EMERSONTest Platform for Automation System

called OSI (Open Systems Interconnection). An OSI model divides the functions of a 

protocol into several layers. Each layer uses only the functions of the next layer, and 

only exports functionality to the preceding layer. The main feature of the OSI-model is in 

the interface between layers which dictates the specifications on how one layer interacts 

with another. This enables that a layer written by one manufacturer can operate with a 

layer from another. The OSI model contains seven layers, but the fieldbus model is only 

based on three major layers (layers 1,2 and 7 in the OSI-model). [3]

AS-Interface (AS-i) 

AS-i is the most simple fieldbus. It is designed for connecting binary devices. 

The OSI-model consists of three layers; Transmission Control, Execution Control and 

Application Layer Interface. 

The Transmission Control includes the wiring of the field devices and other components 

in the process. This layer receives messages and encodes them to physical signals. 

The signals which are in NRZ code (non-return-to-zero code) are encoded using 

Manchester II coding and then implemented with APM coding. 

The principle of Manchester encoding is that every bit period has one transition in the 

middle of the period. A positive transition represents a logic one and a negative 

transition represents a logic zero. Transitions that are not in the middle of the bit period 

do not carry any useful information. These transitions only have the purpose to set the 

signal in the state where the mid-bit transition can take place. 

With APM, pulses are created; a positive pulse is created at a low to high edge and a 

negative pulse is created at a high to low edge. In figure 1.4 the coding principle for both 

manchester encoding and APM is shown. [4][5] 

Figure 1.4. Illustration of the principle for the Manchester encoding and APM encoding. The data is coded 

to Manchester code so that a transition occurs in the middle of every bit period. A positive transition 

represents a logic one and a negative transition represents a logic zero. Transitions that are not in the 

middle of the bit period do not carry any useful information. These transitions only have the purpose to set 

the signal in the state where the mid-bit transition can take place. 

The Execution Control is of Master/Slave characteristic. The AS-i card works as the 

network master. The master automatically controls all communication over the AS-i 

cable. The master interrogates all the available AS-i addresses and repeats the process. 

The Application Layer Interface makes it possible to download the DI and DO functions 

to the controller. This enables communication between the controller and the devices. [6] 

Foundation Fieldbus 

The Foundation Fieldbus card used in this project is H1. As compared to another 

foundation fieldbus card H2, the H1 network is a lower speed and lower cost network 

than H2. 

The communication for foundation fieldbus follows the OSI model with three layers; 

Physical Layer, Communication Stack and User Application. 

The Physical Layer includes the wiring of the field devices and other components in the 

process. This layer gets encoded messages from the next layers and converts them into 

physical signals. The signals are encoded using the Manchester Biphase-L technique. 

In manchester Biphase-L code a positive transition in the middle of a bit period 

represents a logic zero and a negative transition in the middle of a bit period represents 

a logic one, see figure 1.5. 

Figure 1.5. Encoding with Manchester Biphase-L technique. In manchester Biphase-L code a positive 

transition in the middle of a bit period represents a logic zero and a negative transition n the middle of a bit 

period represents a logic one [5] 

The Communication Stack is the layer that manages communication between a device 

and a host or the communication between two devices. 

In foundation fieldbus communication with H1, the H1 card works as a Link Active 

Scheduler (LAS). A LAS is a deterministic, centralized bus scheduler that maintains a list 

of transmission times for all the data buffers in all the devices that need to transmit in 

cyclical fashion. [7] Field devices may also have Link Master capabilities and would in 

the case that the H1 card fails, work as LAS. The H1 card is the only primary Link 

Master allowed on the fieldbus segment. No other Link Master is allowed on the

segment or unpredictable results can occur. DeltaV supports one backup Link Master 

device on each fieldbus segment.[8] 

The communication between LAS and publishers and subscribers can be scheduled or 

unscheduled. 

Scheduled communication-The LAS maintains a list of transmit times for all data buffers 

in all connected devices. When it is time for a device to transmit its data, the LAS sends 

a CD (Compel Data) to that device. The device publishes (sends) data to all devices on