Specifications

PCIe/104 and PCI/104-Express Specification Revision 3.0 February 17, 2015 Page 31
3.6. PCI Signaling Voltage (VI/O) Requirements
3.6.1 PCI Host Module
The PCI Host board will always determine the PCI signaling level on the bus by setting all VI/O pins to either +3.3V or
+5V. If VI/O is set to 3.3V, then the system will use +3.3V I/O signaling and, likewise, if VI/O is set to +5V, then the
system will use +5V I/O signaling. Some PCI host modules may only allow one of the options, while others may provide
a jumper to allow the user to select the signaling level. Once the signaling level is selected, the remaining boards in the
system must use that signaling level.
3.6.2 Add-In Modules
Add-in cards can be 3.3V, 5V, or universal.
3.6.2.1 3.3V Add-In Modules
3.3V add-in modules operate in environments where VI/O has been set to +3.3V by the host module. Using 5V add-in
modules on a 3.3V stack will result in the 3.3V modules being damaged.
3.6.2.2 5V Add-In Modules
5V add-in modules operate in environments where VI/O has been set to +5V by the host module. Using 3.3V add-in
modules on a 5V stack will result in the 3.3V modules being damaged.
3.6.2.3 Universal Add-In Modules
Universal add-in boards can be used on either 3V or 5V I/O signaling buses. Universal boards either use the VI/O signal
to determine its signaling level or are 3V signaling boards that have 5V-tolerant I/O. Many PCI interface chips have a
"VI/O" pin that is the power for the I/O buffers that can be directly connected to VI/O. Universal boards will work on
either 3V or 5V I/O signaling buses.