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RS485 is a differential bus that gives the capacity of long distances. Many control devices use the hardware interface. RS485 is just a hardware specification and there are many protocols that are used on it. This device is used to convert USB to RS485. With the virtual com port the PC can run the protocol on the RS485 hardware.
The converter is based on a Microchip PIC24FJ64GB002. This is a small chip but has a USB port and serial port. There even left over pins if you want other functions. The firmware configures the USB and serial ports. The baud rate of the device is set at 19200 as this was the baud rate of the device I was interfacing to. I have built a couple of these devices. I gave one to the customer I was working with so they could test the interface.
The following schematics are of the PIC24FJ64GB002.
USB to RS485 schematic