Maxiflex P3 PAC User Manual
55
-© Omniflex
UMM126XR18.pdf
8. The Subscription Service Explained
8.1 Introduction to Subscriptions
Central to many applications involving communications across networks is the need to
replicate data between nodes on the network.
Examples include a SCADA system acquiring data from remote telemetry units in the field; or
a point-to-point telemetry application, where inputs are transmitted from one location to
outputs at another location.
In all these cases, the traditional method is for a controlling master node to poll the slave
nodes regularly for data in case something has changed. This crude method is an inefficient
use of the limited network bandwidth, and is inherently slow in typical and worst case update
times. It also limits the number of master nodes in the system to one.
The Maxiflex P3 CPU provides a far superior mechanism to accomplish this commonly used
function through its Subscription Service. This Service operates as follows:
The node requiring the data sets up a subscription with the source node, very much like you
would subscribe to a magazine through your newsagent. You establish a magazine
subscription by telling the newsagent which magazine you want, your home address, and
how often you want it, and then the newsagent takes the responsibility on himself to send
you the magazine whenever a new issue becomes available.
Subscriber
New sagent
Request subscription
August issue
September issue
October issue
etc...
Figure 8.1 The Magazine Subscription Analogy.
In the same way, the P3 CPU’s Subscription Service allows the CPU (acting as a node on
the network) to subscribe to a range of DIT registers on a remote node.
The remote node will then send the data at an agreed time interval, plus, if required, when
the data changes state. The receiving node expects these regular updates, and will flag an
error if the subscribed data is interrupted for any reason.
8.2 Setting up subscriptions
Just like the magazine subscription, the receiving CPU is responsible for setting up and
maintaining subscriptions with other nodes to replicate data across the network. The
advantages of using subscriptions over regular polling mechanisms are as follows:
1. Only one message is required on the network for a data update as opposed to two in a
Request/Reply polling method. This reduces network overhead allowing more data
throughput on the network.
