Yokogawa DO202 2-Wire Dissolved Oxygen Analyzer - Manual (Page 15)

IM 12A00A01-61E
3-10 Foundation Fieldbus
3-3-5 Communication Setting
To set the communication function, it is necessary to change the database residing in SM-VFD.
3-3-5-1 VCR Setting
Set VCR (Virtual Communication Relationship), which specifies the called party for communication and
resources. EXA has 33 VCRs whose application can be changed, except for the first VCR, which is used for
management.
EXA has VCRs of 3 types:
Publisher(-Subscriber) VCR
Publisher-Subscriber VCR’s are designed to link Function Blocks. When a publishing Function Block runs,
its output data is stored in the buffer of the Publisher VCR. Then the LAS (LM) sends a CD to this VCR to
force it to transfer the data. Subscriber VCRs receive this data and gives this to the subscribing Function
Blocks. Typical example is a linkage from an output of an Analog Input (AI) block to the process value input
of the PID control block.
Publisher-Subscriber model is one-to-many one-way Communication. Subscribers are able to know whether
data is updated since the last publish. This mechanism is important because Data Link Layer transfers data
as scheduled regardless the publishing Function Block updates the data in the buffer.
(Client-)Server Model
Client-Server model is universal and used in many communication technologies. An application called
“Client” requests another application called “Server” to do a specific action. When the Server finishes the
requested action, its result is transferred back to the Client. It is an one-to-one two-way communication.
Typical example is a human-machine interface (Client) to read data of a Function Block (Server). The Client
sends a Read request to the Server and then the Server sends back the data to the Client. This communi-
cation is unscheduled and is handled during the unscheduled interval in the macrocycle. A Client may want
to issue many requests at a time. A Client-Server VCR has a queue to store those requests and sends the
requests one by one when the node has the token.
Source(-Sink) Model
A Source-Sink VCR is designed to broadcast messages. It is one-to-many one-way communication without
any schedule. This model is sometimes called “Report Distribution Model.” A Source VCR transfers a mes-
sage in the queue to an assigned global address when the device has the token. Sink VCRs are set to the
same global address and receive the same message from a Source. Foundation devices use this model for
two specific purposes. One is to report alarms or events detected in the Source and the other is to transmit
trends of Source Function Blocks. Alarms are acknowledged through a Client-Server VCR. It is desirable for
an alarm logger to receive alarms from all devices with just one VCR. A Sink can receive messages from
many Sources if the Sources are configured to send messages to the same global address.
A Source VCR transmits data without established connection. A Sink (QUU) VCR on another device can
receive it if the Sink is configured so. A Publisher VCR transmits data when LAS requests so. An explicit
connection is established from VCR(s) so that a Subscriber knows the format of published data.
Each VCR has the parameters listed in Table 3.5. Parameters must be changed together for each VCR
because modification for each parameter may cause inconsistent operation.