Chapter 7 Communication function
7-100
4)
Why is the interlock program used
Slave station in dedicated mode responds to only one request at a time. Thus, even if later different request frame is
received before an earlier request frame is processed by the slave station, the slave station dose not answer to the
later request frame until the earlier request frame is processed. In case that several request frames are transmitted at
the same time prior to the response of the slave, later request frames are ignored by the slave station causing data
loss. [Figure 7.18] describes the principles of this operation.
[Figure 7.18] Example of data loss
Master station
Slave stations
RQT
1
RPS
1
RQT
N
RQT
2
RPS
N
RPS
2
…
RQT
3
RPS
3
Slave stations
Master station
RQT
1
RPS
1
RQT
2
RQT
3
RQT
4
RPS
2
RQT
5
RQT
6
RPS
5
…
DATA LOSS
DATA LOSS
Ex. of normal communication
Example of data loss
Master station requests data in order
to respond normally without data loss
in slave stations.
If master station requests data
continuously without waiting for the
response of slave station, thus, parts
of the request frames are lost in slave
stations, normal response to No.3, 4
or 6 frame is impossible with data
abandoned.
DATA LOSS