Dual Serial USB Adapter


This is a small adapter board I have designed to provide two standard TTL serial interfaces.

It is based on a Microchip PIC18F24J50 microcontroller, equipped with a USB interface and two hardware serial interfaces, suitably programmed as CDC serial emulator.

Since the PIC microcontroller is a 3.3 volt device, a voltage divider was added on the RX and CTS input pins in order to be safely connected to devices operating at 5 volts.

The adapter can be powered directly from the computer’s USB port, or through one of the two serial ports. It is also possible to power one or both serial devices via the USB port, in this case it is necessary to make sure that the devices do not require more than about 350 mA overall.

It does not require dedicated drivers for the operating system, the two serial ports are seen as COM devices on Windows and /dev/ttyACM on Linux. The exact numbering depends on the operating system and the presence of other similar devices but the two serial numbers are always consecutive, for example COM7 and COM8, or /dev/ttyACM0 and /dev/ttyACM1.