CHAPTER 6 COMMUNICATION FUNCTION
6-26
Figure 6.31 shows PLC program scan time and the sending point according to communication
scan time.
PLC-A station PLC – B station
PLC – C station
T1
PLC-A T
scanA
T
scanA
T
scanA
Scan time PLC scan delay time(T
delay_plc1
)
Communication scan sending delay time(T
delay_com
)
T2
Communication T
com_scan
T
com_scan
T
com_scan
Scan time
PLC scan delay time(T
delay_plc2
)
T3
PLC-B T
scanB
T
scanB
T
scanB
Scan time
Figure 6.31 Relation of PLC scan time and communication scan time
In Figure 6.31, PLC-A station delivers the sending data by communication module in T1 that
is the point that PLC-A station program is finished, therefore, the time delay as much as
Tdelay_plc1 may occur. After receiving the data from PLC, communication module can send
the data after waiting the communication scan delay time(Tdelay_com) and max.delay time is
as much as Tcom_Scan1.
Even for PLC-B, as communication module can send the received data to PLC after waiting
for Tdelay_plc2time, max. delay time as much as Tscan2 may occur. As shown Figure 6.30
and Figure 6.31, communication delay time is determined by a variety of variables such as
total number of communication station, program size and O/S scan time of communication
module. As it is not easy to calculate the value of such variables, here the simple and easy
calculation method is provided for the user.
2)
High speed link speed calculation method
High speed link speed is defined as max. time required to send one block data from PLC-A to
PLC-B as an example of Figure 6.31 and the high speed link speed calculation is divided into
2 ways for: complicated system that total number of sending data to more than 10 communication
stations exceeds total 512 bytes and the simple system less than 512 bytes, and the
calculation method is as follows :