You can easily connect serial devices to the USB port using this USB RS485 adapter.  It supports various devices with RS-485 communication interfaces like Programmable Logic Controllers (PLCs).

Specification:

Modbus USB 2.0 RS485 Converter
Screw terminals are used for connection and are backward compatible with USB 1.1.
Baud rates supported: 75bps - 115200bps / up to 6 Mbps
There is no need for an external power supply because of the low power consumption.
Operating systems supported: Windows XP/Vista/Windows 7/Windows 8 # Mac # Linux # Windows CE 5.0
60 x 18 x 14mm Dimensions
Raspberry Pi compatible (no driver required)
Color: Black

Windows driver "CH341SER": needed you can download it by clicking here 

Package includes:

1 x RS485 Converter Bus Adapter Serial