SerialExpress heritage

General principles

SerialExpress can be viewed as an extension of the processor/memory bus, extending outside of the motherboard chassis. Although point-to-point cabling is used, the protocols reflect a backplane heritage, with the following characteristics:

• Read/write transactions. Read and write transactions are used to transfer data between nodes.
• Standard CSRs. Standard control and status registers are used to initialize and configure nodes.
• Resets. After a bus reset, nodes appear at well defined local-bus addresses.
• Bridges. Addresses can be extended and read/write transactions are forwardable through bridges.

The assumption of a memory-mapped read/write interface significantly different from networks, like Ethernet, or channels (like Fibre Channel). The intent is to efficiently support small and large data transfers, eliminating the processor from the task of intervening in most transfers.

Many of the SerialExpress protocols represent refinements of other cable-friendly IEEE 'bus' Standards, including:

• IEEE Std 1596 Scalable Coherent Interface (SCI)
• IEEE Std 1394 High Performance Serial Bus.

Leveraged Serial Bus protocols

Properties of SerialExpress that were derived from Serial Bus, rather than SCI, include the following:

• Unaligned. Larger byte-aligned transfers are supported.
• Isochronous. Fixed-rate limited-latency isochronous data transfers are supported.
• Duplex. Duplex cables and adaptive electronics reduces cable-installation complexities. SerialExpress even includes loop breaking protocols, not supported by Serial Bus.
• Multispeed. Multiple speeds can be supported on different links.

Leveraged SCI properties

Properties of SerialExpress that were derived from SCI, rather than Serial Bus, include the following:

• Scalability. Arbitration and acknowledge timeouts adapt to a wide range of cable lengths.
• Concurrent transfers. Data can be concurrently sent over nonoverlapping segments.
• Small packets. Large transfers are fragmented into smaller 64-byte packets.
• Fast arbitration. Arbitration is immediate when no packets are passing through; latencies only affect arbitration recovery time after transmission conflicts.
• Ringlets. A logical bus is implemented as a small ringlet, around which packets are sent.
• Multicast isochronous. Flow-controlled multicasts simply distribute information to multiple nodes.

Distinct SerialExpress properties

Properties of SerialExpress that were derived from SCI, rather than Serial Bus, include the following:

• Reverse arbitration. Reverse flow arbitration efficiently resolves arbitration conflicts.
• Sequential ordering. Command codes allow selective enforcement of ordering constraints.
• Local fault retry. Ringlet local hardware fault retry protocols are simple yet robust.
• Dual ringlets. Counter-rotating ringlets double the throughput in closed daisy-chain topologies.
• Instant restart. Nodes use context info to instantly and safely restart after bus reset conditions.
• Speed matching buffers. SerialExpress uses speed matching buffers, rather than having links dynamically adapt to multiple speed transmissions.
• Precise time. Wall-clock synchronization protocols compensate for wiring delays, assuming identical out-and-back delays, yielding improved clock-synchronization accuracies.

Send mail to dbg@scizzl.com with questions or comments about this web site.
Last modified: May 4, 1997

Send mail to dbg@scizzl.com with questions or comments about this web site.
Last modified: May 4, 1997