SCRAMNET NETWORK
Copyright 2007
2-4
VME6U HARDWARE REFERENCE
2.4 Network Ring
The SCRAMNet
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Network is a ring topology network. Data is transmitted at a rate of
150 Mbits/s over dual fiber-optic cables. The two lines together produce the incoming
data clock. Due to the network speed and message slot size, the network can
accommodate over 1,800,000 message slots passing by each node every second. There is
an approximate 247 ns (minimum) delay at each node as the message slot works its way
around the ring. The maximum delay depends on the selection of fixed or variable-length
message packets. A fixed-length message packet has a maximum delay of 800 ns, a 256-
byte variable-length message packet is 16.2 µs, and a 1024-byte variable-length message
packet is 62 µs. Delay can be imposed when a node must complete the transmission of a
native message packet before retransmitting a foreign message packet. A SCRAMNet
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Network can accommodate up to 256 nodes per network ring.
2.4.1 Protocol
The protocol is a register-insertion methodology and is NOT a token ring. Depending on
the protocol selected, all message packets are the same size or are variable (as in the
PLUS modes), and multiple nodes can transmit data simultaneously. There is no master
node, and all nodes have equal priority for network bandwidth. The message protocol is
designed specifically for real-time applications where data must be passed very rapidly.
When the node operates in BURST or BURST PLUS mode, the node will never re-
transmit its own messages for error correction. When operating in PLATINUM or
PLATINUM PLUS mode, error detection is enabled, and re-transmission can occur.
BURST MODE
BURST
mode is an open loop, non-error-corrected communication mode. This mode
allows multiple 82-bit messages (46-bit header plus 32-bite data and four parity bits) per
node on the ring at a time. The limited packet length enhances the data latency
characteristics of the network by providing the shortest possible media access delay. The
messages are transmitted as fast as the system will allow.
PLATINUM MODE
PLATINUM mode is BURST mode with error correction enabled. The messages are
transmitted as fast as the system will allow, but error checking is used to detect and re-
transmit corrupted packets.
PLUS MODES
The PLUS mode protocol enhancement can increase the maximum network throughput
from 6.5 MB/sec to approximately 15.2 to 16.7 MB/sec by the use of variable-length
message packets. Each SCRAMNet
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message packet has a 46-bit header plus the data.
The user-selectable maximum packet size increases the data size from the normal 32 bits
to either 256 or 1024 bytes of data. Data must be written to sequential longword
addresses.
2.5 Auxiliary Control RAM (ACR)
The Auxiliary Control RAM (ACR) provides a method of external triggering and
interrupt control by offering a choice of four actions to occur when a particular
SCRAMNet
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shared-memory address is written into. Each shared-memory location has
its own action or set of actions associated with it.
Содержание SCRAMNet+ SC150 VME6U
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