RSLogger 485

RSLogger 485 – Dual-Channel Isolated RS485 Data Logger with Modbus RTU Decoding

RSLogger 485 is a compact hardware RS485 data logger designed for passive monitoring, diagnostics and long-term recording of two RS485 communication channels. It can simultaneously receive data from two differential signal pairs and store both data streams in the internal 512 MB ECC Flash memory.

The logger operates in receive-only mode and does not actively transmit data onto the monitored RS485 buses. Recorded data can be stored in one common file with channel indicators or in two separate files, one for each channel.

RSLogger 485 can record the original binary communication stream or use its built-in Modbus RTU decoding mode to create readable text logs containing timestamps, TX/RX direction, device addresses, function codes, payload data and CRC values.

Main Features

• Two RS485 receive channels – simultaneous passive monitoring of two RS485 signal pairs.
• Galvanically isolated RS485 section – helps reduce ground loops, electrical noise and moderate ground-potential differences.
• Built-in Modbus RTU decoding – converts binary Modbus frames into readable TXT logs with direction, address, function code, payload and CRC.
• Raw binary recording mode – stores the original unmodified data stream for proprietary and non-Modbus protocols.
• 512 MB ECC Flash – circular recording buffer with error correction for reliable long-term data logging.
• USB Mass Storage support – automatic data backup to FAT32 USB drives in TXT or HEX format.
• USB Virtual COM (FTDI) – text menu, AT commands, configuration, data access and real-time channel forwarding.
• Precise time-stamping – RTC-based timestamps with resolution down to 1 ms.
• Rechargeable RTC backup battery – maintains date and time when external power is disconnected.
• Wide 5–31 VDC power range – convenient integration with control systems and external DC supplies.
• 5 V Micro USB power option – operation from a standard USB power source.
• Detachable terminal connectors – secure wiring of RS485 signals, power and auxiliary functions.
• Programmable GPIO – standard firmware provides a UART-level copy of RS485 Channel 1 data.
• External switch input – duplicates the function of the built-in push button.
• Two status LEDs – USB storage activity and system/recording status indication.
• Firmware update support – secure firmware installation using USB Mass Storage.
• Factory firmware recovery – built-in recovery function for restoring factory firmware and settings.
• Compact aluminum enclosure – durable construction suitable for laboratory, service and embedded installations.

Dual-Channel RS485 Recording

RSLogger 485 provides two differential receive channels:

• Channel 1: A1 / B1
• Channel 2: A2 / B2

Both channels can be monitored and recorded simultaneously. Received data can be saved in one common log file with channel or direction indicators or in two separate files, one for each channel.

This allows RSLogger 485 to monitor two separate RS485 data streams or record both directions of communication when the transmitted and received signals are available on separate signal pairs.

The RS485 transceivers operate in receive-only mode. The logger does not actively drive or modify the monitored communication lines.

Built-in Modbus RTU Decoding

RSLogger 485 can record RS485 communication either as an unmodified binary data stream or decode recognized Modbus RTU frames into a clear, human-readable text format.

When Modbus RTU decoding is enabled, every recorded frame can contain:

- precise timestamp,

- TX or RX direction,

- device address,

- Modbus function code,

- frame payload,

- CRC value.

Example decoded Modbus RTU communication:

P 6:29:04.378 TX [ADR 01][FC 01] 00 00 00 08[CRC 3D CC]
P 6:29:04.386 RX [ADR 01][FC 01] 01 4D[CRC 91 BD]

This format makes diagnostics easier because the operator does not need to identify the address, function and CRC fields manually inside a continuous binary data stream.

The recording mode is selected in the CONFIG.INI file.

MODBUS_RTU=0  Records the original raw binary data without protocol decoding or translation.

MODBUS_RTU=1  Enables Modbus RTU frame detection and human-readable text decoding.

Raw binary recording remains available for proprietary protocols, non-Modbus RS485 communication and applications requiring an exact copy of the original data stream to be analyzed/archived  with an external software.

Modbus decoding affects only the saved log format. RSLogger 485 remains a passive receiver and does not transmit or modify data on the monitored RS485 buses.

