Specifications

PCIe/104 and PCI/104-Express Specification Revision 3.0 February 17, 2015 Page 16
2.4.3.1 DIR Line on Host
The state of the DIR line is always determined by the Host so that the Devices can be designed without regards to the
design of other Devices. On the top side connector this line must be tied to ground on the Host. On the bottom side
connector the DIR line must be tied to +5 volt power during all implemented power saving modes except power
completely off. A Host that supports suspend modes may want to use +5V_Aux diode Ored with +5V in case the power
supply does not provide +5V_Aux. Shown in Figure 2-1 is the required PCB connection for the DIR line on the Host.
Link 0
Link 2 Link 1 Link 3
DIR
PCI Express Host Chipset
DIR
GND
+5V
Host CPU Module
Link 0
Link 2 Link 1 Link 3
Figure 2-1 Required Circuitry for a Host Module Configuration for Automatic Link Shifting
2.4.3.2 DIR Line on Device
On the Device the DIR line is an input to the resident auto-switching multiplexers. A resistor divider is required on the
DIR line on each Device to adjust the SELECT line voltage for the switches. Each Device must sink or source less than
300uA of current. Therefore, the divider resistors R1 and R2 shown in Figure 2-2 must total 15K or greater.
Select
PCI Express
Device
DIR Link 0
Link 2 Link 1 Link 3
Add-On Module
DIR
Link 0
Link 2 Link 1 Link 3
R1
R2
PCI Express
Bi-direct ional Switches
Figure 2-2 Required Device Circuitry for Automatic Link Shifting
If the Device is positioned above the Host, then the DIR signal would be grounded and the SELECT line of the
multiplexers would allow Link 0 to connect to the PCI device. Links 1, 2, and 3 are then allowed to shift and pass over so
that Link 1 is in the Link 0 position, Link 2 is in the Link 1 position, and Link 3 is in the Link 2 position. A left-most
Link is now available for the next Device card to be stacked above the first Device.