Scan Synchronization Function - Mitsubishi Electric MELSEC QS Series User Manual

4
FUNCTIONS

4.2.13 Scan synchronization function

Whether to make the link scan synchronized with the sequence scan or not is selected.
[Synchronous mode]
IMPORTANT
The sequence scan time must not exceed the time given in the listed below. If the
scan time exceeds the limit, a time out error occurs at each station.
If the sequence scan time exceeds the above-listed value, reduce the transmission
rate or change the mode to Asynchronous mode.
Especially, when using 10Mbps or 5Mbps, the sequence scan time may exceeds
50ms due to instantaneous power failure of the master station. Therefore, take
preventive measures using external devices such as a UPS (Uninterruptible Power
Supply).
Data flow in the synchronous mode is shown below.
(a) Sequence scan
Safety CPU module
(Sequence scan)
Safety master module buffer memory
(Remote input RX/
remote output RY)
Link scan
Response time of first station
Response time of target station
Response time of last station
Safety remote I/O station response
a: Delay time due to a response delay of a safety remote I/O station
b: Delay time due to data transmission from a safety remote I/O station to the safety
master module
c: Delay time elapsed from the time the safety master module receives the data until it
stores them to the buffer memory
d: Delay time elapsed until the data on the safety master module are refreshed to the
safety CPU module
e: Execution time of sequence processing
Link scan is synchronized with sequence scan. Therefore longer sequence scan
causes longer link scan interval.
Transmission rate
156kbps
625kbps
2.5Mbps
5Mbps
10Mbps
Link scan
Input
a b
Transmission delay time (reception)
4.2 Detailed Explanation of Functions
4.2.13 Scan synchronization function
Sequence scan time
800ms
400ms
100ms
50ms
50ms
f
c d e
Sequence processing
time
Output
g
h
Transmission delay
time (transmission)
4
1
2
3
4
5
6
7
8
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