Troubleshooting Serial Client Display: No Output - Logic Analyzer Verifies Output Sent
This article focuses on a common issue faced when using a serial/USB adapter (FTDI breakout) with an old oscilloscope and the picocom client. We will walk through the troubleshooting process step-by-step and cover key concepts to help you identify and solve the problem.
Scope and Background
The problem arises when the scope requires hardware flow control, and the current setup does not allow for proper output display in the picocom client.
Picocom Configuration and Flow Control
Picocom is a minimal serial communication program for Unix-like operating systems. It supports hardware and software flow control, RTS/CTS and XON/XOFF, respectively. You can check if your picocom client configuration includes hardware flow control by verifying the options provided when starting picocom:
picocom -b9600 /dev/ttyUSB0 -fh
In this command, -b specifies the baud rate, -f enables local flow control, and -h enables hardware flow control. Make sure hardware flow control is indeed enabled in your picocom settings.
Testing Flow Control Settings with a Logic Analyzer
Although the picocom client seems correctly configured for hardware flow control, the output display is not as expected. At this point, it is essential to employ a logic analyzer to validate if the output is being sent but not displayed correctly.
Connect your logic analyzer to the FTDI breakout and set it to monitor the RTS and CTS lines while running the serial communication. If the analysis shows that the RTS line is triggered when the system sends data, and the CTS line responds correctly, then the issue is most likely on the oscilloscope side.
Issues with the Oscilloscope
It is possible that the old oscilloscope you are using does not support or has trouble interpreting hardware flow control signals. If your logic analyzer confirms that the RTS and CTS lines are operating fine, yet, the serial communication data does not show correctly on the oscilloscope, it is time to consider the oscilloscope limitations.
Compatibility: Ensure your oscilloscope supports the baud rate and hardware flow control signals.
Settings: Check the oscilloscope's settings, such as decimation, trigger level, and threshold, to make sure they are appropriately set.
When you experience a "no output" issue in a serial client while using a serial/USB adapter and an oscilloscope where the logic analyzer verifies output has been sent, it involves a systematic troubleshooting process. Covering aspects like picocom configuration, flow control, using a logic analyzer, and exploring issues within the oscilloscope helps you resolve the problem.