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I need to debug an RS485 bus (record analog signals) on a remote installation with a pocket Oscilloscope or Logic Analyzer. Can I simply use a voltage divider to bring down the RS485 signal to within the ADC range of the pocket Oscilloscope / Logic Analyzer? I was thinking about two 1k Ohm resistors in series and take the signal between the two resistors to bring the signal down to within +/- 10V (see attached image).

EDIT for clarification: I need to record the full range (or at least within +/- 20V) of the analog signal (check its shape) and use a software to decoded the analog signal.

Better ways to solve this tasks are very welcome as well.

Background:

  • Abnormal signals are expected on some installations.
  • The baud rate is in the range of 9600 to 115’200.
  • I would like to avoid buying a pocket oscilloscope/Logic Analyzer and not being confident about the recorded analog signals. I would prefer to buy a Saleae Logic Pro with +/- 10V ADC, because I already used Saleae devices during my studies.
  • Please be indulgent for my (most likely) foolish question. Thank you

Planned implementation with a voltage divider of two 1k Ohm resistors

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  • \$\begingroup\$ You can build an RS485 sniffer circuit using a MAX485 chip. It will interface to the A/B lines correctly and not load them. It produces a single-ended output suitable for an unearthed or floating oscilloscope. You will need to supply a low voltage power supply possibly from a battery and voltage regulator. \$\endgroup\$
    – Andy aka
    Jan 23 at 13:38
  • \$\begingroup\$ @Andyaka: Thank you! Am I correct that you suggest something like this: links? I assume the MAX485 output is UART/TTL? I would like to decode (sniff) the communication, but I also need to observe the analog signal. Am I correct that I cannot do this with the MAX485 sniffer? \$\endgroup\$
    – walfab
    Jan 23 at 14:38

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Taken directly from the Saleae Support website.

Decode RS-232, RS-485, & RS-422

The new Saleae Logic products (Logic 4, Logic 8, Logic Pro 8, Logic Pro 16) can be used to read and decode RS-232, RS-485, and RS-422 data up to +/- 25V. Our older Logic products (Original Logic, Logic16) have a 0V to 5V absolute maximum range. Therefore, neither product can be used to measure signals outside of this range. That limits its usage for directly recording RS-232, RS-485, and RS-422, which exceed this range in many cases

Precautions to Avoid Damage to Saleae Logic

These protocols typically operate at higher voltages, sometimes up to +/- 25V. For this reason, there are several precautions and setup requirements to consider beforehand, which you can read more about below.

Decode Differential and High Voltage Data

Use the Async Serial Analyzer

RS-232, RS-485, and RS-422 transmit data via asynchronous serial communication. For this reason, you can use our Async Serial Analyzer to decode the data. You can view our setup guide below.

Async Serial Analyzer - User Guide

Source: https://support.saleae.com/protocol-analyzers/analyzer-user-guides/using-async-serial/decode-rs-232-rs-485-and-rs-422

RS-232, RS-485, and RS-422

  • All four of the new Saleae devices include over-voltage protection to +/- 25 volts. It’s perfectly safe to connect any signal up to this range directly to its inputs
  • The original Logic and Logic16 cannot be directly connected to these signals. They also have over-voltage protection, but it was not designed to be used with these signals continuously. Either use a voltage divider or a dedicated line transceiver/receiver to convert these signals to CMOS/TTL levels.
  • When recording any of these signals, it is important to properly connect the ground from the logic analyzer to either the ground of the transmitter or the receiver. Do not connect ground to one of the signal wires, as this could damage your equipment. Neither RS-232, RS-485, nor RS-422 are isolated, which means that all transmitters and receivers on the bus must share the same ground to operate. In most cases, a ground wire is included in the bus wiring, which could be tapped with Logic. Otherwise, you will need to find another ground connection nearby to connect to.
  • For RS-422 and RS-485, it's generally not necessary to record both the + and - signals. In most cases, recording only one-half of the differential pair is sufficient. However, it's usually a good idea to record both sides, at least at first, to evaluate the differences in the recording quality of the two signals. Because the threshold voltage of the logic analyzer is not matched properly for differential signals, it's likely that one side of the differential pair will have a cleaner recording than the other.

Source: https://support.saleae.com/protocol-analyzers/analyzer-user-guides/decode-differential-and-high-voltage-data

And no, this is far from being a foolish question.

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  • \$\begingroup\$ Thank you! I read the documentation from Saleae and wasn’t sure if the analog signals larger than +/- 10V can be recorded. As I understand it, the device is not damaged within +/-25V but the ADC can only measure +/-10V. Therefore, I assumed that voltages above a set voltage (e.g. 2.5V) are considered logical 1 for the RS485 decoding, but the ADC saturates outside +/- 10V and I cannot observe the shape of the signal outside of +/-10V. Because of the expected saturation, I wanted to use the voltage divider to observe a broader voltage range. Does this make sense or am I missing a point? \$\endgroup\$
    – walfab
    Jan 23 at 14:32
  • \$\begingroup\$ The receiver input range is -7..12V and the receiver input sensitivity is ±200 mV. Sure, the ADC saturates outside ±10V. But what makes you believe that a single-ended signal between +10V and +12V is of importance? When looking at \$V_{AB}\$, only the range > -200mV to < +200mV is problematic and undefined. \$\endgroup\$
    – Velvel
    Jan 23 at 15:11
  • \$\begingroup\$ The problem when you're adding the voltage dividers you've proposed is that you can introduce reflections on the bus. But you could probably get away with it on a "short" bus. \$\endgroup\$
    – Velvel
    Jan 23 at 15:18
  • \$\begingroup\$ Thank you! You are right, the range from +/-10V should contain all the information. This means: I will directly use the Saleae Logic 8 Pro to record the analog signal. I was afraid that I will forget some phenomena (like the reflections) with the voltage divider, which will distort the signal. \$\endgroup\$
    – walfab
    Jan 23 at 16:18

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