network
The o network is a high speed dual ring interface inspired by the proposed IEEE2100 standard (a hybrid between Firewire and SCI).Each node is connected to two neighbors with links that can have up to 8 bits in each direction. Since each signal is transmitted with a differential electrical connection, this means that up to 32 wires are needed. If only one bit in each direction is used, then 4 wires are needed. The possible bit sizes are 1, 2, 4 and 8 and don't have to be the same in both directions.
The connection speed can be set from 31.25MHz up to 500MHz in multiples of 2. That corresponds to rates from 25Mbps to 400Mbps per wire pair, so that the total bandwidth of a link can vary from 50Mbps to 6.4Gbps. At initialization, the link starts out supposing the worst case. Then further bits are tested and the link is made wider if possible. Then higher clocks are tried until the error rates become high enough to be detected.
Note that since the Spartan III chips have built-in LVDS (low voltage differential signalling), the only costs of the network are the connectors and cables. It might seem foolish to expose an expensive FPGA to damage by not having some cheap interface chip but in practice it is simply too much trouble to replace surface mounted circuits making the cost of the repair roughly the same in either case.
A set of nodes can be simply daisy chained and a single ring will form automatically. If the loop is closed, however, then two rings will form increasing the performance and allowing the network to handle the failure of one node. An alternative is to connect nodes (or groups of nodes) using a hub which will form a ring of rings. This is more expensive but will result in the same advantages that people are used to with twisted pair Ethernet.
It is important to point out that this is not a LAN but rather a communication system for a parallel machine, so response times for messages are orders of magnitude smaller than for LANs of similar bandwidth.
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- mechanical specifications last edited on 24 March 2003 at 7:06:06 pm by gandalf.merlintec.com
- main board last edited on 11 October 2004 at 9:11:11 pm by gandalf.merlintec.com
- disk last edited on 25 November 2003 at 5:25:16 pm by gandalf.merlintec.com