USB to Serial Cable Troubleshooting: Fix RS232, RS485, TTL, COM Port, and Modbus RTU Problems

Although a USB to Serial Cable has ceased to work, the problem does not necessarily lie in the cable. There are numerous RS232, RS485, TTL, and Modbus RTU applications whereby USB to Serial Cable debugging will involve testing of the USB connection, driver, COM port, wiring, serial parameters, protocol settings, and field electrical environment, step by step.

The USB to Serial Adapter can be used to link newer computers with PLCs, CNC devices, barcode readers, gauges, sensors, controllers, onboard computers, and various other serial equipment. Nonetheless, when the adapter is not recognized, the COM port is not visible or the device does not react, the users tend to be quick in assuming that the cable is faulty. Actually, most of these issues arise due to mismatched drivers, wrong interface, wrong pinout, RS485 polarity, Modbus RTU configuration or industrial interference.

The guide provides information on the most frequent issues with the USB to Serial Cable and how to effectively solve them.

1. Check Whether the USB to Serial Adapter Is Detected

First of all, you will need to establish if the computer can recognize the USB to Serial Adapter. With Windows, go to the Device Manager and see if a proper COM port is displayed in the Ports (COM & LPT) section. In case the adapter looks like an unidentified USB device, has a yellow warning sign, or is not listed at all, the problem probably lies with the USB port, driver, chipset or hardware.

When the USB to Serial Adapter does not show up, use a different USB port, do not use an unpowered USB hub, test the cable on a different computer, and see if the USB connector is loose or damaged. In case it fails to be detected on various computers the bridge IC, PCB, or internal connections of the adapter can be defective.

2. Fix USB Serial Port Not Showing or Driver Problems

One of the most frequent issues is that the USB device can be identified but the USB Serial Port is not listed as an available COM port. It normally indicates that the operating system is unable to establish a virtual serial port properly.

The various USB to Serial Cables available in the market may have different USB-UART bridge ICs, including but not limited to FTDI, CP210x, CH340 or PL2303. Every chipset may need the right driver. To achieve a stable operation, use the driver provided by the adapter supplier or the chipset manufacturer rather than relying on random third-party driver websites.

When the COM port does not appear even after the driver installation, remove the driver, connect the cable again, and reboot the computer and retest it. In case a different known-working USB to Serial Cable does work on the same computer, the old adapter could be faulty due to its chipset or hardware.

3. Fix COM Port Problems in Serial Software

Although a COM port can be seen, serial software does not necessarily open it. A serial port typically could not be accessed simultaneously by several applications. Your desired software might not even connect to the serial port in case PuTTY, Tera Term, Modbus Poll, PLC software or another background process has already used it.

Shutdown any serial communication applications, reattach the USB to Serial Cable, and load only the application that is needed. Check also that the software chooses the right COM port.

Certain vintage PLC, CNC, POS or instrumentation software can only see COM1 through COM8. The software might fail to identify the port if Windows designates COM18, COM25 or greater. Here, you should manually alter the COM port number in Device Manager to any lower available number.

4. Confirm Whether the Device Uses RS232, RS485, or TTL

An important point in resolving USB to Serial Cable problems is to ensure that the correct interface type has been identified. RS232, RS485 and TTL cannot be used interchangeably.

DB9 connector is not necessarily standard RS232. Terminal block is not necessarily RS485. Pin header is not necessarily TTL UART. Most industrial units have customized pinouts, therefore before making connections always refer to the unit manual.

In case of a failed USB to RS232 Cable, ensure that TXD, RXD, GND, pinout, baud rate, parity, stop bits and the need to use any of the RTS/CTS or DTR/DSR handshake signals are checked. A basic TX/RX/GND adapter might not be sufficient in some cases with legacy devices requiring hardware flow control.

In case a USB to RS485 Cable is not functioning, you need to inspect the following: polarity of A/B, baud rate, slave ID, bus structure, termination resistors, and grounding. Most communication issues with RS485 are due to reversed A/B cables or incorrect Modbus RTU settings as opposed to the failure of the adapter.

In case of non-working of a USB to TTL Cable, check if the target device is using 3.3V TTL or 5V TTL. TX should be connected to RX, RX to TX, and GND to GND. VCC should not be connected unless it is certain that the target board requires power supplied by the adapter. The USB to TTL Cable cannot be directly plugged into RS232 devices.

5. Fix COM Port Opens but No Device Response

In case of the COM port being open and the device not responding, then it is most likely that the USB driver and COM port are functioning. Wiring, interface mismatch, serial parameters, or protocol configuration are typically the cause of the problem.

