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Serial Communications

Serial Protocol Analyser

Browser native protocol analyser for RS-232, RS-485, RS-422, UART, SPI, I²C, CAN / J1939, DMX-512, and M-Bus. Live capture over Web Serial, Saleae and Logic 2 CSV import, full Modbus RTU interpretation with exception and register decode, timing and clock tolerance analysis, RS-485 unit load and bias network checks, and CRC reverse search.

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Top bar with the four analyser modes — Decoder, Timing, Signal, Errors.
Top bar with the four analyser modes — Decoder, Timing, Signal, Errors.
Timing analysis with bit / frame time, throughput, efficiency, and clock tolerance margin.
Timing analysis with bit / frame time, throughput, efficiency, and clock tolerance margin.
RS-485 signal mode with noise margins, unit load compliance, and bias network values.
RS-485 signal mode with noise margins, unit load compliance, and bias network values.
CRC catalogue computation with reverse polynomial search for unknown integrity checks.
CRC catalogue computation with reverse polynomial search for unknown integrity checks.

Overview

Serial communication is where most field commissioning time disappears. The radio is fine. The PLC is fine. The cable is fine. But the Modbus master returns an exception 02 on a register the vendor documentation insists exists, the RS-485 segment drops every fifth poll on a long cable run, the SCADA tag stays at the default value because the byte order in the register is reversed, and nobody on site can prove which side of the link is at fault. The protocol analyser is the one tool that closes the argument: it shows the wire, decodes the frames, and verifies the integrity of every byte against the protocol specification.

The noIM₃ Serial Protocol Analyser brings a commissioning grade serial analyser into the browser. Four modes share one workspace. The Decoder mode parses UART, RS-485 / RS-422, SPI, I²C, CAN / J1939, DMX-512, and M-Bus frames from a live Web Serial capture, a Saleae or Logic 2 CSV import, or a pasted byte stream. The Timing mode validates a baud generator divisor against TX and RX clock tolerance, computes the bit time, frame time, throughput, and efficiency for the configured frame format, and surfaces the safety margin against the maximum tolerable mismatch. The Signal mode handles the electrical side: RS-232, RS-485, RS-422, TTL, LVTTL, and LVCMOS noise margin (V_OH minus V_IH and V_IL minus V_OL), RS-485 differential threshold, TIA-485 unit load compliance for 1 / ½ / ¼ / ⅛ UL devices, and bias network voltage at the idle bus. The Errors mode runs the captured bytes through every CRC algorithm in the catalogue (CRC-16/Modbus, CRC-16/CCITT, CRC-32/Ethernet, CRC-8/Maxim, and more), checksum-8, LRC, XOR, sum-16, and supports a reverse CRC search to identify which algorithm a vendor device is actually using when documentation is wrong.

Live capture is the headline feature. With a Chromium based browser on an HTTPS origin (or localhost), the Connect button opens the browser native port picker, opens the selected USB serial adapter at the configured protocol settings, and streams bytes into the decoder in real time. Bytes accumulate into the rawInput pane, the live decode pipeline runs against every chunk as it arrives, and a one click Send to decoder button hands the captured stream to the Modbus / NMEA / MAVLink interpreters. CSV import covers the offline path: a Saleae Logic 2 export with `time, value` columns or an async serial digital channel export is parsed, framing and parity errors are flagged inline, and the same downstream interpretation runs on the imported bytes. Outputs include a hex / decimal / binary / ASCII byte table, a per frame breakdown with bit level diagrams for UART and RS-485, and a Modbus RTU panel that distinguishes query / response / exception roles and renders register and coil tables for FC 1 to 4 responses with exception reason text for codes 01 to 0B.

Capabilities

Live capture via Web Serial

The Connect button opens the Chromium native port picker, opens the selected USB serial adapter (or Bluetooth serial profile) at the configured protocol settings, and streams bytes into the decoder in real time. Live decode runs against every chunk so the Modbus / NMEA / MAVLink interpretation updates as bytes arrive. The browser remembers granted ports per origin so reconnecting is one click. Requires Chrome, Edge, or Opera served over HTTPS (or localhost); Firefox and Safari fall back to CSV import and paste workflows.

