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Chapter 7 Communication function
7-99
9
In program of [Figure 7.17], RD_START is a contact point to start READ F/B and is set after WRITE
F/B is executed. An initial value of it is set to operate READ F/B at the first scan of program.
WR_START is a contact point to start WRITE F/B and is set after READ F/B is executed to run WRITE
FB. All two variables are allocated to auto-variables. After one dedicated Function Block is executed
through the contact point and 1 sec. timer above, operation in order by the next Function Block is
available in a cycle of I sec. 1 sec. timer is used to prevent unnecessary communication overload by
setting the min. execution cycle of Function Block to 1 second. This timer is adjustable based on
application conditions of user’s.
9
If program of [Figure 7.17] is downloaded to PLC set to master station, programming for 3 stations
communication in [Figure 7.16] is completed. No need to prepare communication program or to set
frame in slave PLC. Communication will be started if PLC program is switched to operation Run by
setting Cnet I/F module to dedicated mode as connected with RS-422 cable. If communication is
normal, NDR output of ‘GM_RD’ & ‘GM_WR’ Function Blocks is turned ON in accordance with the
execution cycle of Function Block, and if communication is not normal, ERR output is ON with error
code displayed on STATUS. [Table 7.25] describes error status according to output STATUS values in
case error occurs. Refer to those actions to take to check for the cause of errors.
[Table 7.25] Error codes and Actions to take when dedicated F/B is executed
STATUS
value(dec.)
Meaning Action to take
10 No response from correspondent station.
1) Check setting of correspondent station No.
2) Check dedicated mode for operation of
correspondent station.
3) Check communication basic parameters
17
Position of Cnet I/F module is incorrectly
specified.
1) Check if SLOT_NO value is correct.
21 No response from Cnet I/F module
1) Check operation mode of local Cnet I/F module
2) Check channel No.
35
NAK from correspondent station has been
received.
1) Check variable name (%MB,%QB,%IB – BYTE
only available)
37 Input parameter setting is incorrect 2) Check data length