Flowserve MX/QX Profibus DP/PA Field Unit - Manual (Page 15)

15
PB DPV1 / PA Field Unit Installation and Maintenance FCD LMENIM2336-03 – 12/12
flowserve.com
Figure 2.3 – MX/QX PB PA Field Unit
The following commands and feedback information are transmitted through this unit:
• OPEN, CLOSE, and STOP commands.
• ESD (Emergency Shutdown) commands.
• Go-to-position commands.
• Redundancy switch-over commands (Profibus DP Redundancy option).
• Position feedback.
• Actuator status, alarm, and diagnostic messages.
• User analog input feedback.
• Discrete input feedback.
• Discrete output relays.
2.2.3 Network Host Station
The PROFIBUS master is considered to be the network host station, which is typically a DCS, PC,
PLC or other microprocessor-based PROFIBUS-compliant device. In a mono-master network, the
network host device is the only active network node. This is common in a standard Master-Slave
PROFIBUS network. In a multi-master network, there are two or more active nodes. This is managed
in a token ring, where the token, a uniquely structured message, circulates continuously among the
active network nodes. In the case of multiple Masters, only one Master has read/write privileges to
its Slaves (passive nodes) at any one time, and the control token is passed continuously in ascending
order to all other active network nodes.
2.2.3.1 Token Bus and Token Passing in a Multi-Master Network
During the bus initialization and startup, the bus access control creates the token ring by recognizing
the active network nodes in ascending order. The bus access control automatically determines the
addresses of all active nodes on the bus, and records them together with its own node address,
creating a List of Active Stations. The Lowest Station Address (LSA) begins with the active token,
allowing it to fetch and send data messages to its passive slaves (referred to as polling). At comple-
tion of its request frame (polling telegram), and acknowledgement or response frame returned from
the slave, the token is passed to the Next Station (NS) with a token telegram. The active node from
which the node was passed is called the Previous Station (PS). This continues until the token is being
passed from the Highest Station Address (HSA). At completion of the HSA polling telegram, the token
is passed to the LSA. The List of Active Stations is required during network operation to remove a
faulty active node, or to add a node, without disturbing data on the bus.