Rockwell Automation 1757-FFLD Linking Device Configuration Manual - Manual (Page 44)

Publication 1757-PM002A-EN-E - June 2006
3-6 Basic Function Blocks
Relationship of Output Function Blocks to the Transducer
Initial Configuration: Virtual
Communication
Relationships Virtual Communication Relationships (VCRs) are communication links on an
H1 network. A total of 128 VCR’s are possible within the linking device, and
these VCRs are restricted to 32 per H1 channel. This is further restricted to 16
VCRs going to the Control Logix processor from H1 devices and publishers,
and 16 VCRs coming from the Control Logix processor to H1 devices and
subscribers.
These restrictions limit the number of inputs and outputs within an H1 line.
However, there is no restriction on the type of signals. They can all be analog,
all discrete, or a mix of the two. The only restriction is the 16 publisher and 16
subscriber limits.
If the VCR limit is exceeded, a download will still be permitted, although when
the download reaches the 17
th
VCR, a “download failure” will occur. The first
16 VCR’s will still be in place, but the 17
th
will not exist.
VCR Classes
The Foundation Fieldbus specification Fieldbus Access Sublayer (FF-875-1.4)
defines three classes of Virtual Communication Relationships that are possible
for communications with an H1 field device. These classes are:
• BNU: Buffered Network-Scheduled Unidirectional
• QUU: Queued User-triggered Unidirectional
Output
Transducer
Block
Manufacturer
Specific
I/O
Subsystem
Channel put
rameters
Output Snap
of Transducer
Block
Inputs
Block
Algorithm