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9 Tips for Selecting the Right USB-to-Serial Converter

4/29/2022


Why USB to Serial Conversion?

Serial communications are still used in many industrial devices as well as in legacy capital equipment in factories and enterprises. Older devices were designed to connect through serial ports, but of course won’t work with modern IoT systems. As the demand for using big data to optimize manufacturing processes and reduce energy consumption grows, decision makers are seeking solutions to allow data from the OT domain to be available to the IT domain. USB-to-serial converter allows serial equipped legacy devices such as PLCs, to communicate with a PC via a USB port for data visualization and management.

How Does USB to Serial Conversion Work?

USB to serial conversion allows a new laptop to communicate with serial devices via a USB port. The USB-to-serial converter acts much like a serial port expansion card, except it uses the USB interface instead of a PCI or PCI express slot. When the converter is plugged into the USB port, the driver sets up a serial COM port in Device Manager that can be accessed by your software applications. Legacy peripherals and equipment remain fully operational, but can now connect via USB.

USB to Serial Conversion Options

USB-to-serial converters come in a wide variety of shapes and sizes, so it's important to make sure you're selecting the right one for your application. Just because a connector seems to have the correct pin configuration doesn't guarantee that the converter can make the necessary conversions, or meet the required baud rates.

For example, a USB-to-serial converter connecting to a 9-pin DB9 port will typically have the capacity to connect to RS-232 devices. However, supported operating systems, baud rates, cable types, buffer rates, duplexing capabilities and a number of other factors can vary greatly. The same type of variables hold true for USB to RS-422 and RS-485 converters.

Because of these differences, it is important to understand the exact specifications needed for the conversion process.

Selecting the Right USB-to-Serial Converters

Here are some essential elements you should consider before purchasing USB-to-serial converters:

  1. What is the serial data type of the device you are connecting to? 
  2. How many serial ports do you want supported from one USB port? 
  3. Are you operating in an electrically noisy environment?
  4. What other environmental or safety aspects are you looking for?
  5.  What physical serial connector best suits the application? 
  6. Is there a particular mounting format or physical size that fits the application best? 
  7. Are you considering a high retention USB connector for vibration applications? 
  8. Do you have all the products and accessories needed to make the connections?
  9.  Does the converter need to be shared between multiple computers?

(1)  What is the serial data type of the device you are connecting to?

Is the serial protocol RS-232, RS-422, RS-485, or do you need a device that can handle them all? You should also consider what serial data rates are required for your applications, 460.8Kbps or 921.6Kbps? If you are using RS-485, make sure it’s full-duplex or half-duplex. The answer to these questions will greatly affect the price of the converter you select. Advantech offers RS-232, RS-422/RS-485 (2 wire or 4 wire connections), and TTL (3.3V or 5V).

(2)  How many serial ports do you want supported from one USB port?

Needs vary depending on application and purpose. Do you need to consider this before purchase? Advantech offers 1, 2, 4 and 8 port solutions.

(3)  Are you operating in an electrically noisy environment?

If so, isolation is almost always required to prevent damage to your PC. Consider serial isolation related products to help protect against surges, spikes, and grounding issues. Some Advantech models offer 2kV or 3kV isolation on each serial port.

More about USB to Serial isolation protection:

When running long serial data lines, there is a chance that a difference in ground potential could damage your equipment. There is also a chance that the long cable will pick up unwanted signals, so including isolation in the USB to Serial Converter helps alleviate these situations. When using USB isolation, you need to understand that the maximum USB speed that is supported is 12 Mbps. High Speed (485 Mbps) and Super Speed (4.8 Gbps) are too fast for the isolation chips that are used. Luckily, this does not impact USB to Serial Converter applications since serial speeds are much lower than 12 Mbps, most application work fine with 9.6 Kbps.

Advantech provides port-to-port isolation:

   


Most isolated USB to Serial Converters isolate the upstream device from the downstream device. This is fine when you are working with a single port unit. However, with multi-port devices, you need the additional protection offered by port to port isolation. Simply put, port to port isolation isolates the upstream device from the downstream device, as well as downstream devices from each other. This is the only way you can be sure that ground loop or surge cannot be transferred through Port 1 to a device connected to Port 2.

(4)  What other environmental or safety aspects are you looking for?

Is the device meant for field service, or will it be left alone in a harsh environment? Do you need a device that supports UL Class1/Division 2 minimum ratings? Are shock, vibration, or drop testing specifications needed for the application?

(5)  What physical serial connector best suits the application?

Are you dealing with a terminal block or DB9/DB25? Male or female? Most prefer a DB9 for an RS-232 connection and a terminal block for RS-422/485 connections.

(6)  Is there a particular mounting format or physical size that best fits the application?

Inline and compact designs are common for field service applications. DIN-rail mountable products are suitable for in cabinet designs. Panel mountable are often used on desktops.


(7)  Are you considering a design with a high retention USB connector for high vibration applications?


The world is fast adopting USB as a cost-effective tool. But, without a strong retention mechanism for the connector, it can be deemed inadequate for some industrial applications. Manufacturers have responded with proprietary connectors and cables featuring a custom thumbscrew. While this attempts to address the issue, it adds cost and size. Worse yet, it requires the purchase of a custom USB cable that you will not find at a local store when you are stuck on a job site. Advantech designs high-retention USB port connectors into most USB products. The high retention ports are 50% stronger than conventional ones, so they hold tight to standard USB cable connections. The interface meets Class1/Division 2 minimum withdrawal requirements of 15N and it retains a positive connection so data communications will not be disrupted.

(8)  Do you have all the products and accessories needed to make the connections you need?

Do you have all the necessary accessories such as cables, power supplies, and cabinets? Also, chipsets and drivers are important so make sure you select a stable platform. If you want to overcome the problem of long distance, you can add a serial repeater which extends communication distances up to 1.2 km or chose a serial-to-fiber converter which is available in both single-mode (up to 15km) or multi-mode (up to 4km) configurations, which not only provides distance but also prevents ground loops and adds noise immunity. Fiber converters support point-to-point plus multi-drop, allowing one serial device to communicate with up to 31 other devices in a fiber optic ring.

(9)  Does the converter need to be shared between multiple computers?

In this case you should use one that locks in the COM port assignment. Advantech’s "locked serial number" model such as ULI-341TCK provides this functionality and is great for field service technicians.

Advantech locked serial numbers explained:

      


We configure our single-port USB to serial converters in two ways. In standard format, each product has a unique serial number. “Locked serial” format uses the same serial number that is associated with a model type. If your converter will always be used with the same computer, the standard serialized model is all you need. If the converter is shared among several computers, like field service laptops, the locked serial number model lets you plug and play without having to worry about matching the two.

      


Connecting your Legacy Serial Assets with the IoT World

Bridging the gap between serial and USB is easy when you've got the right converter. Simply attach the converter to a USB port on the computer and connect your peripheral to the other end. You'll have to install drivers the first time, but after that you'll be able to hot swap elements and components as necessary.

USB-to-Serial Communication Resource 


Selection Guide

Brochure

IoT Academy

For more information about Advantech's USB-to-serial solution offerings, please contact us at IIoT.marketing@advantech.com.tw

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