Roger Miller, of the Samtec Optical Group, walks us through a PCI Express® over optical FireFly™ demonstration at SC17 in Denver. In this application, instead of inserting a video card directly into the host system PCI Express slot, the Firefly optical interconnect allows the motherboard PCI Express bus to be “extended” to a remote location.
In this demo, a server, or “Host” system, plays a 4K video. Plugged into one of the PCI Express expansion slots is a Samtec PCI Express compliant add-on card that contains two FireFly PCUO optical modules, which implements a x8 PCI Express Gen 3 (8 Gbps) link. The add-on card is the PCOA series. It is also possible to use four PCUO modules to implement a x16 PCI Express link. By the way, there is no video card in the host system.
Second is an endpoint system with another PCOA card, which also contains two Firefly optical PCUO modules. A video card is plugged into the backplane, which is the actual PCIe endpoint device. The video card output is connected to a 4K monitor.
The optical interconnect between the host and target PCOA cards is a spool of 100 meters of multi-channel fiber optic patch cable. A patch cable with MPO connectors plugs directly into the bulkhead optical adapters on the PCOA optical cards.
PCOA can be used as either a simple transparent bridge to extend the PCI Express bus (as shown in this demo), or as a non-transparent bridge to enable multi-host systems and other application-specific functions.
Challenges of PCI Express Signaling Over Fiber
PCI Express is ubiquitous, and is a popular solution for many types of system architectures, including video transfer, data acquisition, surveillance, computing, storage systems, and many others. This is usually achieved through copper cabling. But, as link distances increase, there reaches a point where optical interconnect becomes the only viable solution.
But, transmission of native PCI Express signaling over optical interconnect does have its challenges. These challenges are mainly related to the unique characteristics of the PCI Express protocol and its physical layer signaling, which can be incompatible with optical transmission technologies. This includes electrical idle periods, receiver detection sequencing, fast transmitter squelching, and low speed out-of-band signaling.
Samtec has addressed these issues by incorporating hardware and firmware solutions in this version of the Firefly optical engine, named PCUO. PCUO is designed to transmit and receive native PCI Express data over optics, at distances of 100+ meters, and includes support for PCIe sideband signaling.
Firefly is not constrained to panel mounting, and allows the designer to place the optical module closer to their FPGA, ASIC or Switch device, anywhere on the PCB design. This results in better signal integrity characteristics, which in turn reduces the channel equalization requirements, and the system power consumption. PCUO uses the same electrical PCB connector system as all optical Firefly modules.