Rockwell Automation Allen-Bradley ControlLogix 1756-MVI Programming Reference Manual page 25

Assembly Instance/ Max. Size (words)
Connection Point
1
2
5
6
7
8
(1)
The first 4 bytes are overwritten with "FF" when the connection is not open or broken (This applies only to Assembly Instance 1).
(2)
The first 4 bytes (2 words) of 250 are status words, which the MVI API strips off. (Note that the CIP API does not.)
(3)
The maximum number of words of data that can be transferred using MVI messaging will depend upon the path to the MVI module. The value shown
assumes that the controller and module are located in the same physical rack.
Figure 3.1 shows the relationship between the API components.
Control Processor
(Logix 5550)
The MVI Backplane API implements a predefined configuration of the
assembly object. The configuration is shown in table 3.B.
Table 3.B - MVI API Assembly Object Implementation
Connection Type Description
(1) Class 1
250
(2) Class 1
248
3 Class 1
0 Class 1
(3) Unscheduled
239
Unscheduled
(3)
239
Figure 3.1 API Component Relationship
ControlBus (Backplane)
Midrange ASIC
Backplane Device
(MVI56BP.EXE)
CPI API Library
MVI Driver
(MVI56DD.EXE)
MVI API Library
Thread Thread
Multi-threaded MVI Application
1756-MVI Module
Input data accessed via MVIbp_WriteInputImage
Output data accessed via MVIbp_ReadOutputImage
Status input (not accessible from MVI application)
Status output (not used)
Message input data accessed via VIbp_SendMessage
Message output data accessed via
Publication 1756-RM004B-EN-P - October 2000
MVI Backplane API
Driver
Thread
MVIbp_ReceiveMessage
3-3