Homework - Work at Home
When engineers first launch into a cellular data pilot it can be a bit like Christmas with the excitement of new toys, future trends and being "on top of it". However, I encourage anyone interested in using cellular or satellite-based IP systems to do some home work first ... literally "work at home". You'll save lots of cash and avoid many headaches by learning the basics at home first.
Most of you have cable or DSL router/modem at home, so start there. Take a PLC or controller home. If it has an Ethernet port you are all set; however if your device is RS-232 based, then beg, steal, borrow, or purchase a simple Device Server such as the Digi One IAP (fancier, Modbus and Rockwell protocol aware) or the Digi One SP (much cheaper but just a raw Ethernet-to-serial converter). Your goal is to connect from your office computer over the Internet to this device at home ... if you cannot succeed at this, then you won't succeed at cellular access either! But unlike with cellular, all of your trial-and-error over your Cable/DSL Route won't be costing you by the byte.
Just remember that your "Home Cable/DSL Terms and Service Agreement" likely forbids running "servers" so don't go and try to setup an e-commerce shop once you see how easy it is to access your home from the Internet.
Get to Know Your Cable/DSL Router Box
Hopefully you all have an external commercial router box that you either got from your ISP or bought at any big-box store for $39 to $59. If your computer connects directly into your modem or you were fooled into using Microsoft's "Internet Sharing" tool on one computer, save your sanity and go buy a cheap router box! For your $39-59 you get a 4-port switch, a professional stateful-firewall and NAT (more about that later), a wireless access point, and it all consumes maybe 8-10 watts of power so costs you a few $ a year to run. If for no other reason, you just don't want the mindless broadcasts and hacker probes taking a percentage of your home computer's bandwidth. For my VPN testing I have some Linux boxes up exposed like this and they see up to 50 broadcasts per second and a few dozen probes for open Windows and Unix services per hour. There is NO REASON to expose your home PC to this rubbish - use an external router box ... period.
Step 1: Learn how to log onto your Cable/DSL Router.
- Under Windows 2000 or newer, open a command window and type the command "ipconfig". You should be shown your computer's current IP Address and the Default Gateway, which is another name for your Cable/DSL router. Most likely the router has an IP such as 192.168.0.1 or 192.168.1.1.
- Confirm you can ping your Cable/DSL router with this IP
- Open your web browser and browse to the address - as example type the URL "http://192.168.1.1". You should be asked for a user name and password.
- Check with your router documentation or go on line to the vendor and read the user guide. For example, at home I have an ActionTec router/wireless access point supplied by Qwest, and when it first came it has no user name and a password of "admin". This is actually not so insecure since by default you can ONLY access this web page from inside your firewall/router. But common sense says changing this name/password is wise.
- There is no way I can explain how all Cable/DSL routers work, but once you can log in you should be able to find a status web page which gives your currently assigned external IP address and 2 DNS addresses. This is how the world sees your home system - write this info down. For example, my home Cable/DSL router (as of today) has the temporary (dynamic) IP of 63.228.51.x.
So at this point, you know how to access your raw "face" exposed on the Internet. Now we want to give ourself a nice, memory-friendly DNS name to represent that face.
- As mentioned above, my Qwest IP is dynamic and liable to change at any time. So while I could go to the office and try to point my OPC server or PLC software at 63.228.51.x, I can never be sure how long this will work. In reality it only changes every few months or if I power-cycle my router, but the solution to this problem is very easy so we should solve instead of work-around it.
- Sign up with one of the many free online Dynamic DNS providers - I use dyndns.org. The Digi Connect WAN (cellular router) family directly supports this, as do many LinkSys and DLink-class home products. In a nutshell, they allow you to create a domain name such as sillyjoe.gotdns.org or sammy345.dyndns.org and then a client tool on your home system automatically updates this DNS name every time your ISP changes your dynamic IP address.
- While the above service is free, you may want to pay the $10 or so per year for a minimum account. This makes the service more tolerant of errors on your part - for example many services automatically delete your free account if it is untouched for 45 days and so on.
- If you have a Windows computer, the easiest DDNS update client is just to download the Windows tool recommended by your Dynamic DNS provider. This client automatically monitors the Cable/DSL router's IP as it accesses the internet. If your IP has changed, it correctly updates the DDNS (dynamic DNS servers). I stress the word "correctly" since many external Cable/DSL Router boxes which support DynDns and such services come with bugs which cause your free service to be deleted within hours of setup. So if you chose to use your Cable/DSL Router to maintain your DDNS name, make sure you have the latest firmware upgrade on it!
- Within an hour of setup, anyone in the world should be able to ping your new DDNS name and get a response.
We are almost ready to try access - but if you point your OPC server at your DDNS name ... nothing will happen since your Cable/DSL Router does NOT understand Modbus or other industrial protocols. Remember, the IP your DDNS name represents is the IP address of your Cable/DSL Router and NOT the IP of your home computer nor is it the IP address of your PLC/controller device.
- Log back into your Cable/DSL Router and locate the setup for port forwarding. Some routers call it setup for applications and games. We need to configure the router to FORWARD specific TCP and UDP ports to Ethernet-based devices you have at home. If you don't know what that means, you are in for a tough time using wide-area-network technologies - I suggest you go to any bookstore and buy a book on basic networking that covers what TCP, UDP, IP and NAT are. This is really key to success in this area. You don't need to be an expert, but you do need to understand the basics!
- In summary, if you think of the IP address as being synonymous with the main phone number in a building (aka - how to telephone the building), then the TCP and UDP port numbers are synonymous with phone extensions within that building (aka - how to reach a certain department or service). So for example, a Modbus/TCP OPC server will connect to your router using the IP (main phone number) attached to your DDNS name, and then request a connect to TCP port 502 (the service). We need to configure the router to accept and forward the Modbus/TCP traffic to your Modbus PLC. So take the example of a Modbus/TCP device on my local network with the IP 192.168.1.105. We need the router (at say IP 63.228.51.x) to accept any incoming connection on TCP port 502 and forward the packets to the local Ethernet device at IP 192.168.1.105 TCP port 502. Since the PLC has a web server and most ISP block access to home web servers, we'll tell the router to forward TCP port 8080 to local port 80. Depending on the brand of router you have the configuration can get fancier than that - but basically we'll end up with a line in the table looking something like:
|Incoming port||Service||Local IP||Local Port|
Step 4: Get to Work Learning
That's it - at this point you should be able to use a Modbus/TCP OPC server to poll your PLC indirectly by polling your DDNS name on the standard TCP port 502. Pointing your browser to http://your DDNS name:8080 will pull up your PLC's web pages (the ":8080" tells the web server to use TCP port 8080 instead of default 80).
Of course there is no security offered here - anyone in the world can access your PLC so this is just for educational purposes. Here are some common port numbers to use:
- Modbus/TCP uses TCP port 502
- Digi Ethernet-to-Serial products use ports like 2101, 2102, etc to access a serial device by raw TCP or UDP sockets.
- Digi RealPort uses TCP port 771 (TCP 1027 for SSL/TLS secure connection).
- Rockwell AB PLC5E and SLC5/05 use TCP 2222 for the older legacy CSPv4. This is often called AB/Ethernet or AB/TCP by 3rd party vendors
- Rockwell ControlLogix and ODVA Ethernet/IP uses TCP port 44818, UDP port 44818, and UDP port 2222. But be warned Rockwell tools are very poorly designed for wide-area network use.
- Siemens S7 protocol uses TCP port 102
- GE SRTP uses TCP ports 18245 and 18246
- GE QuickPanels use TCP port 57176 for configuration