SPI-USB Adapter Board
A lot of times I need to talk with some custom equipment throught the PC. Most of the time I prefer to use SPI interface on a hardware side, because it is so simple and does not consume a lot of space on digital devices like FPGAs to implement. On the PC side, for some small interface control I like to use USB.
FTDI makes different kinds of USB-to-SPI/I2C/UART like adapter ASICs. They are easy to use when you do not want to waste space on your for example FPGA with a USB protocol implementation.
Here is the picture of the adapter board. With this board I can achieve up to 30 Mbps SPI-USB transfer speed.
For a PC-side interface I am using a C# Library from FTDI, examples can be downloaded here.
The schematics are shown below