Global Data

Introduction

Global Data, which is supported by the BMX NOE 01x0 modules, provides an automatic data exchange for the coordination of PLC applications.

NOTE: Global Data is not supported by the Ethernet port of BMX P34 20x0 CPUs.

Global Data

Key features of Global Data:

Data exchanges are based on a standard producer/consumer protocol, giving optimal performance while maintaining a minimum network load.
The Global Data service provides real-time exchanges between stations in the same distribution group, which share variables used for PLC coordination.
Global Data can synchronize remote applications or share a common database among several distributed applications.
A maximum of 64 stations can participate in Global Data within the same distribution group.

Operation

An application variable is a group of contiguous words from a PLC. Communication module stations can publish or subscribe to application variables:

publish: Communication modules can publish one 1024-byte local application variable for other communication modules in the time-based distribution group. The publication period can be configured from 10 ms to 15000 ms (in 10 ms increments).
subscribe: A communication module can subscribe to between 1 and 64 application variables that are published by other modules in their distribution group, regardless of their location. The validity for each variable is controlled by health status bits, linked to a refresh timeout configurable between 50 ms and 15 s (15000 ms). Access to a variable element is not possible. The maximum size of the subscribed variables is 4 Kbytes.

Through Global Data configuration you can define:

the number of valid published and subscribed variables
the group with which these variables are associated for the communications module

Once the module is configured, exchanges between the communication modules sharing the same Distribution Group are automatically carried out when the PLC is in RUN mode.

A distribution group is a group of communication modules identified by the same IP address. Multicast exchanges distribute Global Data. Several independent distribution groups can co-exist on the same subnetwork with different multicast addresses.

Important points:

A publish/subscribe protocol on UDP/IP is used for data distribution.
The content of the publication variable is synchronized during the out section of the MAST task.
The content of the subscribed variables are copied to the PLC application memory during the in section of the MAST task.
The PLC memory zones that receive the various subscription variables should not overlap. Do not use these variables for any other purpose.

Health Bits

A health bit (status bit) is associated with each application variable.

This bit indicates the validity of each subscription variable. It is 1 if the variable has been published and the subscriber has received it in the configured validity time. Otherwise, it is 0.

Multicast Filtering

Global Data uses . In accordance with the nature of broadcast messages, the multicast frames transmitted by a module are repeated on all switch ports, generating network congestion.

When the switches possess this function, allows multicast frame spreading to be limited to only the ports that require it.

The protocol establishes the list of ports involved in data traffic.

Operating Modes

The operating modes are as follows:

Stopping the PLC stops Global Data exchanges.
The use of the I/O forcing system bits (%S9, %SW8, %SW9) does not stop Global Data exchanges.

Limits

There are no theoretical limits for the number of stations that share a distribution group. There is a limit on the number of variables (64) that can be exchanged within the distribution group. For more information, refer to the table describing the parameters for the BMX NOE 01x0 modules later in this user manual.

NOTE: It is recommend to use fewer than 200 modules in any single distribution group.