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USB Serial Port: How to Setup an RS232 serial link through a USB port.

USB Serial Port: How to setup and use an RS232 serial port when you only have USB interfaces available. There are two types of digital interface to consider...

I use a USB serial port for RS232 communications all the time on my projects RS232 since it is a very useful standard communication module which can be found built-into PICs and most other microcontrollers.

Computers used to come with RS232 and parallel port built-in as standard, but not any more as new computers have USB ports instead.

There are three useful features about RS232:

  1. It is found in internal hardware modules of microcontrollers.
  2. It has minimal connection requirements (minimum is three wires).
  3. It has robust communication over fairly long wires i.e. noise tolerant.
The way to get it working is to use a very cheap USBadaptor unit that you can easily buy on ebay:

There are two types that do a similar job

  1. A digital serial to USB adaptor
  2. An RS232 to USB adaptor

USB Serial Port: Digital Serial

The digital serial unit does away with all RS232 comms (and cables) and outputs only digital signals. This is the generates the same as the signals that you get at the microcontroller, after they are decoded at the MAX232 chip. Using this unit means that you can plug the signals directly to the microcontroller  RX TX pins (connect digital serial Rx to uC TX and digital serial Rx to uC Rx to communicate correctly) and you do not even need a MAX232. The disadvantage is that you do not get noise immunity over long distance which the the RS232 interface does provide, but this is not a problem when working directly beside the units as I do. I use the digital serial all the time.

Digital Serial to USB on ebay :

This is the one I use - if you move the connectors a lot you can buy new duponts to replace them - it also as a low power 5V and 3V output for very simple circuits and it installs without a disk! It also has a couple of LEDs on board shown when comms is active - sometimes useful.

PL2303HX USB To RS232 TTL Auto Converter Adapter Module For arduino W/ Cables

Search for above text on ebay. There are bound to be others available if you can not see that exact one.

This USB serial port unit looks like this:
Digital Serial Interface RS232

RS232 to USB on ebay

The RS232 unit provides all RS232 signals (between +-3V to +/-12V) and will plug into the MAX232 device. Since RS232 voltage levels are generated it will allow operation over long cables. For this method you will need the MAX232 chip (or equivalent) connected as shown in the circuits to decode/encode the RS232 signal levels for the PIC's digital i/o.

USB 2.0 TO RS232 SERIAL DB9 9 PIN ADAPTER CABLE GPS PDA Adapter Converter Cable

Search for above text on ebay. There are bound to be others available if you can not see that exact one..

This RS232 USB Serual port unit looks like this:

usb to rs232 serial interface

I have tested this on the on-site project "serial LCD" and it works as expected.

Either of the above USB serial interfaces will do the job.

Note: The RS232 version means more work in wiring up a level translator chip e.g. MAX232 and you won't see much benefit other than operation over longer distance, for normal use. The desktop advantage can be seen in that the uint will generate all signals (not just TX and RX) so you can observe the signals DTR and RING and these can be made to power a small device (with a few diodes) or provide hardware handshake operation.


To progress, next you need a terminal program running on your PC so that you can send data to, and receive data from the microcontroller. A suitable program is "Tera Term" which is a free.

All you do is plug your chosen USB to serial interface into the USB port and connect to the development board and then start The terminal program.

Then all you do is select the serial interface that has appeared as a result of plugging in the serial to USB interface and everything works the same as if you had a built in serial link. (Tera Term menu Setup-->Serial Port...) - click the "Port" drop down and select the new Serial com port (depends on which USB port you plug into - mine is COM11)

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