There is however an attractive alternative to placing a server in embedded field devices. That alternative is to use a field device that is a client (or master) that sends data automatically on a scheduled or on demand basis. There are many advantages to reversing the transfer direction in publicly accessible networks, making the field devices act as clients and having the receiving computer be the server. A client does not have to be polled; it simply sends you data. There is a sensibility in having a central server collect data from numerous distributed clients. As we will see, these advantages carry over to one-on-one data exchanges between a remote client and a standalone server (perhaps a laptop computer) as well.

Getting Through Firewalls

Some years back, a number of major chemical suppliers established a plan to supply their field personnel with laptops running web browsers. The idea was to access embedded webservers in the chemical delivery equipment installed in their customer’s facilities. This scheme hit the proverbial brick wall – the firewalls that protected their customer’s access points. They found that security measures established at these target facilities would not permit outsiders to access data within their protected networks. They quickly learned that the key to success was to reverse the flow of data and let the embedded equipment report as a client.

Typically, firewalls are designed to let their internal (inside the firewall) clients send traffic out through the portal. These firewalls recognize the IP address of the reply message as the one that was addressed by the outgoing message and let the reply pass through. When a browser internal to the protected network accesses a web server, it permits the selected web site to send pages to the browser. Similarly, clients embedded in field devices can send data safely out through a firewall and in return receive acknowledgements and even further requests for more data.

Putting aside the technical issues, it is far more acceptable to the managers that ultimately grant permission for data exchange to have devices transmit predefined and functionally limited data (as clients) than grant access to outsiders to penetrate and remove data from servers  (and potentially unintended other sources) within their facilities.

Consider also that in any system that is accessed from the outside, there is risk of intrusion.
A slave (server) must always be listening — ready to be polled by the master. It has been said that the only truly secure system is one that is not connected. Perpetually accessible systems invite intrusion. By contrast, a client-initiated system is not vulnerable to attack or unauthorized access because it does not accept unsolicited, externally originated messages. A client (communications master) can open its communications channel only when it has a message  to send and then only accept reply messages in its expected format from the server that it addressed.

“Report by Exception” can Cut Response Time
Client initiated data transfers provide another major benefit. Alarm conditions and service requests are reported immediately by the monitoring equipment that call out or “report by exception”. Report by exception eliminates the delays that are commonplace in polled systems. Report by exception has another important benefit in the reduction of message traffic.

Polled systems are constantly generating messages in the attempt to determine the status of remote stations. Client initiated messaging only generates traffic when there is something significant to report. Reduction of the number of messages is especially
economical in systems in which you pay for each call or pay for bandwidth.

Internet Access can be Expensive or Difficult

Readily Available Client Transfer Functionality

An Internet Client may be as simple as a master I/O driver that transmits register data on a scheduled or event driven basis. Here are some examples of free (or otherwise inexpensive) client-initiated data solutions that will deliver information over an Internet or Ethernet link.
These client-based solutions have no per-controller licensing fees.