A Siemens S7-200 PLC that has been in service for ten years has only an RS232 interface. An engineer needs to connect a laptop from the control room for remote programming, but the laptop no longer has a serial port. A USB-to-serial cable solves the problem temporarily — switching to another laptop may require reinstalling drivers, and cables over 15 meters need to be replaced. Connecting serial devices to Ethernet is a retrofit many factories are undertaking, and the core device for this is the serial-to-Ethernet converter.

Products on the market range from a few dozen yuan to over a thousand. Parameters look similar, but stability varies significantly. This article presents three practical dimensions for evaluating options in the PLC communication scenario.

1. Start with Port Type: RS232, RS485, or Dual-Port?

Different PLCs and sensors use different physical interfaces. The first step in selection is to confirm what interface the field devices use, rather than looking at price first.

• RS232: Point-to-point communication, typically no more than 15 meters. Suitable for connecting programming computers, HMIs, and barcode scanners. For remote programming from a laptop to a single PLC, a single-port RS232 converter is sufficient. PUSR's USR-TCP232-302 provides one RS232-to-Ethernet port with a simple design and low cost.

• RS485: Supports multi-drop bus, with 32+ slave devices on a single twisted pair, up to 1,200 meters. Modbus RTU instruments, temperature sensors, and variable frequency drives all use RS485. The corresponding product is USR-TCP232-304 (1-port RS485).

• Dual-Port (RS232 + RS485): When a project has both programming computers and RS485 instruments, a dual-port device avoids purchasing two separate converters. USR-TCP232-410s provides one RS232 and one RS485 port, both operating simultaneously.

Selection method: List all devices that need network access, grouped by interface type. For more than 4 RS485 devices, consider multi-port products like the USR-N540 (4-port RS485), where each port can be configured with an independent baud rate.

2. Protocol Support: Transparent Transmission and Modbus Gateway Are Different

This is the most common pitfall in selection. Serial-to-Ethernet converters have two primary operating modes with entirely different use cases.

Transparent transmission mode: Raw serial bytes are encapsulated directly into TCP packets for forwarding, without protocol parsing. This is equivalent to replacing a long serial cable with an Ethernet cable. The benefit is no software changes required. The drawback is that Ethernet jitter may break the 3.5-character silent interval that Modbus RTU relies on, causing CRC errors and timeouts.

Modbus gateway mode: The converter actively parses Modbus RTU frames, strips the CRC, adds the MBAP header, and converts to standard Modbus TCP format. The SCADA side sees standard Modbus TCP (port 502) without needing a virtual COM port driver. PUSR's USR-TCP232-410s and USR-N540 both support multi-host polling, allowing multiple SCADA systems to read the same RS485 device simultaneously without bus contention.

Selection method: If the host software only supports Modbus TCP (most modern SCADA systems), choose a product with Modbus gateway functionality. Older software (such as early WinCC or Wonderware versions) only recognizes COM ports, where transparent transmission with a virtual COM port driver is more suitable.

3. Industrial-Grade Design: Workshop Environments Are Not Laboratories

The "operating temperature" specification on a datasheet is often the key determinant of long-term stability.

A commercial converter may perform well in a lab. But factory power cabinets exceed 55°C in summer, and unheated workshops in northern regions drop to -10°C in winter. Commercial units typically operate at 060°C, while industrial-grade units support -4085°C — a significant difference.

Other factors to consider:

• Housing material: Metal housings dissipate heat better, which is critical in enclosed power cabinets.

• Power input range: Industrial sites primarily use 24V DC. Converters supporting 5~36V wide-range input (such as USR-TCP232-410s) handle voltage fluctuations and include reverse-polarity protection.

• Electromagnetic isolation: At least 1.5KV opto-isolation between RS485 and Ethernet ports prevents motor surges from damaging switches or computers.

• Watchdog: Dual hardware and software watchdogs auto-reset on program lockup.

• Mounting: DIN-rail mount for power cabinets; wall mount for surfaces.

Ask suppliers for operating temperature range and EMC ratings — specific, verifiable parameters indicate industrial-grade design.

Deployment Considerations

After selection, a few field deployment details affect success rate.

IP address planning: Assign static IPs. Do not rely on DHCP. Record the IP-to-device-location mapping.

Serial parameter consistency: Baud rate, data bits, parity, and stop bits must match the PLC side exactly.

Step-by-step testing: After power-on, ping to confirm IP reachability, then use serial debugging software to send a Modbus request and verify the serial side, followed by an end-to-end test. Before batch deployment, run a 72-hour ping stability test. PUSR offers sample testing services.

Selecting a serial-to-Ethernet converter does not require a complex methodology. Narrow the field by interface type, determine whether Modbus gateway functionality is needed based on protocol requirements, and filter out commercial-grade products using industrial specifications. PUSR's product line covers single-port RS232 (USR-TCP232-302), dual-port RS232+RS485 (USR-TCP232-410s), multi-port RS485 (USR-N540), and Edge IoT gateways (USR-N720). All products are CE, FCC, and RoHS certified with -40~85°C industrial-grade design.

Frequently Asked Questions

1. What is the difference between a serial-to-Ethernet converter and a Modbus gateway?

A serial-to-Ethernet converter performs transparent transmission — it encapsulates raw serial bytes into TCP packets and forwards them without protocol parsing. It acts like a "virtual Ethernet cable," allowing remote devices to see the same serial data they would over a physical cable.

