 communication port

• Set TCP Server mode, opening a port for MES polling

• Configure Modbus RTU to Modbus TCP protocol conversion, mapping machining parameters (spindle speed, feed rate, tool life, etc.)

• For cloud reporting, choose a model with MQTT support (e.g., PUSR USR-TCP232-410s) to convert directly to MQTT format without an additional gateway

Key selection criteria:Confirm the CNC port count—some systems require both RS232 and RS485 ports. A dual-port model (RS232 + RS485) saves space and wiring costs.

2. Textile / Dyeing Process Equipment Monitoring

The PLC control system on a dyeing line typically connects multiple temperature controllers, flow meters, and level sensors via an RS485 bus, with data uploaded to the host computer in Modbus RTU format. For remote monitoring via WiFi or LAN:

• Connect the serial server's RS485 port in parallel to the RS485 bus

• Configure TCP Client mode to connect to the factory's WiFi AP or switch

• Set edge computing rules (e.g., USR-TCP232-410s supports up to 300 data points) to perform Modbus RTU to Modbus TCP or JSON conversion at the device side

• Report via MQTT to the cloud for remote alerts and trend analysis

Key selection criteria:Industrial environments experience large temperature fluctuations. Choose wide-temperature models (operating range -40℃~+85℃) with DC 9-36V wide-voltage power supply to avoid disconnections during summer heat or winter cold.

3. Legacy Equipment / Heterogeneous Protocol Retrofit Projects

A common challenge: a workshop with multiple brands and protocols of legacy equipment (Modbus RTU, DL/T 645, proprietary protocols), needing standardized cloud data upload without modifying existing programs.

In such cases, an edge gateway solution is more suitable:

• Use a multi-protocol conversion edge gateway (e.g., PUSR USR-N720 with 2×RS485 ports) to connect different devices

• Configure protocol conversion rules inside the gateway to unify different protocols into Modbus TCP or MQTT JSON

• Supports SD card offline storage: data is cached during network outages and auto-resumes after reconnection, preventing data loss

• Supports up to 2 TCP connections + 2 MQTT connections, enabling simultaneous integration with multiple systems

Key selection criteria:Data point count—if collection points exceed 500, prioritize a gateway with stronger edge computing capability (USR-N720 supports 1000+ data points). Lower-end models with 25~300 data points may be insufficient.

Installation and Configuration: Three Key Points

1. Serial Parameter Matching

This is the most common issue. The serial server's baud rate, parity, data bits, and stop bits must exactly match the device's serial parameters.

• Look up serial parameters in the device manual (often labeled "Serial Communication Settings")

• Write parameters to the serial server via web configuration interface or companion tool

• If uncertain, use a serial debugging tool to scan common parameters (9600/19200/38400/57600/115200)

Common pitfall: CNC equipment often uses even parity (Even), while standard serial debugging tools default to odd parity or no parity. Mismatched parameters produce garbled data.

2. IP Address and Network Configuration

• Directly connect a computer to the serial server and access the default IP (typically 192.168.1.100 or 192.168.0.100)

• Configure DHCP auto-obtain or static IP

• Confirm gateway and DNS are correct (for TCP Client mode connecting to cloud)

• Use ping to test connectivity to the target device (MES server or cloud)

3. Protocol Conversion Configuration

• Confirm whether the device outputs Modbus RTU or another protocol

• Configure the corresponding conversion mode in the serial server:
  • Modbus RTU → Modbus TCP:suitable for connecting to existing MES systems
  • Modbus RTU → JSON:suitable for direct cloud platform integration
  • MQTT mode:publish directly to MQTT Broker (requires device support)

• For data encryption, enable SSL/TLS (supported in TCP Client, HTTPD Client, and MQTT modes)

Troubleshooting Common Issues

When encountering connection problems after adding a serial server, troubleshoot in this order:

Step 1: Verify Physical Connection

• Ensure RS485 A+ and B- are not reversed

• Confirm bus terminal resistor (120Ω) is installed (for long-distance wiring)

