Calculating The Maximum Message Response Time - Omron CJ1W-DRM21 Operation Manual

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Calculating the Maximum Message Response Time

The message response time is the time required from the time a DeviceNet Unit starts to receive an I/O
memory read request message (BYTE DATA READ explicit message) until the DeviceNet Unit com-
pletes sending the response message to the client.
The message response times described below are based on the following conditions:
• Only one DeviceNet Unit is mounted.
• The I/O memory read request is returned from CPU Unit in 2 CPU Unit task periods.
• Baud rate: 500 Kbit/s
• Communications cycle time: 3 ms (no remote I/O communications)
• CPU Unit task period: 10 ms
• DeviceNet Unit's refresh time on the CPU Unit: 2 ms
Request from One Client (BYTE DATA READ)
CPU Unit
Shared memory
DeviceNet internal buffer
Communications cycle
Client
Shown below is the response time required to return data for 100 words.
• BYTE DATA READ request sending time: T
DeviceNet header (4 bytes) + Parameters (3 bytes) = 7 bytes (no fragment)
Therefore, T
• CPU Unit's processing time: T
T x 2 = CPU Unit task period x 2 = 10 x 2 = 20 ms
CPU
• BYTE DATA READ response receiving time: T
DeviceNet header (3 bytes) + Number of words to be read x 2 = 203 bytes
Therefore, T
time
• DeviceNet Unit refresh time: T
The message response time will be as follows:
= T + T
RM CPU
= 132 ms
Shown below is the response time required to return data for 100 words to 8 clients.
• BYTE DATA READ request sending time: T
DeviceNet header (4 bytes) + Parameters (3 bytes) = 7 bytes (no fragment)
Therefore, T
CJ-series DeviceNet Units Operation Manual for NJ-series CPU Unit(W497)
Task
execution
BYTE DATA READ processing time:
CPU Unit task period x 2
Read request
T
CPU
= 1 communications cycle time (T
BRS
CPU
= 1 + 203 bytes/6 communications cycle time = 35 communications cycle
BRR
= 2 ms
RF
x 2 + T x 35 + T x 2 =3+20+105+4
RM RF
= 1 communications cycle time (T
BRS
8 Communications Timing
Task
execution
T
CPU
BRS
) = 3 ms
RM
x 2
BRR
BRS
) = 3 ms
RM
Read result
8
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