Omron CJ1W-DRM21 - Calculating the Maximum Message Response Time

8-11
8 Communications Timing
CJ-series DeviceNet Units Operation Manual for NJ-series CPU Unit(W497)
8-2 Message Communications
8
8-2-2 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
z Request from One Client (BYTE DATA READ)
Shown below is the response time required to return data for 100 words.
• BYTE DATA READ request sending time: T
BRS
DeviceNet header (4 bytes) + Parameters (3 bytes) = 7 bytes (no fragment)
Therefore, T
BRS
= 1 communications cycle time (T
RM
) = 3 ms
• CPU Unit's processing time: T
CPU
x 2
T
CPU
x 2 = CPU Unit task period x 2 = 10 x 2 = 20 ms
• BYTE DATA READ response receiving time: T
BRR
DeviceNet header (3 bytes) + Number of words to be read x 2 = 203 bytes
Therefore, T
BRR
= 1 + 203 bytes/6 communications cycle time = 35 communications cycle
time
• DeviceNet Unit refresh time: T
RF
= 2 ms
The message response time will be as follows:
= T
RM
+ T
CPU
x 2 + T
RM
x 35 + T
RF
x 2 =3+20+105+4
= 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
BRS
DeviceNet header (4 bytes) + Parameters (3 bytes) = 7 bytes (no fragment)
Therefore, T
BRS
= 1 communications cycle time (T
RM
) = 3 ms
8-2-2 Calculating the Maximum Message Response Time
CPU Unit
Task
execution
Task
execution
BYTE DATA READ processing time:
CPU Unit task period x 2
Shared memory
DeviceNet internal buffer
Communications cycle
Client
Read request
Read result
T CPU T CPU