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Digi-Connect WAN modem for cellular connection of dataloggers
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Digi Connect WAN operation manual
The following are instructions to set up cellular communications between a computer and a remote serial port interface device, such as a CR1000 Datalogger. The Digi Connect WAN includes software called Real Port, which will create a virtual COM port on a local computer which is linked to the cellular modem located in the field. In this way, the software controlling the datalogger sees the datalogger as a local serial port device so long as the cellular modem is connected and powered on.
Overview of cellular communications:
Cellular communications allow for access to data anywhere cellular service is available, utilizing standard cellular data packages. To be able to successfully set up a communications link, however, a basic understanding of how cellular data plans work is required. Any device on the internet is located by utilizing an IP address. An IP address is a characteristic string of numbers unique to a device on the internet. However, there are a limited number of IP addresses available and they are costly to own, and because not everyone is utilizing their cellular device at the same time, a cellular provider does not own as many IP addresses as they have customers. To make this work, the data plan provider utilizes dynamic IP addresses, which means that when you try to access the internet, you will be assigned a temporary IP address for your device for the duration of time you are using it. When you are done with it, it is released and can be assigned to another user. In addition, most cellular packages utilize a set of private IP addresses, which means that you cannot â€œdial inâ€ they are only able to â€œdial outâ€ into the internet.
This system works well in device initiated situations, where the device can dial the central server, ask for an IP adress and then continue to access the internet with this address. However, in the case of data collection, the communications is initiated by the computer collecting the data, and it needs to know how to find the device, the address of which is changing constantly.
In order to acheive this, the modem has support for a DNS hostname. A DNS hostname allows the device to dial the cellular provider, receive an IP address, and then register it under a constant hostname. For the snowstudy, http://snowstudy.dyndns.biz/ was used, it was registered at a free site at www.dyndns.com. Now, by pointing the browser to the DNS hostname of the device, it automatically retreives the current Dynamic IP address and can find the modem.
The second problem is to bypass the private IP address. In order to do this, a more expensive VPN (Virtual Private Networking) plan must be purchsed. This provides a dynamic public IP address.
Attaching the Modem to the Datalogger
The Digi Connect WAN modem can work seamlessly with the Cambell Scientific Datalogger utilizing the RealPort feature on the modem. This feature allows the serial port on the modem to act like a local serial port (COM 1-5 usually) on any computer which has the software installed. The software will dial the DNS hostname of the modem, and establish a virtual link with its serial port. The datalogger then operates as if it is attached to the local machine.
-Purchase a SIM card for a standard data plan from any wireless retailer that supports GSM and VPN data packages, and install it into the unit.
-Go to www.dyndns.com and register a unique DNS hostname
-Go to www.digi.com and download and install the latest version of RealPort
-Attach the DigiConnect WAN modem directly to the computer using the ethernet port
-Direct the browser to 22.214.171.124, this will bring up the configuration menu
-Follow Config->Network->DynDNS update settings
-enter the DNS name, Username and password
-set DynDNS DDNS to Dynamic DNS
-set "use wildcards"
The modem should now be adressed to the DNS hostname. Disconnect the modem from the ethernet port on the computer, and direct the browser to the DNS hostname, the configuration screen should appear.
Now set up the realport configuration
Take note of the current IP address and the cellular number of the modem, for reference when dealing with the cell phone company.
If there is a communications issue, there is a method to check the operation of the serial port to narrow down the problem to the datalogger or the remote connection.
-Download and install TeraTerm Pro. This is a terminal emulator which lets you communicate directly with serial devices
-Open TeraTerm Pro, and direct it to the appropriate COM port. If you are unsure of which COM port the device is on, you can look it up in device manager under serial devices
-Unplug the RS-232 cable from the modem, and physically short pins 2 and 3 using a flat head screwdriver. This shorts the send and receive terminals, which will return any message sent to the port.
-Type a few characters into the TeraTerm Pro console, you should see them repeated back on the screen in a few seconds
-If this doesn't work, there is likely an issue with the realport or DNS setup, if this does work, it means that communications to the remote modem are functioning properly, and the issue is with the configuration of the components attached to the modem.