Modbus decode on Pico USB scope pins down PLC problems

July 12, 2017 // By Graham Prophet
Pico Technology (St. Neots, UK) has added to the capabilities of its PicoScope with the addition of Modbus ASCII and Modbus RTU decoders. It is, Pico says, the only oscilloscope range capable of analyzing Modbus (RS-232/RS-485) signals in industrial PLC applications – from installation and commissioning to maintenance and repair operations.

PicoScope is a USB-connected PC oscilloscope, offering six test and measurement instruments in one – including protocol analysis, logic and spectrum analyzers and a signal/arbitrary waveform generator. This capability makes PicoScope outstanding for signal integrity measurements on serial buses. The Modbus decoder is available for all PicoScope models, from the low-cost 2000 Series through to the top-of-the-range 6000 Series. For sources that are not referenced to earth, such as RS-485, the recently introduced PicoScope 4444 Series permits differential measurements on up to four channels without the expensive external active differential probes typically needed in industrial applications. PicoScope includes a comprehensive set of automated and ruler-based on-screen measurements as standard.


The option has protocol analysis for Modbus, a serial communications protocol for use with programmable logic controllers (PLCs). Managed by the Modbus Organization, this simple and robust protocol is widely used to connect industrial electronic devices. The PicoScope decoder supports master and slave devices, standard and custom baud rates, and one or two ID bytes. Hundreds or thousands of Modbus messages can be captured with each acquisition using the deep memory provided in most PicoScope models. Each message is stored with start/end timing information, device IDs, function codes, transmitted data and CRC information as well as data validity checks. The table of acquired messages can be analyzed directly within PicoScope or can be exported in CSV format for offline analysis. A link file helps speed analysis by cross-referencing hexadecimal field values, such as the slave ID, to plain text. Clicking on a decoded packet takes you to the corresponding point on the analogue waveform, making it easy to troubleshoot bus errors.


Modbus decoding and analysis is included as standard in the latest PicoScope beta version 6.13, which now supports a total of 18 serial protocols. The software is available free of charge for all existing and new PicoScope users and can be downloaded from