With the adoption of the USB interface on industrial and consumer equipment, there is an increasing requirement to add support for older legacy serial connections on modern PCs. A typical example of these unsupported interfaces on modern PCs is the RS-232 serial interface which has largely been dropped.

Simpler serial communication solutions often cost less than complex alternatives. The basic components of an RS-232 solution are a UART and driver and a simple handshaking protocol can be easily implemented in even the most resource-limited of microcontrollers.

The nature of RS-232 makes it a robust point-to-point communications solution over short distances in harsh environments. However in applications where the controller is a PC, supporting RS-232 may become an unwelcome issue, due to the lack of RS-232 support in modern PCs. Likewise, replacing an RS-232 connection with USB in an application requires significant design effort.

There is another option to supporting RS-232 peripherals that requires less design effort. The enabling technology is a range of ICs from FTDI Ltd that implement all of the complexity associated with embedded USB support, with the addition of a standard UART port.

Figure 1 – Legacy RS-232 modem connection to Modern PC.

These fifth generation devices from FTDI support USB 1.0 and USB 2.0 (low, full and high speed) and provide a range of asynchronous and synchronous serial interfaces including UART (RS-232/RS-422/RS-485) and FIFO.

Integrated at the PCB level the devices can be used to create cables that, to a PC, behave as a USB connection, while simultaneously appearing as an RS-232 interface to a peripheral. USB operates at a much higher baud rate than RS-232, in the region 6000:1, so the device must provide not only a compliant USB interface but must be able to buffer, analyse and reassemble the data in accordance with the RS-232 connection’s expectations. It achieves this through FTDI’s proprietary IP, which comprises of a USB Protocol Engine, a UART Controller, a Serial Interface Engine and a USB Transceiver.

Figure 2 – A block diagram of an example device, the FT232R

While the software being run on a legacy device doesn’t change, the application running on a PC needs to be aware that a USB connection is being used instead of an RS-232 connection, but only in terms of the which COM port to use. When a USB to RS-232 cable is connected to a PC (USB port), the PC operating system creates a virtual COM port. It is with this COM port that the application communicates with the USB interface. This requires no software changes to any application running on the PC. The application only needs to know the COM port which the cable is connected to in order to communicate with it.

The use of USB in embedded devices is still determined by how easily it can be supported by microcontrollers. This implies that implementing a USB interface on a microcontroller needs to be done in addition to the incumbent ports such as UART, SPI and I2C. This makes the microcontroller more complex and therefore expensive for integrated device manufacturers.

However, using FTDI’s USB interface devices, adding a USB port to an embedded device requires only a UART interface on the microcontroller to communicate with the USB IC. So, with the addition of just a few simple instructions any embedded device using a microcontroller can now support USB.

Contact Information

Future Technology Devices International Ltd.

7235 NW Evergreen Pkwy
Hillsboro, Oregon, 97124
USA

tele: 503-547-0988
fax: 503-547-0987
us.sales@ftdichip.com
www.ftdichip.com