Multi protocol decode

UART / RS-232, RS-485 / RS-422, SPI (CPOL / CPHA modes 0 to 3), I²C (Standard, Fast, Fast-Plus, High-Speed, 7 and 10 bit address), CAN / J1939 (PGN catalog and frame parser), DMX-512 (E1.11), and M-Bus (EN 13757). Each protocol has the correct frame ordering (UART start / data / parity / stop with LSB first transmission and MSB or LSB display), the right wire convention, and the right inter character / inter frame gap interpretation. Inverted logic (idle / active polarity flip) is a one click toggle for TTL probes on RS-232 lines and vice versa.

Modbus RTU interpretation with role detection

The Modbus panel distinguishes query, response, and exception roles based on the function code and the payload shape. FC 1 and 2 responses are decoded into a coil bit grid; FC 3 and 4 responses are decoded into a register table with unsigned, signed (i16), hexadecimal, and binary columns; FC 5 and 6 write commands surface the target coil or register and the written value (with validation that FC 5 writes 0x0000 or 0xFF00). Broadcast addressing (slave 0) is flagged inline. Exception responses (FC | 0x80) are decoded into the exception code, the name (Illegal Function, Illegal Data Address, etc.), and a plain language reason that names the likely cause on site rather than just the code.

Saleae and Logic 2 CSV import

A CSV exported from a USB logic analyzer (Saleae Logic 2, or another capture tool that produces `time, value` columns or an async serial digital channel export) is parsed in the browser. Hex (0x48) and decimal (72) value formats are both accepted. Framing and parity errors detected during the import are flagged inline against the affected bytes, and per byte timestamps are preserved so the downstream decoder can show inter character gaps. The same downstream interpretation runs on imported bytes as on live captured bytes.

Timing analysis with clock tolerance

Validates a baud generator divisor against TX and RX clock tolerance (parts per million). Computes the bit time, the frame time, throughput in kbps, and frame efficiency (data bits divided by frame bits). Common oscillator presets (XTAL, ceramic resonator, MEMS, RC oscillator) populate ppm values for fast sanity checks against the maximum tolerable mismatch (typically 3 percent for an 8 bit UART frame). Surfaces the safety margin so a borderline RS-485 link over a low cost RC oscillator is identified before it fails on site.

RS-485 electrical and bus loading analysis

Signal mode handles RS-485, RS-422, RS-232, TTL, LVTTL, and LVCMOS. Calculates V_OH minus V_IH and V_IL minus V_OL noise margins, RS-485 differential threshold, the differential signal swing, and the common mode range. TIA-485 unit load compliance is checked against 1 UL, ½ UL, ¼ UL, and ⅛ UL devices on the bus (limit 32 UL). The bias network solver returns the fail-safe pull-up and pull-down resistor values needed to keep V_AB above the 200 mV differential threshold at the idle bus, taking V_CC, termination, and bus load into account.

CRC catalogue and reverse search

The errors mode runs the captured bytes through every algorithm in the CRC catalogue: CRC-8/Maxim, CRC-8/J1850, CRC-16/Modbus, CRC-16/CCITT, CRC-16/XMODEM, CRC-32/Ethernet, CRC-32/Castagnoli, and more, plus checksum-8, LRC, XOR, and sum-16. A reverse CRC search takes an expected CRC value (the value a vendor device returned for a known frame) and identifies which algorithm matches, finally closing the argument when documentation says one thing and the device does another.

NMEA-0183 and MAVLink detection

NMEA-0183 sentences ($TTSSS,...*HH) embedded anywhere in the byte stream are extracted, the talker (GP for GPS, GN for multi-GNSS, etc.) and sentence (GGA, RMC, GSV, etc.) are decoded, and the XOR checksum is verified. MAVLink v1 (0xFE magic) and v2 (0xFD magic, 24 bit message ID, optional 13 byte signature) frames are detected and their headers parsed. Useful for marine, automation, and UAV integration work where serial captures often contain mixed protocol traffic.