Functional Galvanic Isolation

The complete RS485 input section is galvanically isolated from the internal logger electronics, USB interface, external system ground and any computer connected through USB.

The functional isolation helps reduce:

• ground-loop currents,
• electrical noise transferred from communication lines,
• moderate differences in ground potential,
• interference affecting the logger or connected USB equipment.

Both RS485 channels share one common isolated reference and are therefore not galvanically isolated from each other.

Important: the isolation is intended for functional separation of communication equipment. It must not be treated as safety isolation, direct mains isolation or protection against continuous voltage differences in the kilovolt range.

Internal Memory and Automatic USB Backup

Received data is stored first in the internal 512 MB ECC-protected Flash memory. The memory operates as a circular buffer, automatically replacing the oldest data when its available capacity is full.

A FAT32 USB Mass Storage device can be connected at any time to:

• download recorded data,
• create automatic backup copies,
• load and save configuration files,
• install firmware updates.

When a USB drive remains connected, newly recorded data can be copied automatically in the background. This provides practically unlimited recording capacity for low- and medium-traffic communication systems.

The logger can save data in a common file for both channels or in separate channel files. Depending on configuration, data can be stored in binary, HEX or decoded Modbus RTU text format.

Precise Time-Stamping

The built-in real-time clock provides precise timestamps with resolution down to 1 ms. The RTC is supported by an internal rechargeable backup battery, allowing the device to retain its date and time when external power is disconnected.

Available timestamp modes include:

• a timestamp for every received byte,
• a timestamp when the active channel changes,
• periodic timestamps,
• character-triggered timestamps,
• 12-hour and 24-hour time formats.

USB Virtual COM

The Micro USB interface includes an FTDI-based USB Virtual COM port. It can be accessed with standard serial terminal applications and does not require dedicated RSLogger software.

Virtual COM can be used for:

• configuration through the text-based menu,
• AT-command control,
• downloading recorded data,
• real-time forwarding of RS485 Channel 1,
• real-time forwarding of RS485 Channel 2,
• forwarding a mixed data stream from both channels,
• manufacturer service and firmware operations.

Terminal Connections

RS485 terminal connector

• A1 – RS485 Channel 1 signal A
• B1 – RS485 Channel 1 signal B
• A2 – RS485 Channel 2 signal A
• B2 – RS485 Channel 2 signal B
• GND_RS485 – common isolated reference for both RS485 channels

Power and auxiliary terminal connector

• SWITCH – external input duplicating the built-in push-button function
• GND_RS485 – isolated RS485 reference
• GND – logger system ground
• GND – additional logger system ground terminal
• Vcc – external 5–28 VDC power input
• GPIO – programmable auxiliary input/output

The isolated RS485 ground and logger system ground are available on neighbouring terminals. They may be connected externally with a short jumper when galvanic isolation is not required and a common reference is preferred.

The grounds should remain separated when the RS485 installation and logger may operate at different ground potentials.

Programmable GPIO

The programmable GPIO terminal allows RSLogger 485 to be integrated with external control systems, embedded controllers or additional interface modules.

In the standard firmware configuration, the GPIO output provides a UART-level copy of the data received through RS485 Channel 1. Other GPIO functions may be implemented as part of a customized firmware version.

Why RSLogger RS485?

RSLogger 485 combines passive dual-channel communication monitoring with local data storage, precise timestamps, USB data access and functional galvanic isolation. Unlike simple RS485-to-USB adapters, it can operate independently without a permanently connected computer.

The optional Modbus RTU decoding mode also provides immediately readable communication logs, making diagnostics and service work faster and easier.

It is a practical RS485 sniffer, Modbus RTU logger and serial communication recorder for technicians, engineers, developers and system integrators.

Typical Applications

• Passive monitoring of RS485 communication.
• Modbus RTU communication recording and diagnostics.
• Long-term recording of two RS485 data streams.
• Monitoring communication between controllers and peripheral devices.
• Recording data from meters, sensors, PLCs and automation equipment.
• Diagnostics of industrial and embedded communication systems.
• Monitoring communication over long RS485 cable runs.
• Service and troubleshooting of serial communication systems.
• Laboratory, educational and development projects.
• OEM integration requiring local storage and USB access.

