Message Communications Characteristics - Omron DRT2 SERIES DEVICENET SLAVES - OPERATION MANUAL 04-2008 Operation Manual

Message Communications Characteristics

Program Example
8-2 Message Communications Characteristics
8-2-1 Message Communications Time
As shown in the preceding table, it takes time for DeviceNet communications
to start up. This programming uses flags in the Master Status Area to prevent
the Slaves' I/O processing from being performed until the Master Unit and
remote I/O communications have started up.
Note Refer to the operation manual of the Master Unit being used for details on the
Master Unit Status Area.
The following program example is for a CS1-series PLC and a Master Unit
with a unit number of 00.
2001 2001
15 14
I/O Data
Communications
Flag
Error/
Communications
Stopped Flag
The message communications time is the time required from the time a Mas-
ter Unit starts to send a message over the Network to another node until the
Master Unit completes sending the message (SEND(090)/RECV(098)
instructions to send/receive data and CMND(490)/IOWR(223) instructions to
execute FINS commands).
Note If the CPU Unit attempts to send another message or receives a message
from another node before the message communications time has finished, the
response message being sent or the message being received from another
node may be destroyed. Always perform message communications at inter-
vals longer than the message communications time and use message instruc-
tions (SEND(090), RECV(098), CMND(490), and IOWR(223)). Never send
messages to any one node at intervals less than the message communica-
tions time.
If send or receive messages are destroyed, the error record will be placed in
the error history of the Master Unit. If an error occurs, read the error history
using the FINS command or monitor the error history from the Configurator.
The following equation can be used to calculate the approximate message
communications time.
Message communications time = Communications cycle time ((No. of mes-
sage bytes + 15) ÷ 6 + 1)
The number of message bytes is the number of data bytes following the FINS
command code. The communications cycle time depends on whether remote
I/O communications are being used.
(004)
JMP #0001
Slave's I/O
processing
(005)
JME #0001
Section 8-2
453