Electrical systems generally exist at different potentials from each other unless they are electrically connected. When two electrical systems come into contact, a current flow through that contact which equalizes the potential of the two systems. If the first contact is ground to ground, the equalization current flows through the ground system, which is generally very robust, and there is seldom any damage. But if the current flows through a signal net, damage can occur as the current is forced into the net.
Thus the preferred connection sequence between an Intel® FPGA download cable and a circuit board or host computer is to force the grounds to connect first. If the Intel® FPGA download cable is already connected to either the host or the board, and the host and board share a common earth ground – such as through a power strip ground pins – then there is no problem. Note that many laptops are not tied to earth ground, even when plugged into their power adaptor.
The USB connector used the Intel® FPGA download cable to the host is well designed to force the grounds to connect first, but the standard JTAG connector lacks this feature so that any signal could connect first. The following connection sequence is recommended:
- Touch the shroud of the USB connector to circut its ground on the board. This equalizes the potential between the board and the Intel® FPGA download cable.
- Holding the USB connector on board ground, plug the JTAG connector into the board. This connects the board ground to the Intel® FPGA download cable ground.
- Release the USB connector from the board ground, and plug the USB connector into the host USB connector. Since the USB connector forces the ground to connect first, the potential difference between the board-cable system and the host system will be discharged through the USB connector ground.