Connecting two Nexys 2 boards with USB

[sunny441]

We are currently using Nexys 2 boards in our class and we're trying to transfer data between two boards using USB. In the reference manual of Nexys 2 it says there are public APIs to access USB connection but I failed to find it anywhere. Are there any drivers that we can use to serve this purpose?

[zygot]

@sunny441

I think that perhaps you need to research USB protocol and infrastructure a bit more. SPI and I2C are directional in that there is a master and a slave controller RS-232 is directional in that there are DTE and DCE terminals. USB is a bit more complicated with upstream and downstream devices. Implementing a USB HUB and a USB Host Controller require different hardware. You can easily connect two Nexys2 boards using LVCMOS UART TxD and RxD compatible signals. Connecting two FPGA baords using a USB port... that's a tough or impossible proposition. An alternate plan might be to use the Ethernet PHY ( I haven't checked to verify that your boards have one....). A far better approach would be to design your own interface and use a PMOD on each board. Best wishes.

[jpeyron]

Hi @sunny441,

The reference manual here on page 7 under USB port is referring to the Adept SDK which allows users to make custom software here to interact with a Nexys 2 through the USB. This software would not facilitate two Nexys 2 boards to communicate. You could use the serial port as zygot suggested. Here is the resource page for the Nexys 2 which has reference designs as well as reference components. The RS232 interface reference component looks like it would be helpful for your application depending on how fast you are needing to communicate between the two boards. You can also use the Hirose FX-2 high density 100 pin connector which is discussed on page 16 of the Reference Manual.

cheers,

 

Jon

[zygot]

@jpeyron,

Thanks for the additional information. I usually take the time to view a board's schematic before making comments. Being in a hurry never seems to work out... The RS-232 connector uses normal RS-232 signalling levels and not FPGA IO compatible levels. You'll need a NULL MODEM adaptor to connect two ends of an RS-232 cable when both ends are DTE or DCE terminals. If you are using any hardware control signals beware having both ends drive them. Fortunately RS-232 interface devices are pretty robust as there is a good likelihood of issues. You will not be getting high data rates out of the RS-232 interface on your boards; 115200 or maybe 230400 baud. Since PC companies haven't spent the money on real RS-232 connectors for a while now you might have to do some research.

Just some more passing commentary. The FX-2 connector could be used as a reasonably high speed interface but the pins have no protection from shorts or contention and might be a fast way to ruin two boards. The PMODs at least have some protection to the FPGA from accidental abuse. I don't know of any cables for the FX-2 but you need to think about Vcc pins and dedicated signals that are mean't to have one driver before connecting two boards together with a cable. Unfortunately, your boards don't have an Ethernet PHY. RS-232 and Ethernet make cabling a snap ( well RS-232 can be a pain figuring out DTE, DCE and various gender and DB-9, DB25 connectors)