Download 3 Before You Begin

Before You Begin
PLC Serial Communications
Communications Scan Times
As a rule of thumb, the scan frequency of the algorithm block
should be set at a rate that is half the communications scan frequency. This ensures that the block obtains data from every
communications scan. Experiment with various scan frequencies to obtain optimum communication efficiency.
Figure 3.7 shows the relationship between the communication
port scan time and the algorithm block scan time.
Data from all the comm port scans
is read by block #1
Block #1 scan
frequency (1 sec.)
COM port scan
frequency (2 sec.)
0 sec
Block #2 scan
frequency (3 sec.)
2 sec
4 sec
6 sec
8 sec
?
Data from the comm port
scan at 4 seconds is
never read by block #2
Figure 3.7 Relationship between algorithm block scan time and the
communication port scan time
PC-30 User’s Manual
3-25
Before Starting
Drivers perform communication transfer differently. Most PLC
communications attempt to transfer data one group at a time. It
takes a fixed amount of time (per driver) to scan one group of a
device block. However, this does not mean that all data is transferred in one request or that data can be transferred in a fixed
amount of time.