USB to Serial Cable for PLC Programming: What Engineers Need to Know

The latest models of laptops do not come with a COM port. To an engineer required to program a legacy PLC on a factory floor, that gap is a daily fact of life and the only feasible link is a USB to serial cable.

The guide will help you know which USB to serial cables are compatible with which PLCs, reasons why drivers are prone to failure more frequently compared to hardware, and how to test them out before purchasing as it will ensure that the work is completed on the initial visit.

Why Legacy PLCs Still Need a USB to Serial Cable

Step into most manufacturing plants constructed prior to 2015 and you will come across Siemens S7-200 controllers that continue to control production lines. Packaging machines are controlled by the Mitsubishi FX2N units. Conveyor systems controlled by Omron C200H PLCs remain in operation due to their inability to fail over a period of fifteen years, and therefore, no one wishes to replace them.

The PLC replacement cycle in industrial settings is deliberately slow. The controller will not be exchanged just because there is a newer model available. It will be replaced in the event of failure, inability to obtain spare parts, or if a full line redesign is already being done due to other reasons.

These legacy PLCs have either RS232 or RS485 programming ports. The only method to connect a modern laptop into such an installed base is to use a USB to serial cable, a cable that will transform a USB port into a virtual COM port that programming software can understand. The first step to getting that connection correct is to determine precisely what interface each individual PLC uses.

Matching the Right USB to Serial Cable to Your PLC

Here is where most purchasing errors occur. Electrical standard and physical shape of the connector are different issues and they are not necessarily consistent with what panel label suggests.

Siemens S7 Series

The S7-200 is the prevalent origin of cable selection mistakes in practice. The programming port is a female DB9 connector, which has the same shape as a conventional RS232 port on older computers. Engineers who are accustomed to using RS232 devices think that they must have a USB to RS232 cable. The S7-200 programming port is RS485. A USB to RS232 cable will not cause any damage to the PLC, but it will also not communicate.

To program S7-200 and S7-200 SMART, you will require a USB to RS485 cable. The corresponding software is STEP 7 Micro/WIN in case of the original S7-200 and STEP 7 Micro/WIN SMART in case of the SMART series.

S7-300 and S7-400 are using MPI or PROFIBUS ports that are operating on RS485 physical layer but with Siemens-specific MPI protocol. This is not going to be solved by a generic USB to RS485 cable, as these models need a specific USB-MPI adapter. Communication in S7-1200 and S7-1500 is done via Ethernet and does not require any serial cable at all.

Mitsubishi FX and Q Series

The Mitsubishi FX legacy models are the ones with which people tend to be most careless when choosing a USB to serial cable to use in PLC programming. The FX1S, FX1N and FX2N series use RS422 via a Mini DIN 8-pin connector. RS422 is electrically incompatible with RS232 as well as RS485. These models cannot be communicated with using either a USB to RS232 or USB to RS485 cable no matter what configuration is used.

FX1S, FX1N and FX2N need either the original Mitsubishi SC-09 cable or a direct replacement USB to RS422 cable having proper Mini DIN 8-pin pinout. GX Developer is the compatible software.

The FX3U, FX3G and FX3S moved to USB Mini-B to allow direct laptop connection and no longer had to use serial cable at all. Q series older models work with RS232 through DB9 male port using standard USB to RS232 cable and GX Works2.

Omron CP, CJ and Legacy Series

The current Omron CP and CJ series controllers that are CP1E, CP1H, CP1L, CJ2M and CJ2H use the standard USB-B connection. These models do not need a USB to serial cable.

The USB to serial cable is also useful when working with older devices within the Omron series. The C200H and C200HE series have RS232 using a DB9 male connector and need a USB to RS232 cable. The CQM1 has a DIN 5-pin RS232 port and needs a USB to RS232 cable with the correct adapter. Once the COM port is set properly CX-Programmer does support both generations.

Other Common Platforms

The Delta DVP series controllers - DVP-SS2, DVP-SE and DVP-SV2 - are operated through a DB9 male RS232 connection. WPLSoft and ISPSoft will both work with a standard USB to RS232 cable as long as the COM port is set right.

The current Schneider Electric Modicon M221 and M241 both use USB Mini-B. The older TSX Micro and Nano series have RS232 so you need a USB to RS232 cable and the right programming adapter.

Driver Installation: Where Most USB to Serial Cable Problems Start

When you have the proper cable type, driver compatibility is where real-world failures occur. Its cause is in most cases either of three things: incorrect chip inside cable, incorrect COM port number that software does not recognize, or incorrect baud rate.

FTDI, CH340 and CP2102: Why the Chip Matters

Each USB to serial cable is made using one of three converter chips. The behavior of a driver and its performance with industrial programming software depend on the chip.