• Measure RS485 port voltage with a multimeter: A-B should be between 2V~6V

Step 2: Verify Serial Parameters

• Check whether baud rate, parity, and data bits match the device

• Verify flow control (XON/XOFF or RTS/CTS) settings

• Some CNC devices require flow control to be disabled

Step 3: Verify Network Configuration

• Confirm IP addresses are on the same subnet

• Check if the firewall allows the target port

• Verify the TCP port is not occupied by another device

Step 4: Verify Protocol Mapping

• Confirm Modbus function codes and device addresses match

• Verify data register addresses (refer to device manual)

• Check byte order (Big Endian / Little Endian) settings

Selection Summary

Scenario	Recommended Model	Key Capabilities
Single CNC / Small Equipment	USR-TCP232-410s	RS232+RS485 dual ports, MQTT+SSL, 300 data points edge computing
Multi-Device / Gateway	USR-N720	2×RS485, 1000+ data points, SD card offline storage
Compact Installation Space	USR-DR132/134	Lipstick size, 25 data points edge computing, quick deployment
Budget / Simple Transparent Transmission	USR-TCP232-304/302	Virtual COM port, custom data packets, auto-reconnect

Frequently Asked Questions

1. What is the difference between a serial server and a regular network cable converter?

Regular network cable converters (e.g., RJ45 to USB) only handle physical interface conversion and do not support protocol conversion. A serial server not only converts RS232/RS485 to TCP/IP, but also performs Modbus RTU to Modbus TCP or MQTT protocol conversion at the device side, and supports edge computing (data filtering, aggregation, rule triggering), reducing cloud platform load.

2. My device only has an RS232 port, no RS485. Can I still use a serial server?

Yes. Choose a model with an RS232 port (e.g., USR-TCP232-410s provides both RS232 and RS485 ports). If the device uses an RS232 serial port, connect directly via DB9 wiring to the serial server's RS232 port.

3. How many devices can an RS485 bus support?

A standard RS485 bus supports up to 32 nodes. With isolated, signal-amplifying industrial RS485 repeaters, this can extend to hundreds of nodes. In practice, it is recommended to keep no more than 20 devices per bus for communication stability.

4. Does a serial server offer longer transmission distance than RS485?

The standard RS485 transmission distance is approximately 1,200 meters (at baud rates below 19,200). After converting to Ethernet via a serial server, transmission distance depends on the network architecture—LAN has no distance limit, while WAN or Internet transmission depends on network bandwidth and latency.

5. Will data be lost during a network outage?

It depends on the model. Gateway models with SD card offline storage (e.g., USR-N720) support data caching during outages and auto-resume after reconnection. Standard models (e.g., USR-TCP232-410s) support auto-reconnect but cannot cache data during outages. For complete data records, choose a gateway with storage functionality.

6. What protocols does a serial server support?

Mainstream serial servers support:

• Modbus RTU ↔ Modbus TCP bidirectional conversion

• Modbus RTU → JSON format conversion

• MQTT publish/subscribe (some models support SSL/TLS encryption)

• TCP Server / TCP Client / UDP / HTTPD and other basic network protocols

• Virtual COM port (RFC2217 standard)

7. What is the difference between serial servers for industrial and home environments?

Industrial-grade serial servers differ in the following aspects:

• Operating temperature:Industrial -40℃~+85℃, consumer 0℃~50℃

• Power supply:Industrial supports DC 9-36V wide-voltage input, consumer typically DC 5V or 12V

• Isolation protection:Industrial provides 1.5KV~3KV electromagnetic isolation, consumer typically has none

• Mounting method:Industrial supports DIN rail mounting, consumer is usually wall-mount or compact design

8. Will adding a serial server affect the operation of existing equipment?

Normally, it will not. The serial server passively reads device data on the serial side and does not send write commands to the device (unless write rules are explicitly configured). As long as serial parameters (baud rate, parity, etc.) are set correctly, device operation is unaffected. Backing up device parameters before configu