Decoder presets and quick samples

One click presets for the common configurations: RS-232 9600 8N1, RS-485 115200, Modbus RTU at 9600 8E1, NMEA GPS at 4800, Arduino 115200, SPI Mode 0. Sample data buttons load known good frames (a Modbus FC 3 request, the ASCII string "hello", and a synthetic frame) so the analyser can be evaluated immediately without a live capture or a CSV import.

Browser only computation

Runs entirely in your browser. Captured bytes, imported CSV files, and decoded frames are not submitted to any server. Useful for commercially confidential infrastructure work, OT / SCADA networks under air gap policy, and any environment where information security policy prohibits sending engineering data to third party services.

Standards & methodology

  • TIA / EIA-232-F (RS-232) electrical and signalling standard
  • TIA / EIA-485-A (RS-485) differential signalling standard with unit load and bias network conventions
  • TIA / EIA-422-B (RS-422) differential signalling standard
  • Modbus RTU Application Protocol V1.1b3 and Modbus Serial Line Protocol V1.02
  • NMEA-0183 (marine electronic devices interface standard)
  • MAVLink v1 and v2 message protocol (autonomous vehicle telemetry)
  • ISO 11898 (CAN) and SAE J1939 (heavy duty vehicle network)
  • ANSI E1.11 (DMX-512) entertainment lighting control
  • EN 13757 (M-Bus) heat / utility metering
  • I²C specification (UM10204) for Standard, Fast, Fast-Plus, and High-Speed modes

When to use this tool

  • Debugging a Modbus RTU exception 02 (Illegal Data Address) on a vendor PLC with unclear documentation
  • Identifying which CRC algorithm a vendor device actually uses when the datasheet is silent or wrong
  • Verifying RS-485 unit load compliance before adding a 33rd device to an existing bus
  • Sizing the fail-safe bias network resistors for a Modbus RTU segment that drops frames at idle
  • Capturing an NMEA-0183 stream from a marine GPS over USB and verifying GGA, RMC, GSV checksums
  • Validating an SPI Mode 0 captured frame against the configured CPOL / CPHA settings
  • Decoding an I²C transaction (address + R/W + ACK + data) from a logic analyser export
  • Checking baud rate clock tolerance for a Modbus RTU segment using a low cost RC oscillator
  • Recovering register values from a captured Modbus FC 3 response when the master log only shows a CRC error
  • Verifying a MAVLink v2 telemetry capture from a UAV ground station
  • Investigating a CAN / J1939 PGN against the bundled catalog during heavy duty vehicle network debugging

Is this the right tool for you?

Reach for the Serial Protocol Analyser in any of the following situations.

  • You arrive on site to commission a Modbus RTU energy meter for a SCADA integration. The master logs an exception 02 on a register the vendor documentation says exists. Live capture the bus with a USB RS-485 adapter, decode the master query and the slave exception response, see the exception name and the plain language reason, and identify whether the register address is off by one or whether the meter has a different register map for the firmware revision installed.
  • You are debugging an RS-485 segment that drops every fifth poll over a 600 m cable run. Pull a Saleae capture into the analyser, decode the framing and parity errors flagged on the affected bytes, compute the clock tolerance against the documented oscillator on the slave device, and confirm whether the issue is the oscillator margin, the bias network, or the cable capacitance.
  • You are integrating a CAN / J1939 telematics gateway with a fleet management system. Live capture the CAN bus, decode each frame against the J1939 PGN catalog, and verify the engine speed, fuel rate, and odometer PGNs are being broadcast at the expected rate.
  • You are commissioning a DMX-512 lighting controller for an entertainment venue. Live capture the universe, decode the start code and channel data, and verify each fixture is receiving the expected channel slot.
  • You are an OT / SCADA integrator working under an air gap policy. The browser native analyser runs entirely client side so the captured byte stream is never uploaded to a third party service; everything from capture through decode happens on the engineer's laptop without leaving the OT network.
  • You are commissioning an M-Bus utility meter on a smart building deployment. Live capture the bus, decode the telemetry frames per EN 13757, and verify the heat / water / gas measurement is being reported correctly before signing off the installation.
  • You are an amateur radio operator debugging a CAT serial control link to a transceiver. Live capture the USB serial port, decode the UART frames, and verify the polling cadence and the frequency reporting against the manufacturer protocol.
  • You are an SI helping a customer reverse engineer a legacy serial protocol over RS-232 where the vendor has gone out of business. Capture the byte stream, run the reverse CRC search to identify the integrity check, and document the protocol structure for the customer's in-house team.