When working with RS232 and TTL, verify that TX and RX are properly crossed and that GND is connected. In case of reversed TX/RX or absence of GND, the COM port can open as usual, but the device will not be able to communicate.

On RS485, test A/B polarity. There are different labels used by various manufacturers to identify RS485 lines: A/B, D+/D-, or 485+/485-, and these labels may not always be consistent. When all settings seem to be right but nothing happens, the swapping of A and B is a regular field troubleshooting technique.

6. Fix Garbled Serial Data

In case the serial monitor displays scrambled serial data or illegible characters, it might indicate that the connection is only half-functional. The most frequent reasons are incorrect baud rate, parity, data bits, stop bits, flow control or TTL voltage level.

Ensure that both parties utilize the same serial parameters, including 9600 8N1, 19200 8E1, or 115200 8N1. Incorrect baud rate may produce random characters. In case of incorrect parity or stop bits the device can either ignore the commands or output abnormal data.

With most simple serial applications, it is recommended that the flow control be set to None unless otherwise specified by the device manual as RTS/CTS or XON/XOFF.

7. Troubleshoot Modbus RTU Timeout

A timeout of a Modbus RTU indicates that the master has sent a request but it has not received any valid response within the time frame allowed. It is not always indicated by a faulty USB to RS485 Cable.

Verify the values of the Modbus slave identification number, transmission rate, parity, function code, register address, time-out and retry configuration. In case of a wrong slave ID, the device will not respond. In case of a mismatch between the baud rate or parity, the master can demonstrate timeouts or CRC errors.

Register addressing is also a popular issue. Certain programs use 0-based addressing whereas other software uses 1-based addressing. If a register is indicated as 40001 in the manual, it can be keyed in differently based on the Modbus tool.

In the first test of the system, only one RS485 slave device should be connected using a short cable and starting with a low bit rate, i.e. 9600 or 19200.

8. Troubleshoot Modbus RTU CRC Error and RS485 Problems

The error of a Modbus RTU CRC usually indicates that data has been received but was corrupted in the course of transmission. It may also indicate RS485 signal quality, cable routing, grounding, shielding, termination or electrical noise.

RS485 networks are usually configured in a daisy-chain bus topology. Poor quality cable routing, long star wiring, or excessive branches can result in reflection of the signals and unstable communication. In order to provide stable operation at longer distances or higher speeds, RS485 networks may require that 120-ohm termination resistors are installed at both ends of the bus.

The solution to this problem is probably associated with signal quality or interference, as reducing the baud rate decreases the CRC errors. Shielded twisted pair cables should be used and the RS485 lines must be separated from motor cables and VFDs outputs, and grounding checked properly.

9. When the Cable Works on Desk but Fails in Factory

There are times when a USB to Serial Cable is functional on a desk but does not work in an industrial cabinet. Here, the issue can be due to EMI, ground loops, VFDs, motors, relays, switching power supplies, or poor grounding.

Use a shorter wire, slower baud rate, shielded twisted pair wire, proper grounding, and good wire routing. An isolated USB to RS485 Adapter can also be used to enhance performance in a noisy environment or when there is a ground potential difference.

10. Run a Loopback Test Before Replacing the Cable

Prior to installing the new USB to Serial Cable, perform a loopback test. To test RS232 or TTL, connect TX with RX, open a serial terminal, and verify if the transmitted characters are returned back. In case the loopback test is successful, it means that the basic transmit and receive operations of the adapter are functioning normally.

When testing with RS485 use two USB to RS485 Adapters or one known-good RS485 device. The cable should be short, have one device, proper A/B wiring and basic Modbus RTU commands.

Conclusion

The method of troubleshooting a reliable USB to Serial Cable should always be done in the communication chain layers by layers: USB detection, driver installation, COM port state, type of interface, wiring, serial settings, Modbus RTU configuration, and industrial field environment.

When it comes to RS232, consider TX/RX wiring, GND, pinout as well as handshake signals. In case of RS485, test A/B polarity, Modbus RTU setup, bus wiring, termination, grounding and noise. With TTL UART, verify the logic level (3.3V or 5V) prior to connection.

The majority of the time when a USB to Serial Cable does not work there is a problem with one of the communication setup stages. Through the careful examination of each layer, you will be able to determine the actual cause more quickly and refrain from replacing the faulty part.

Author

Franck Yan
Founder | Farsince Connectivity Solutions

Franck Yan is the founder of Farsince and has more than 13 years of experience in the cable and connectivity industry, working closely with global customers on data center, industrial, and network connectivity solutions.