DIY USB to RS232 adapter

Sometimes you need to connect some device to a COM port, but the computer doesn’t have one, especially a laptop. There are two ways – go and buy a USB to COM adapter or build one. So if you decided to build a USB to COM(RS232) adapter, there is one solution.

USB to RS232 adapter

This adapter is straightforward to build as there is a single-chip USB to UART bridge (CP2102). This chip from Silicon Labs supports USB2.0 full speed. Internal Resistors are required for the USB interface, integrated clock, internal 1024 -byte EEPROM for Vendor ID, Product ID serial number, power descriptor, and other information. SP2102 USART part support almost all standard features of RS232 communication including handshaking, Databits (5, 6, 7, and 8); 1 or 2 stop bits; odd, even, mark, space, and no parities; baud rate from 300bps to 1Mbits.

The device is powered by the USB port and requires about 10mA. Chip drivers enable connecting this adapter to most Operation systems like Win98, Win2000, WinXP, and Linux.

USB to RS232 adapter circuit

As you can see, the signal level from TTL to RS232 is converted via the MAX3243 chip from Texas Instruments. This chip converts all available signals: DSR, DTR, RTS, CTS, RI, DCD, RXD, and TXD.

It is better to make a double-sided board this allows to minimize the size of the adapter.

USB to RS232 adapter PCB

To make the adapter work, for all download drivers for your operation system (Quick drivers for WIN2000_WIN2003_XP; Linux; Win98SE; OS_MAC). Then extract it somewhere. Now connect the adapter to the USB port and wait until the system asks for the driver. Brose to extracted driver location and let the finish installation. When the device is ready, the LED indicator should light ON.



  1. and about te components?

  2. plz somebody upload the component list …

  3. Please advise where I can buy the chip and how much?

  4. Hi, can u post a parts list please!!!

  5. Why don’t you just buy a ready made lead from eBay for about £1.60??

  6. Hi, can I modify usb to parallel converter as usb to serial 232 converter?
    What should I do??

  7. I want to connect a Roland DXY-1300 plotter to my Windows 7 laptop.
    But I can’t find a proper USB – RS232 interface with the correct output voltages. This DIY USB to RS232 adapter is the one I need! Where can I buy one or how can I place an order for such a device? Kind regards, Dick van Fulpen, Houten (NL).

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