Frequently asked questions

Which browsers support the live capture feature?

Live capture uses the Web Serial API, which is supported on Chromium based browsers (Chrome, Edge, Opera) served over HTTPS or localhost. Firefox and Safari do not currently implement Web Serial; on those browsers the live capture panel is disabled and the analyser falls back to the CSV import and paste workflows. The Web Serial spec requires a user gesture (a click) to open the port picker, so the Connect button must be clicked rather than triggered programmatically.

What protocols can the analyser decode?

UART / RS-232, RS-485 / RS-422 (Modbus RTU), SPI (CPOL / CPHA modes 0 to 3), I²C (Standard, Fast, Fast-Plus, High-Speed, 7 and 10 bit address), CAN / J1939 with the PGN catalog, DMX-512 per ANSI E1.11, and M-Bus per EN 13757. NMEA-0183 sentences and MAVLink v1 / v2 frames are detected automatically anywhere they appear in the byte stream, so a mixed traffic capture can be decoded without manually splitting the protocols.

How does the Modbus interpretation handle response frames?

The decoder looks at the function code and the payload shape and infers the role: query, response, or exception. A 4 byte payload after FC 1 to 4 is a query (start address + quantity). A payload where the first byte equals the byte count and the rest matches the documented response shape is a response: FC 1 and 2 are decoded into a coil bit grid, FC 3 and 4 are decoded into a register table with unsigned, signed (i16), hexadecimal, and binary columns. Exception responses (FC | 0x80) decode to the exception code, name, and a plain language reason for codes 01 through 0B. Broadcast addressing (slave 0) is flagged inline because broadcast frames receive no response.

What CRC algorithms are in the catalogue?

CRC-8/Maxim (1-Wire), CRC-8/J1850, CRC-16/Modbus, CRC-16/CCITT (XMODEM and FALSE), CRC-16/XMODEM, CRC-16/USB, CRC-16/DNP, CRC-32/Ethernet, CRC-32/Castagnoli, and more — each with the documented polynomial, initial value, reflection settings, and final XOR. The reverse CRC search takes an expected CRC value and runs the captured frame through every catalogue algorithm to identify which one matches, which is the fastest way to identify the integrity check on a vendor protocol when documentation is silent.

Can I import a Saleae Logic 2 CSV?

Yes. The CSV import accepts either a `time, value` two column export or a Saleae async serial digital channel export. Both hex (0x48) and decimal (72) value formats are accepted. Per byte timestamps are preserved through the decoder so the byte table can display the actual capture time of every byte; framing and parity errors flagged during the import are surfaced inline against the affected bytes so the operator can see which captured byte caused the decode failure.

Is the captured data uploaded to a server?

No. All decoding, all CRC computation, and all Web Serial capture runs entirely in your browser. The captured byte stream, the imported CSV file, and the decoded frames are never uploaded to a server. This is important for OT / SCADA environments under air gap policy and for commercially confidential infrastructure work.

Does it work for SPI and I²C captured from a logic analyser?

Yes. SPI mode handles the four CPOL / CPHA combinations (modes 0 through 3) with 8 or 16 bit word sizes. I²C mode handles the standard speed modes (100 kHz Standard, 400 kHz Fast, 1 MHz Fast-Plus, 3.4 MHz High-Speed) with 7 or 10 bit addressing. Both protocols expect a logic analyser export with the captured bytes; the analyser interprets the byte stream against the configured mode.