USB to serial (TTL/RS232) converter comparison

There has recently been quite some fuss about the FTDI disabling their driver to work with counterfeit devices. This is completely legal step, however made many people unhappy when their cheaply purchased devices suddenly stopped working (there is a workaround to get them working again with the old driver). FTDI has a rich selection of USB to different serial protocols converters. The most simple one is FT232R which is a single serial port converter. The bigger FT2232 and FT4232 chips feature multiple serial ports and may also be used for more complicated protocols like JTAG. These chips (or RS232 USB converters) are really a winner if you want multiple serial ports on USB as they take only one USB port 🙂

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FTDI chips are themost expensive ones, but also the most feature-rich. FT232R chip has inbulit 3.3V regulator that can supply the external circuitry 50mA of current. It has full serial port implementation with all the control pins and also a few additional GPIO pins that can be software controlled. The serial and GPIO pins can work from 1.8V to 5.25V by using external voltage source, they can work at 5V with USB power supply or at 3.3V from internal regulator. There are also some advanced functions that may be programmed into chip, like inverting specific pin output signals. Also driver strings and PID/VID can be customised if the customer wants to use custom / rebranded drivers.

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The most general FTDI drivers are available on producers website and also on windows update so they will be autoinstalled. Android/Win/Mac drivers are available. The drivers will pick up VID 0403 and PID 6001.

I think the FTDI chips also have the fastest response time of the control pins. This is not important in normal serial communication but can be useful if you use the control pins as general purpose inputs / outputs or to emulate other communication protocols.

A nice feature of the driver is that the same chip / product will keep its COM port number if plugged into different USB ports.

FTDI chips are mainly used in products where price is not the most important criteria for choosing a chip. (In cheap devices many counterfeit chips were used.) You can find FTDI in high end USB-RS232 cables, better GPS devices, OBD cables, some arduinos use them and also BusPirate (red circuit in picture) uses it.

 

Prolific also had problems with product clones. Their early drivers gave occasional BSOD (later changed to yellow excalmation mark error message) if using a counterfeit device which made them look bad, as the customers of course blamed the company for bad drivers (most of people didn’t even know there were counterfeit devices on the market). When the product clones became indistinguishable from originals they even dumped the old line of their product, blocking the entire family of chips – even the legitimate ones – from working in the newest driver. If you own a product with the old chip there is a workaround for this also. Their most used chip is PL2303HX. (Image shows both sides of the RS232 converter, on one side is PL2303HXA with crystal, on the other side is a sipex level converter – max3232 equivalent.)

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If designing a new product be sure to use the PL2303HXD chip and not the old deprecated HXA version (they are identical however the latter will not work with the driver). The chip requires external crystal oscillator for operation. It also has inbulit 3.3V regulator however the datasheet does not state if it can be used to power external circuitry. The serial pins can be used with 3-5V, usually supplied from internal regulator or 5V USB power. VID and PID can be customised.

The drivers are again available for all platforms and they will pick up VID 067B, PID 2303. If you desire some exotic baudrates the driver might not work as it is and needs to be adapted (note that this may not be the case with recent drivers).

Prolific chips were quite popular before the clone issues (and while the clones still worked with the driver). They could be found in many USB-RS232 cables, GPS devices, development boards…

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One comment on “USB to serial (TTL/RS232) converter comparison
  1. Rob S says:

    Thanks for the very good lesson on these devices. I have had mixed results with a few of then and always wondered why.

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