Debug Serial Comms

Sean Conway walks you through the process of troubleshooting communication problems between a PC application and a serial device.

The itch for an article on troubleshooting USBto-serial connections needed to be scratched, after working on a request for assistance from a local model railroad group (see LXF239). This tutorial will provide the knowledge to troubleshoot communication (comms) issues from an app on a PC to a serial device, using a USB-to-serial cable/interface.

In the diagram shown below, there is a PC running an application that requires a serial interface. The PC is connected to the serial device, shown on the far right side of the drawing, using a USB-to-serial cable/ interface. The drawing contains a few additional details that will be introduced as our discussion expands.

Hardware in the PC supported serial comms; at one point during the evolution of the PC, a minimum of two DB9 ports (the 9-pin COM port) was considered standard. The red highlighted text in the diagram are some examples of COM port communication needs. But all things come to an end, and the ubiquitous USB port soon replaced COM ports. To consolidate all the different serial device comms requirements, a USB-toserial cable/adaptor came into play. Unfortunately, this generic serial comms doesn’t always work, and it can be frustrating trying to isolate the root cause of serial communication failure.

After connecting the USB-to-serial cable to the host PC, the first step is to determine if the OS recognises the USB-to-serial adaptor. In Linux, the dmesg command reveals information regarding hardware detection, driver assignments and communication port allocation. It is important to take note of what COM port (COM1, 2, 4 or so on) is used – this will be needed to support our efforts, as well as set up the application.