# Fieldbus Definitions

An Item is a digital signal. It can for example be signals, as they are described in the controller reference section:

- A single bit of information, like Tightening OK.

- Several bits of information, like an Event code.

An Item can be several bits of information representing a counter value, an identification number or a tightening result.

An Item can be a string of characters.

Fieldbus items
Information | Description
--- | ---
Fieldbus Item | Boolean: The signal can be a simple boolean expression, occupying one single bit position. Examples are digital IO signals.
Integer: The data occupies one or more bits of data in the array.
Character or string: The signal can be a alphanumeric character string, occupying one or several byte positions.

Fieldbus definitions
Information | Description
--- | ---
Fieldbus Map | An array of data. The Size must be an even number of bytes. The fieldbus map is created in the Fieldbus window in the Fieldbus menu. Fieldbus items are placed in the fieldbus map to create a unique array for the master-subordinate communication. Items are placed into the array by defining the starting position and the length, expressed in number of bits.
Process Data Frame | An array of data. The size must be an even number of bytes. The Process Data Frame is created in the Fieldbus window in the Fieldbus menu. Fieldbus maps are placed into the Process Data Frame when they are assigned to a Virtual Station. The position of the map within the frame is defined by the Offset, which is calculated from the beginning of the frame.
Fieldbus Configuration | Fieldbus mapping configurations are done in the Fieldbus window in the Fieldbus menu. Configuration parameters consist of both common parameters and type specific parameters. A list of parameter entry boxes are displayed after the fieldbus type has been selected.

Filedbus item positions
Information | Description
--- | ---
Offset | A pointer variable. The signal can be a simple boolean expression, occupying one single bit position. Examples are digital IO signals.
Starting Byte | A pointer variable. Points to a byte within the fieldbus map that contains the LSB of an item.
Starting Bit | A pointer variable. Points to a bit in the previously defined byte within the fieldbus map that contains the LSB of an item.
Item Length | A size variable expressed in a number of bits.
LSB | Least Significant Bit or Byte.

Name | Description
--- | ---
Fieldbus Transmission | A datagram sent over the network. The transmitting node combines a Frame Header, containing address information, with a Process Data Frame that is sent to the network. The receiving node detects its address in the Frame Header and retrieves the Process Data Frame.
Frame Header | Network address information and other data to ensure an error free transmission over the network
Fieldbus Master | Typically a PLC that initiates the communication to a fieldbus subordinate. The master transmits a data frame and receives another frame from the subordinate.
Fieldbus Subordinate | A remote node responding to a transmission from a fieldbus master. When the subordinate detects a transmission, it receives a frame and responds by sending another frame in return to the sender.