P&E Microcomputer Systems Cyclone LC Series, Руководство пользователя

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User Manual For Cyclone LC Programmers 111
10 ETHERNET CONFIGURATION
This section describes the mechanism used by the Cyclone device to transact data over an
Ethernet network. It primarily focuses on the User Datagram Protocol (UDP), which is a popular
method for sending data over a network when the speed of a data transaction is of more concern
than the guarantee of its delivery. The Cyclone takes advantage of the UDP protocol’s penchant
for speed, and adds an extra layer of logic to guarantee the delivery of UDP packets in order to
offer a best-of-both-worlds solution.
10.1 Network Architectures
Before delving into the innards of Ethernet message passing, it is prudent to briefly describe the
different network architectures in use today, and how they pertain to the operation of the Cyclone.
Computers are, of course, connected to one another through intermediary devices in order to form
networks. There are several classes of these intermediary devices, but they generally fall into one
of the following three groups:
Hubs
At the most basic level, computers are connected to one another through a Hub. A Hub is
a device with several ports that are used to connect multiple computers together. It is a
repeater device – a Hub simply copies the data incoming on one port as data outgoing on
the other ports. In this manner, if there are four computers connected through a Hub, and if
the first computer is sending data to the second computer, then the third and the fourth
computers will also receive an identical copy of that data. Hubs are usually used to set up
a small Local Area Network (LAN), which may have on the order of 10 to 20 computers.
Switches
The aforementioned type of process, where the data is simply replicated onto every
available port, quickly becomes inefficient for larger sized networks. For this reason, a
larger sized LAN employs the usage of Switches instead of Hubs. A Switch is essentially a
smart Hub, in that it limits the input and output of data to the two transacting computers.
Routers
Larger networks, such as Wide Area Networks (WANs), or the Internet for that matter, use
progressively more sophisticated devices to transact data. At the core of these devices is
the Router, which functions as a switch between networks.
The Cyclone performs irrespective of the connection mechanism, with one very important caveat:
it needs to be set up with the appropriate network parameters for the underlying network
architecture.
10.2 Network Parameters
A typical network becomes operational not after the physical connections have been established,
but after network parameters in the form of IP (Internet Protocol) numbers have been assigned to
the individual computers. An IP number is a unique string that consists of four numbers ranging
between 0 and 255, separated by dots, e.g., 192.168.1.2. Every computer that is on a network
needs to have a unique IP number. The computer uses this IP number to identify itself on the
network, and also to address the recipient of its data.
Assignation of this IP number is sufficient information to transact data on a simple network
connected by a hub. On a more complex network, however, routing information becomes
important. The routing information consists of two more IP numbers. The first of these is called the
Subnet Mask, and is used to determine whether or not the destination address resides on the
same subnet (i.e., doesn’t need to be forwarded to another network). The other IP number is the
Gateway Address, which is the address of the computer that handles forwarding and receiving of
packets to and from other networks.
Before first use, the Cyclone needs to be programmed with a unique IP number, the Subnet Mask
IP number, and also the default Gateway’s IP number. This can be done via the USB or the Serial
port, and is described in greater detail in the “Configuring the Cyclone” section of this manual.