Information Technology Reference
In-Depth Information
These can either be cyclic or an explicit user request. A cyclic message is when the system
configures object names. These exchanges are automatically sent without the user requesting
them. An explicit user request involves requesting variables and the related response.
20.3.1 Addressing
WorldFIP has two addressing modes:
•
Variable addressing - this is a global addressing scheme, where each variable in the
distributed system has an associated identifier, which uniquely identifies the variable.
Each identifier is a 16-bit integer value. A device requesting the variable does not need to
know the location of the variable, and uses broadcasting to all connected devices.
•
Message addressing - this is an addressing scheme which uses a 24-bit address for each
device on the segment. Each address identifies the network segment and the node address
on that segment.
20.3.2 Application layer-physical layer interfaces
The data link layer provides an interface between the application layer and the physical layer.
It consists of a number of produced and consumed buffers, which contain the latest values
updated by the user or by the network. These buffers are overwritten when the value is
updated, and are automatically created on the initial configuration of a station.
Transactions involve passing an ID_DAT frame, which is followed by an RP_DAT
frame. The format of these is illustrated in Figure 20.4. Frames begin with a control byte
which allows network devices to determine the frame type. It is used to identify variable
transfer requests, acknowledgement frames, and so on. Frames end with a frame check
sequence (FCS) which is used to provide error detection.
FTR
Control
Identifier
FCS
DTR
ID-DAT
2 bytes
1 byte
2 bytes
2 bytes
1 byte
FTR
Control
Data
FCS
DTR
RP-DAT
2 bytes
1 byte
n bytes
(< 128B)
2 bytes
1 byte
Application
layer
User data
INDEN
Data link
layer
Control
FCS
Physical
layer
FTR
FSD
DTR
Figure 20.4
WorldFIP frames
Question frames are ID_DAT types, and are ID_RQ (ID request) and ID_MSG (ID
message). These types are identified by the bits set in the control field. The responses are:
