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20-3SYSTNX2.DOC Page 1 Latest update: 27. Februar 2003
2. NX2/NEXUS NETWORK - BRIEF INTRODUCTION
For full information see Appendix A
For practical understanding see § 6 Server
The NX2/NEXUS Network is a high performance, non collision Multi talker Multi
receiver databus especially designed to be used for marine and navigational
applications.
The most important features are the high update rate, fast response times, very
low data latency (25 ms) and very high data security even at very long distances.
Another important feature is that Data transfer efficiency will not degrade even
when used in large and complex systems. It utilises the RS485 standard with up
to 32 senders and/or receivers to form a Local Area Network.
Data is transmitted asynchronously with 1 start-bit, 8-data-bits, 1 parity-bit, two
stop-bits in 9600 baud.
Start Stop
0 d7 d6 d5 d4 d3
d2d1
d0 P 1 1
2.1 PROTOCOL
All data is exchanged in PACKETS. There are two main types of PACKETS:
Data-Packets and New-Sender-Packets .
The
Data-Packet consists of a ”header byte” followed by 1, up to max. 24 bytes of
data, ended with a checksum byte. The Data packets ”header byte” is identified by
having the
parity bit = 1 and bit d7 = 0 . The remaining 7 bits are identifying the
type of data that will follow (0-127). The bytes that follows the ”header byte” is the
usable data. There is no length-field included, but the receiver will know by the pre-
defined field length from the packet-ID (bit d6-d0 in the ”header byte”). The last byte
is the checksum, equals the XOR-summation of packet-ID and all data bytes.
The
New-Sender-Packet consists of just one byte and has no checksum. When
this packet have been sent, the receiving unit switches over to transmit mode,
and the ”old” sender switches over to receiver mode. The sending rights have
been ”handed-over” to the
New-Sender .
New-Sender-Packet format
:
d7 d6 d5 d4 d3 d2 d1 d0 P
1 Sender ID 1
N.B. the parity bit is not used for error detection, just to identify the ”header byte”
and ”next sender ID” from the data.