15
Technical Information and Terminology Regarding Ethernet
It is important to keep in mind that unlike the serial connection, which is used by a program on the computer, Ethernet
is between the computer itself and the CP4/5. The computer will send packets to the CP4/5 during the initial DHCP
handshake and send packets regularly to make sure the CP4/5 is still there.
All of this is handled by the computer's Ethernet card before you ever run a telescope control program like APCC, the
AP V2 Driver, or any of the other myriad programs that can send commands to the CP4/5 on an Ethernet connection.
Information is being sent the very moment you plug the Ethernet cable into the CP4/5, and is still being sent until the very
moment you unplug the cable or turn the power off.
Compared to the Serial and USB connections, Ethernet and Wi-Fi are ALWAYS in communication. You can unplug a serial
cable from the CP4/5 and the computer will never notice; unplugging Ethernet from the computer is instantly noticed.
Ethernet connection will work with any device that supports Ethernet. This means Mac, Linux, Windows, and even another
CP4/5 could be connected over Ethernet. As stated earlier, Ethernet connection is between the computer and the CP4/5,
NOT between the software and the CP4/5. APCC and the V2 driver can access the Ethernet connection, but they are not
the only software that can. The most basic way to connect over Ethernet is to open the CP4/5 home page. In this case,
the browser program is interacting with the Ethernet connection, sending data to and from the CP4/5.
Over Ethernet, the CP4/5 can not only communicate using the AP Command Protocol, but it can act as a client and send
files to and from the computer. This is how you can connect the CP4/5 to a computer and pull up the CP4/5 home page:
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The computer sends a query for the index file at the IP address typed into the browser.
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The CP4/5 gets the query and sends the HTML file for the home page.
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The browser window displays the HTML file.
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Clicking buttons on the page will send a command in the AP Command Protocol.
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When the CP4/5 gets those commands, it performs the respective routine.
Computers can send large files over Ethernet very quickly, like the CP4/5 firmware updates and license files. Other
software like APCC, the AP V2 Driver, and others behave just like the browser; they tell the computer to send a packet, and
the computer replies with what comes back.
With this all in mind, here are some important terms you need to know to make the most out of your CP4/5 Ethernet
connection.
Term
Definition
Client
A client is a computer connected to a network that requests information from other systems. Clients
initiate connections to servers and await requests. Most computers behave as a client, and will try
to request information from the CP4/5.
DHCP
DHCP (Dynamic Host Configuration Protocol) is the protocol used when the CP4/5 is connected to
a live Ethernet cable. It establishes who talks first, and settles what IP address clients and servers
will use while connected to the network. Before any further communication can occur, these
important topics need to be settled.
Hostname
A label or name that is assigned to a device connected to a computer network and that is used to
identify the device.
IP Address
This is the address by which your computer and CP4/5 talk to each other. Both the CP4/5 and
computer will have an IP (internet Protocol) address, and any packets sent by one will be sent to
the IP Address of the other. It is important to note that devices connected with multiple Ethernet
and Wi-Fi will have an equal number of IP addresses to differentiate the different communication
ports.
LAN
LAN (Local Area Network) is the name given to a network not connected to the internet, or a
network connected to but behaving independently from the internet.
NetBIOS
A protocol passed down from old Windows point-to-point connections. This protocol wraps a
hostname around each TCP packet, so that computers can refer to each other by a human
readable name (hostname), among other reasons. Other operating systems will require
modification to read the NetBIOS protocol. NetBIOS is not routable, so if your CP4/5 is connected
to a router the hostname will no longer show up on your computer.
Network
A network is a connected set of clients, sometimes with routers and servers. A network can be
anything from a computer wired directly to a CP4/5, to a CP4/5 connected to a router that can
access other routers and computers, and possibly the entirety of the internet.
