Flowserve MX FOUNDATION Fieldbus Field Unit - Manual (Page 12)

MX/QX FOUNdaTION Fieldbus Field Unit FCd LMENIM2330-01 – 08/11
12
Figure 2.3 – MX/QX Interface Board
2.2.3 Network Host
The FOUNDATION fieldbus network is connected to the host device, which can be a distributed
control system (DCS), a PC, a PLC, or other microprocessor-based device. The host device can be
the Link Active Scheduler (LAS), which performs the duty of network traffic controller. Networks can
have multiple Link Masters, which have LAS capability, but only one of these can be the LAS at any
time. If the LAS device fails another LAS is automatically created; this redundancy ensures continuing
communication in the event of a failure. The MX/QX FF unit can be configured for Basic or LAS mode.
In BASIC, the MXa/QX will display COMM LOSS, while in LAS it will display HARDWARE FAILURE, if
power is removed from unit.
2.2.3.1 LAS Tasks
The main task of the LAS is to schedule Compel Data (CD) messages to all devices in turn. The CD
message is directed to one device so it can broadcast (publish) data to all other devices. The LAS
also updates the Live List, which contains all devices on the network with the capability to commu-
nicate, and schedules a Pass Token that allows devices to send unscheduled messages such as
setpoint changes and downloads. The LAS does the following:
• Recognizes and adds new devices to the link
• Removes non-responsive devices from the link
• Polls devices for process data at scheduled times
• Distributes a priority-driven token to devices between scheduled transmissions
• Controls the Publisher/Subscriber process using the CD message
• Controls Server/Client communication between pairs of devices
• Broadcasts a time signal to synchronize all devices on the link
CD Schedule
The CD scheduler contains a list of activities that are scheduled to occur on a cyclic basis. At the
scheduled time, the LAS sends a Compel Data (CD) message to a specific buffer in a fieldbus device.
The device immediately broadcasts or “publishes” a message to all devices on the fieldbus.