Typical System Configurations There are two typical network configurations for the wireless telemetry radio-based SCADA systems. They are the point-to-point and the point-to-multipoint configurations.
1.Point-to-Point Configuration 2.Point-to-Multipoint Configuration1.Point-to-Point Configuration The Point-to-Point configuration is the simplest set-up for a telemetry system. Here data is exchanged between two stations. One station can be set up as the master and the other as the slave.
An example is a set-up of two RTUs: one for a reservoir or tank and the other for a water pump at a different location. Whenever the tank is nearly empty, the RTU at the tank will send an EMPTY command to the other RTU. Upon receiving this command, the RTU at the water pump will start pumping water to the tank.
When the tank is full, the tank’s RTU will send a FULL command to the pump’s RTU to stop the motor.Point-to-Point Configuration2.Point-to-Multipoint Configuration The Point-to-Multipoint configuration is where one device is designated as the master unit to several slave units. The master is usually the main host and is located at the control room. While the slaves are the remote units at the remote sites. Each slave is assigned a unique address or identification number.Point-to-Multipoint ConfigurationModes of Communication There are two modes of communication available, namely, the polled system and the interrupt system.
1.Polled System In the Polled or Master/Slave system, the master is in total control of communications. The master makes a regular polling of data (i.e., sends and receives data) to each slave in sequence.
The slave unit responds to the master only when it receivers a request. This is called the half-duplex method. Each slave unit will have its own unique address to allow correct identification. If a slave does not respond for a predetermined period of time, the master retries to poll it for a number of times before continuing to poll the next slave unit.
Advantages: • Process of data gathering is fairly simple • No collision can occur on the network • Link failure can easily be detected Disadvantages: • Interrupt type request from a slave requesting immediate action cannot be handled immediately • Waiting time increases with the number of slaves • All communication between slaves have to pass through the master with added complexity2.Interrupt System The interrupt system is also referred to as Report by Exception (RBE) configured system. Here the slave monitors its inputs. When it detects a significant change or when it exceeds a limit, the slave initiates communication to the master and transfers data. The system is designed with error detection and recovery process to cope with collisions. Before any unit transmits, it must first check if any other unit is transmitting.
This can be done by first detecting the carrier of the transmission medium. If another unit is transmitting, some form of random delay time is required before it tries again. Excessive collisions result to erratic system operation and possible system failure. To cope with this, if after several attempts, the slave still fails to transmit