Modes of Communication

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

• 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 complexity

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 a message to the master, it waits until polled by the master.

Advantages:

• System reduces unnecessary transfer of data as in polled systems
• Quick detection of urgent status information
• Allows slave-to-slave communication

• Master may only detect a link failure after a period of time, that is, when system is polled
• Operator action is needed to have the latest values
  
  Collision of data may occur and may cause delay in the communication