Friday, August 27, 2010

USB to RS232 Converters - An Introduction

USB to RS232 Converters - An Introduction
By Daniel Lorenz

Many people are familiar these days with the use of universal serial bus [USB] ports on their desktop, laptop and notebook computers. The widely used USB 2.0 specification supports a 480 Mbps transfer rate and is popular because computer users can easily connect a wide range of consumer electronics devices and computer peripherals to their computers without having to configure 'dip switches' or install dedicated interface boards.
Although the 'plug'n'play' functionality of USB is its greatest benefit over older and cumbersome ways of connecting and configuring devices, there are still devices that do not support (or physically have installed) USB ports, but do possess RS232 serial ports.
Here is where USB to RS232 converters have a role to fulfill by connecting computer and RS232 peripheral together.
However, there's more to successfully connecting, for example, an RS232-enabled GPS phone to a computer's USB port, than just manufacturing an appropriate cable. Sophisticated converter electronics plus operating system dependent device driver software are, in addition to the cable itself, what make up a reliable USB to RS-232 converter.
The reason for this complexity lies partly in how software engineers and application programmers have interpreted and coded for the RS232 specification over the years.
The complete details are beyond the scope of this article but in essence the fact that the RS232 specification defines only the interface, and not the protocol to be used, has given implementers a lot of freedom.
Bottom line: application or operating system device driver ways of communicating with RS232 ports introduced a 'flexibility' which does not appear in the more inflexible USB interface specification.
This means that communicating with RS232 via a USB port introduces the requirement for RS232-style UART device driver emulations. So we now have two device drivers involved over a USB Bus with the potential for timing conflicts that can interrupt and delay RS232 communications.
On the hardwire side, there are known issues with various RS232 line driver chips that can result in RS232 input port behavior outside of the original specification.
Another problem that can show up in cheap USB to RS232 adapters is an inability to perform the hardware flow control required by some (but no all) applications.
Of course, identifying which applications use software flow control and which use hardware flow control is often beyond the knowledge of the converter manufacturer. And the application itself may make irregular and unclear use of hardware flow control.
The lesson here is to pay careful attention to how the USB to Serial converter handles application timing, RS232 signaling and hardware flow control. This is particularly important in industrial automation scenarios, although don't be surprised to also experience irregular issues with poorly designed converters in office computing locations.
In contrast, the leading USB to RS232 converter manufacturers use well regarded processor chips in their top end products, such as those from FTDI Semiconductors. They also provide simple to install device drivers supporting popular operating systems such as Windows (including Windows 7 and Vista), Linux and Mac.
Visit to find out more about RS232 to RS485 converter products, free Tools and Diagnostic software.
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1 comment:

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