A Modbus gateway adds protocol translation: it actively parses Modbus RTU frames, strips the CRC checksum, adds the TCP/IP MBAP header, and converts data to standard Modbus TCP format. The SCADA or PLC side communicates via standard Modbus TCP (port 502) without installing a virtual COM port driver.

In short: transparent transmission suits scenarios where existing serial-based software is retained; Modbus gateway suits modern systems where the host only recognizes Modbus TCP. PUSR's USR-TCP232-410s and USR-N540 support both modes and can be switched via the web configuration interface.

2. My SCADA software only recognizes COM ports. Can I network it through a converter?

Yes. In this case, use the Virtual COM Port (VCOM) feature. The converter works with a VCOM driver installed on the host PC to map the remote serial port over the network to a local COM port (e.g., COM5). When an older SCADA software opens "COM5," data actually travels over Ethernet to the converter, then to the serial device.

Pay attention to driver compatibility when selecting a product. PUSR converters ship with the VirCom virtual serial driver, which supports Windows 10/11 and includes a bulk configuration tool — all 20+ units can have their IP parameters set from a single PC without on-site individual configuration.

3. Can I use cheap serial-to-Ethernet modules (a few dozen yuan)?

Modules at that price point are generally suitable for labs or temporary debugging, not for long-term factory operation. Key differences:

• Operating temperature: Commercial units typically operate at 060°C. Factory power cabinets often exceed this in summer, causing device lockups. PUSR industrial-grade units support -4085°C.

• Electromagnetic isolation: Budget modules often lack isolation between RS485 and Ethernet. Motor surges in the field can damage switches or network cards. PUSR provides 1.5KV electromagnetic isolation.

• Watchdog: Industrial units have dual hardware and software watchdogs for auto-reset on anomaly. Budget units lack watchdogs — once locked, manual power cycling is required.

• Driver stability: VCOM driver quality is a major pain point for budget converters — disconnections on network jitter and failures after Windows updates are common complaints. PUSR drivers support automatic reconnection on link loss.

For projects requiring 7x24 hour operation, industrial-grade products are recommended.

4. What if serial parameters (baud rate, parity) are set incorrectly?

Serial parameter mismatch is the most common cause of communication failure. The converter's baud rate, data bits, parity, and stop bits must match the PLC or sensor side exactly. For example, if the PLC is set to 9600-8-N-1 (9600 baud, 8 data bits, no parity, 1 stop bit), the converter must use the same settings.

Troubleshooting steps:

• Consult the PLC or sensor manual to confirm serial parameters.
• Open the converter's web configuration page (enter the IP address in a browser) and verify serial parameters match the device.
• Connect the serial device directly to a laptop via USB-to-serial adapter, send a Modbus request with a serial debugging tool to confirm the device responds correctly before connecting through the converter.
• If everything above is correct but communication still fails, check whether the RS485 A/B wires are reversed and whether 120Ω termination resistors are installed at both bus ends (recommended for cables exceeding 100 meters).

5. How many RS485 devices can one converter support?

This depends on the converter's operating mode:

• Transparent transmission mode: The converter itself does not limit device count. The RS485 bus can theoretically support 32+ devices (depending on the chip). However, all devices share the same baud rate, and the host must poll them individually via RS485 protocol.

• Modbus gateway mode: The converter acts as a Modbus TCP master station, actively polling slave devices. The USR-TCP232-410s supports up to 128 data points; the USR-N720 Edge IoT gateway supports 1,000 data points.

For more than 30 RS485 devices on-site, multi-port products like the USR-N540 (4 RS485 ports) are recommended, with each port polling independently without interference.

6. Which PLC brands does the converter support?

PUSR converters are protocol-agnostic and do not distinguish PLC brands. Any device using RS232 or RS485 interfaces with Modbus RTU protocol — whether Siemens, Allen-Bradley, Mitsubishi, Omron, or others — can be connected to Ethernet through the converter.

The key question is not which brand the converter "supports," but confirming the following:

• The converter's serial parameters (baud rate, parity) match the PLC.

• In Modbus gateway mode, the PLC's Modbus register address map aligns with the SCADA configuration.

• In transparent transmission mode with VCOM driver, the driver software is compatible with the PLC programming software in use.

PUSR converters are widely deployed in retrofit projects involving Siemens S7 series, Allen-Bradley CompactLogix, Mitsubishi FX/Q series, and other common PLCs.

7. Does the converter need a static IP or can it use DHCP?

A static IP address is recommended. Reasons:

• DHCP-assigned IPs may change after router restarts or lease expiration, causing host software to lose device connectivity.

• Factory networks typically have strict IP planning, and static IPs simplify troubleshooting and fault localization.

• Static IP combined with IP-MAC binding prevents unauthorized devices from accessing the network.

PUSR converters support web configuration and bulk configuration tools, which can modify IP addresses, subnet masks, and gateway settings for multiple units from one PC — suitable for deployments of 20+ units.

8. What long-term issues occur with converters running in factories? Do they require maintenance?

Industrial-grade converters have a design life of 10+ years and require almost no maintenance under normal operation. Three recommendations for periodic checks:

• Firmware updates: Manufacturers release firmware periodically to fix known issues or add features. Check every six months and verify on a bench unit before deploying to production.

• Configuration backup: Export current settings to a file via the converter's web interface. If a unit fails and needs replacement, the configuration can be imported to the new unit, saving reconfiguration time.

• CRC error counter: Converters with diagnostic capabilities (such as USR-N540) display CRC error statistics on the web page. A rising CRC error count typically indicates degrading RS485 cable quality (loose termination resistors, aging cables, increased electromagnetic interference), which warrants a physical cable inspection.