The "Client" Advantage
Is a SIXNET RTU a Client or a Server?
Inexpensive Ethernet Connections
The term "server" suggests a large or complex central store of data, but this is not necessarily the case. A server is an object (software or hardware with imbedded software) that is available to supply information upon request. On the Internet, many clients (users) can make requests from a server (website). There is however another scenario of great interest. By this same definition, a simple I/O module is a data server because it is a servant (slave) that waits for then responds to requests for I/O status from a master (otherwise known as a client because it makes requests of the servers). Accordingly, in a simple control system, the controller (PLC, DCS or RTU) is the master device and is the client. It polls the I/O modules (servers) for information. It is perhaps a bit strange to discover that the client can be the smarter device and that there can be many servers and as few as one client in a system.
It can be either or both. The choice is yours. A look at the alternatives may help you decide which is best for your system. A SIXNET RTU can simultaneously act as a master and as a slave using the advanced SIXNET "I/O for Windows" protocol. This secure communications method uses packets to insure the integrity of the data transmissions. Both command and reply messages can travel simultaneously over the same channel. The result is that the remote access port (the telephone modem, wireless modem or Ethernet port) is always listening for incoming messages and can report exceptions as they occur. This gives you a unique opportunity to be a client (master) or server (slave), or if you wish both as the same time!
If a SIXNET RTU is setup as a server, it will be necessary to assign a static (fixed) IP address to the device so that clients looking for information can find it on the Internet. Unfortunately, IP addresses are scarce commodities. Assuming that you can find availability, high costs can be encountered in the form of administrative time to be granted an IP address assignment from the local network administrator or in the high cost of commercial broadband service that includes static IP addresses (sometimes as high as US$400 per month).
The alternative is to connect the SIXNET RTU as a client (so that it is equivalent to a browser to the network). IP addresses can be borrowed using DHCP often with no cost for the installation (because the site already has a contract for many Internet connections of this type). This type of connection will also benefit from an Easy connection through the local firewall and the advantages of being able to Report on Exception.
Often, Remote Terminal Units in distributed systems are configured for periodic and usually infrequent connections to the central host site. This is particularly true in large systems using telephone and/or radio links where the considerations of polling time, communications bandwidth and sometimes cost are significant factors. If everything is running smoothly, this background reporting of status and uploading of historical data files is adequate for the application. The problem is that timely reports of alarm conditions are required. Waiting for hours or the end of the day is not acceptable. The solution is to give the RTU "Report on Exception" capabilities so that it can report alarms on an unsolicited basis. For the RTU to "call in" it must become the client (master) and the host must be the server (slave). This is exactly the action a SIXNET RTU will take. When an alarm occurs, the RTU can be configured to send an ASCII message describing the condition or transfer I/O register data. (Please note that a special I/O driver that can act as a slave is needed on the receiving end. Two good choices are the Control Room IOmap or a SIXNET gateway acting as a Master Terminal Unit.)