The transmission of messages across the fieldbus is managed through a deterministic centralized bus scheduler called the Link Active Scheduler (LAS). The H1 card functions as the LAS. Some of the Link Active Scheduler's responsibilities are:
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 software supports one backup Link Master device on each fieldbus segment.
Scheduled Transfers
Scheduled transfers are typically used for the regular, cyclical, exchange of control loop data between devices on the fieldbus segment. The LAS maintains a schedule called the Compel Data schedule, which is a list of transmit times for all the data buffers that need to be cyclically transmitted. The data buffers are in the fieldbus devices. When it is time for a fieldbus device to send a data buffer, the LAS issues a message called a Compel Data message to the device. When the fieldbus device receives the Compel Data message, it broadcasts or publishes the data in the buffer to all devices on the fieldbus and any device that is configured to receive the data receives it. The devices that are configured to receive the data are called subscribers. Although scheduled transfers are the highest priority activity performed by the LAS, it requires the smallest portion of a macrocycle. (A macrocycle is a single iteration of a schedule.)
A link, or Virtual Communication Relationship (VCR), is defined as a connection between a fieldbus parameter in one device on the segment and a fieldbus parameter in another device on the segment. Some devices support publisher and subscriber VCRs and other devices support Free VCRs.
Publisher and Subscriber VCRs
A subscriber VCR is an output from a fieldbus device to an input in another device on the segment. The input device can be another fieldbus device or a DeltaV controller. A publisher VCR is an output from a DeltaV controller to the input of a parameter in a fieldbus device. Here are a few examples of publisher and subscriber VCRs:
The link between a function block running in a controller to a function block running in a device is a publisher VCR.
The link between a function block running in a device to a function block running in a controller is a subscriber VCR.
The link between a function block running in a device to a function block running in another device is a subscriber VCR
The H1 card supports as many as 35 H1 publisher VCRs and 50 fieldbus device subscriber VCRs per port as long as the total number of VCRs does not exceed 50. For example, the card can support 35 H1 publisher VCRs and 15 fieldbus device subscriber VCRs per port or five H1 publisher VCRs and 45 fieldbus device subscriber VCRs.
Many backup Link Master devices cannot support these limits. This means that the configuration can exceed the capacity of the backup Link Master device. If communication is lost between the H1 card and the segment, the backup Link Master device may be unable to handle the segment scheduling and the schedule download may fail. If the backup Link Master device indicates Schedule Download Failure in DeltaV Diagnostics, the device cannot function as a backup Link Master. If an H1 card configuration has more than 25 publisher/subscriber VCRs per port, test the configuration before going online to ensure that the backup Link Master device can support the configuration. To avoid a Schedule Download Failure due to lack of capacity in the backup Link Master consider the following options:
Free VCRs
A Free VCR can function as a publisher link, a subscriber link, or a device alarm. Device alarms require one VCR. For example, if a device can support a maximum of five VCRs, and one VCR is used for a device alarm, the remaining four VCRs can be used for any combination of publisher and subscriber links. Each port on the H1 card can support a maximum of 50 input links and 35 output links as long as the total number does not exceed 50 VCRs.
Unscheduled transfers
Unscheduled transfers are typically used for user initiated changes such as setpoint changes, tuning changes, and downloads and uploads. Unscheduled transfers require the greatest portion of a macrocycle. The LAS gives all devices on the fieldbus a chance to send unscheduled messages between transmissions of scheduled messages. The LAS grants a device permission to use the fieldbus for unscheduled messages by issuing a Pass Token (PT) message to the device. When the device receives the PT, the LAS allows it to send unscheduled messages until it has finished or until the maximum token hold time has expired; whichever is the shorter time. The device can send unscheduled messages to a single destination or it can multicast the message to multiple destinations. The LAS maintains a list of the devices that are properly responding to the PT message. This list is called the Live List.
Live List maintenance
New devices can be added to the fieldbus at any time. Between the times it sends out Compel Data messages, the LAS sends out Probe Node (PN) messages to the addresses not in the Live List. If a new device is present at that address, it receives the PN and answers with a Probe Response (PR) message. When the LAS receives the PR message from the device, it adds the device to the Live List. Whenever a device is added or removed from the Live List, the LAS broadcasts the change to all devices. This allows each device to maintain a current copy of the Live List.