Curtiss-Wright GT200 VME, Справочник по аппаратуре

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OPERATION
4.2.10 Latency
Latency is the delay that a frame accumulates when passing through nodes and cables.
Each node contributes approximately 0.45 µs to a frame’s latency, and each meter of
fiber-optic cable contributes 5 ns to a frame’s latency. A complete discussion of GT200
network latency is beyond the scope of this manual. For more information, see
Curtiss-Wright Controls Technote G-T-ST-SCTN0138-A-0-A1, or consult with our Tech
Support staff.
4.2.11 Message Ordering
Host writes to the network progress in-order; message priorities are not used. Once on the
network, messages are not reordered. However, new messages from other nodes may be
inserted between messages that are in progress.
4.2.12 Memory
In its simplest form, the GT200 Network system appears as general-purpose memory.
The use of this memory depends on the nature of the data stored and conventions and
limitations imposed by the specific host computer system and operating system. On most
processors, this means that the application program can use this memory in the same way
as any other data-storage area of memory. The memory cannot be used as instruction
space.
The major difference between GT200 memory and system memory is that any data
written into GT200 memory is automatically sent to the same GT200 memory location in
all nodes on the network. This is why it is also referred to as replicated shared memory.
When a host computer writes to the shared memory, the GT200 node host adapter
supplies the proper handshaking logic. The shared memory behaves somewhat like
resident or local memory.
4.2.13 FIFO Buffers
The GT200 card contains various FIFO buffers used for temporarily storing information
during normal send and receive operation of the node.
4.2.13.1 Retransmit FIFO
This buffer is used to receive foreign messages from the network, and send them on, or to
hold received foreign messages while inserting a native message from the host onto the
network.
Each node is responsible for receiving foreign messages, writing them to its copy of
shared memory, and re-transmitting the message to the next node.
4.2.13.2 Receive FIFO
The Receive FIFO is designed as a temporary holding place for incoming foreign
messages while the shared memory is busy servicing a host request. This FIFO is
designed so it can never be overrun.
Copyright 2005 4-4 GT200 VME Hardware Reference