Data Consistency Function - Mitsubishi Electric FX5-DP-M User Manual

4.5

Data Consistency Function

This function maintains consistency of I/O data when the data from each DP-Slave is read/written from/to the buffer memory.
Data inconsistency is an issue where new and old two-word size (32 bits) link data is divided in units of words (16 bits) at the
timing of refresh of I/O data.
I/O data consistency and occurrence
■Without the data consistency function
The PROFIBUS-DP bus cycle is asynchronous with the sequence scan by the CPU module.
Therefore, if the CPU module reads input data from the buffer memory while a DP-Slave is transferring the data to it, data
inconsistency occurs. (This problem also applies to output data)
The following example shows data inconsistency that data is read from CPU module while the I/O data is transferring to the
buffer memory.
(3)
1
0
3 2
B A
D C
(1) DP-Slave
(2) Buffer memory of the FX5-DP-M
(3) CPU module
(4) Area already updated with data transferred from a DP-Slave
(5) Area not yet updated with data transferred from a DP-Slave
 Data being transferred
 Data read while transferred
■With the data consistency function
When the data consistency function is used, reading data from CPU module is not performed while the data is transferring
from the DP-Slave to the buffer memory of FX5-DP-M (Input data area). When the data has transferred, reading data starts.
In addition, transferring data is not performed to the DP-Slave until the data has written from the CPU module to the buffer
memory of FX5-DP-M (Output data area). When the write data has completed, transferring data starts.
(3)
1
0
3 2
5 4
7 6
(1) DP-Slave
(2) Buffer memory of the FX5-DP-M
(3) CPU module
(4) Area already updated with data transferred from a DP-Slave
 Data transfer completed
 Data read after transferred
(2)
1
0
(4)
3 2
Ó
B A
(5)
D C
·
(2)
1
0
3 2
Ó
(4)
5 4
7 6
·
(1)
1
0
3 2
Ò
5 4
7 6
·
(1)
1
0
3 2
Ò
5 4
7 6
·
4.5 Data Consistency Function
4
4 FUNCTIONS
35