PLX Technology PEX 8505, Hardware Reference Manual

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PEX 8505RDK Hardware Reference Manual v1.1
6 © 2007, PLX Technology, Inc. All rights reserved.
2.1 PEX 8505 PCI Express Switch
The PEX 8505 is a 5-lane, 5-port (maximum) PCI Express switch, which can be configured using strap
pins or EEPROM. The RDK’s onboard PEX 8505 is set up as a 5-port switch, with one x1 upstream port
(Port 0) and four x1 downstream ports (Ports 1 - 4). Ports 0 - 3 connects to the card edge (P1), while Port
4 connects to x16-sized slot connectors (J1). The PEX 8509RDK plugs into an RDK Port expander which
breaks out four lanes connected to P1 into a PCI Expressx1 cable connector and three downstream slots.
2.2 PCI Express Card Edge P1
The RDK form factor is based on the PCI Express CEM 1.1 specification. The board can directly plug into
the RDK Port Expander, which in turn provides a break out of the lower four lanes. All four lanes from the
The default configuration of the PEX 8505RDK sets Port 0 to be the upstream port.
2.3 PCI Express Connector J1
Connector J1 is a straddle-mount (SMT), x16 PCI Express connector. Cards plugging into this slot will be
in-line with the RDK. Port 1 connects one lane (x1) to the lower four lanes of connector J1. The upper
fifteen lanes are unconnected. Power is provided to connector J1 from the ATX hard disk connector J5.
2.4 Reference Clock Circuitry
Two Cypress CY28400-2 1-to-4 PCI Express clock buffers (U10 and U11) provide on-board REFCLK
distribution to the PEX 8505 and the downstream slot (J1). The first CY28400-2 (U11) input is sourced by
the card edge P1. U11 provides two differential CML (current mode logic) clock outputs. One pair drives
the PEX 8505’s PEX_REFCLKP/N input. This pair is AC-coupled (C74 and C75). This is required to block
the DC information of the transmitter from the PEX 8505 clock receiver, which has on-chip biasing and
termination circuitry. The other pair drives the input of a second CY28400-2 (U10), which is used drive the
downstream slot.
Figure 2-2. PEX 8505RDK Reference Clock Circuit
PEX 8505
PEX_REFCLK
CY28400-2
CY28400-2
RefClk
Card-Edge
RefClk