Windows 8 and above, most Linux distributions, and macOS have FTDI FT232RL and FT232H driver support preloaded and do not require manual installation. False FTDI devices are rarely incompatible with STEP 7 Micro/WIN, GX Works2 and CX-Programmer. When working in the field and the production line is idle, FTDI has the least risk.

CH340 and CH341 chips are popular in low-cost USB to serial cables and they mostly need manual driver installation on Windows 10 and 11. A few CH340 based cables have been reported to cause intermittent COM port recognition problems with STEP 7 Micro/WIN and previous versions of GX Developer. Suitable to use in an office but risky to use when there is limited time in the field.

The CP2102 and CP2104 of Silicon Labs is an in-between. Windows 10 and 11 tend to automatically install CP210x drivers. Relatively dependable when it comes to programming PLCs but not as widely available as FTDI with older software releases.

Setting the COM Port in Programming Software

Once the driver has been installed, go to the Device Manager and record the port number that the device is using before you open any of the programming software. It is located at: Device Manager > Ports (COM and LPT)> USB Serial Port (COMx).

In this step of Micro/WIN: Communications -> Set PG/PC interface -> PC/PPI cable (PPI) -> Properties -> select COM port number to match -> set baud rate to 9600.

Under GX Works2: Project -> Connection Destination -> Direct Coupled Setting -> Serial -> select matching COM port -> baud rate 9600.

In CX-Programmer: PLC Work Online Serial enter COM port, pick Toolbus or Host Link, which depends on the Omron model.

Should the software be unable to locate the PLC with proper port selection, try looking at the baud rate first. The programming port of both the S7-200 and the Mitsubishi FX have a default baud rate of 9600. Mismatch in baud rates is the second major cause of connection failures, the other being the incorrect cable type.

COM Port Number Conflicts

Both Windows 10 and 11 allocate COM port numbers depending on the USB port history. The same USB to serial cable can be found as COM3 on one machine but COM11 on another. Previous versions of STEP 7 and GX Developer are able to recognize only COM1-4. These programs do not see a cable assigned to COM7 or higher even with an installed driver.

The steps are as follows: Device Manager -> right-click USB Serial Port -> Properties -> Port Settings -> Advanced -> select the COM Port Number to be between COM1- COM4 -> OK.

In case the target number is greyed out, the other device has the assignment. Select View -> Hide Hidden Devices, delete the inactive COM port entries and re-allocate.

Three-step diagnosis for "COM port not found" errors:

Cable appears under Unknown Devices → driver not installed correctly

COM number above COM4 → reassign in Advanced settings

PLC not responding after correct port → check baud rate, start at 9600

What to Look for When Buying

Industrial vs. Consumer Grade

USB to serial cables sold by a general electronics store can be used at a desk. Electrical noise created by motor drives, variable frequency drives and switching power supplies in a factory floor creates conditions that are not anticipated by consumer-grade cables.

The optical isolation is the main requirement. The optically isolated USB-to-serial cable interrupts the electrical connection of the laptop with the PLC. Data passes through the isolation boundary. Fault currents do not. With no isolation, a PLC-side ground fault can harm a laptop USB port - an experimentally observed failure mode during field service work, not a hypothetical one.

Optical isolation can be used optionally in a lab or office. It is the correct selection in any factory floor application or an environment with the presence of VFDs.

Cable Length and Build Quality

In the case of a typical USB to serial cable use - connecting a laptop to a PLC programming port at a panel door - 3 to 5 meters will cover pretty much every occasion. Beyond 5 meters, RS232 signal quality deteriorates more rapidly with distance in electrically noisy conditions. The field kit cable should be kept short.

Pre-Purchase Checklist

Before ordering a USB to serial cable for PLC programming, confirm the following:

• Electrical standard — RS232, RS485 or RS422, not just the connector shape
• Physical connector — DB9, DB25, Mini DIN 8-pin or terminal block
• Software COM port range — STEP 7 Classic and GX Developer require COM1–COM4
• Chip selection — FTDI for field service, CH340 acceptable for office use
• Optical isolation — required for factory floor and panel installations
• Cable length — 5 meters or under for portable field kit use

Conclusion

Choosing the proper USB to serial cable to use in PLC programming depends on several particular choices. The S7-200 requires RS485 as opposed to RS232. RS422 with a Mini DIN 8-pin connector is required by the FX2N. In the case where a production line is waiting, an FTDI chip will cause less driver problems than a CH340. In any actual industrial setting, an optically isolated cable is worth the price difference.

If we can get these right before our first site visit then we will remove one entire class of failures which has absolutely nothing to do with PLC programming but has everything to do with the cable in the bag.

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.