Technical Specifications

• Communication standard: RS485.
• Channels: 2 receive channels.
• Operating mode: passive, receive-only monitoring.
• Recording modes: raw binary RS485 stream, HEX output or decoded Modbus RTU text log.
• Modbus RTU log fields: timestamp, TX/RX direction, device address, function code, payload and CRC.
• Modbus configuration: MODBUS_RTU=0 for raw mode; MODBUS_RTU=1 for decoded mode.
• Isolation: functional galvanic isolation between the RS485 section and logger electronics.
• Channel reference: both RS485 channels share one common isolated ground.
• Memory: 512 MB ECC NAND Flash, circular buffer.
• Baud rates: up to 230,400 bps per channel.
• Data interfaces: USB Mass Storage and USB Virtual COM.
• USB storage: FAT32, 1–32 GB.
• Time-stamps: resolution down to 1 ms.
• RTC backup: internal rechargeable battery.
• Power: 5–31 VDC external input or 5 V through Micro USB.
• Connectors: detachable terminal blocks for RS485, power and auxiliary functions.
• GPIO: programmable input/output; standard firmware provides UART copy of Channel 1.
• Enclosure: compact aluminum housing.
• Dimensions: 55 × 16 × 66 mm.

Typical RSLogger 485 current consumption

Values refer to typical current consumption during normal operation, without a connected USB storage device.

What You Get

• RSLogger 485 dual-channel data logger
• Detachable terminal connectors
• User Manual download
• Optional Mounting Kit, if selected

➡ Order RSLogger 485 today and add passive dual-channel RS485 recording, functional galvanic isolation and readable Modbus RTU logging to your monitoring system.

Downloads

Download latest firmware

Download user manual

Frequently Asked Questions

Does RSLogger 485 transmit data onto the monitored RS485 bus?

No. RSLogger 485 operates as a passive receive-only monitoring device. It records received data without actively driving or modifying the monitored RS485 signal lines.

Can RSLogger 485 decode Modbus RTU communication?

Yes. When MODBUS_RTU=1 is enabled in CONFIG.INI, the logger detects Modbus RTU frames and records them in a readable text format containing the timestamp, TX/RX direction, device address, function code, payload and CRC.

Can it record non-Modbus RS485 data?

Yes. With MODBUS_RTU=0, RSLogger 485 records the original binary RS485 data without protocol decoding or translation. This mode can be used with proprietary and non-Modbus communication protocols.

Does Modbus RTU decoding modify the monitored communication?

No. Decoding affects only the format of the saved log. RSLogger 485 remains a passive receiver and does not transmit or modify data on the monitored lines.

Can both RS485 channels be recorded simultaneously?

Yes. Both channels can be recorded at the same time. Data can be stored in one common file with channel indicators or in separate files for each channel.

Are the two RS485 channels isolated from each other?

No. Both RS485 channels share one common isolated reference. The complete RS485 section is galvanically isolated from the logger electronics, USB interface and system ground.

What is the purpose of the galvanic isolation?

The functional galvanic isolation helps reduce ground-loop currents, electrical noise and moderate differences in ground potential between the RS485 installation and the logger electronics.

Can the isolation be used as protection against mains voltage?

No. The isolation is intended for functional separation of communication systems. It must not be treated as safety isolation, direct mains isolation or protection against continuous kilovolt-level voltage differences.

Can the isolated RS485 ground be connected to the logger ground?

Yes. Both grounds are available on neighbouring terminal pins and may be connected externally when galvanic isolation is not required. They should remain separated when ground-potential differences may be present.

What is the standard GPIO function?

In the standard firmware configuration, the GPIO output provides a UART-level copy of data received through RS485 Channel 1.

How can recorded data be downloaded?

Recorded data can be exported automatically to a FAT32 USB Mass Storage device or accessed through the USB Virtual COM interface.

Can the firmware be updated by the user?

Yes. Firmware updates can be installed using a USB Mass Storage device. The logger also includes a protected factory firmware recovery function.