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Message Communications Section 8-2
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 frag-
ment)
Therefore, T
BRS
= 1 communications cycle time (T
RM
) = 3 ms
• PC’s processing time: T
PC
x 2
T
PC
x 2 = PC’s cycle time 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 com-
munications cycle time
• DeviceNet Unit refresh time: T
RF
= 2 ms
The message response time will be as follows:
= T
RM
+ T
PC
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 frag-
ment)
Therefore, T
BRS
= 1 communications cycle time (T
RM
) = 3 ms
• PC’s processing time: T
PC
x 6 + T
PC
x 2 = T
PC
x 8
The following time is required to process data with other clients before the
PC starts processing.
7 other clients/2 = Approximately cycle time x 3
If one processing requires twice the cycle time, the time will be as follows:
Cycle time x 3 x 2 = Cycle time x 6
PC’s processing time: T
PC
x 2 = Cycle time x 2
Total time will be as follows:
Cycle time x 8
• 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 com-
munications cycle time
• DeviceNet Unit refresh time: T
RF
= 2 ms
The message response time will be as follows:
= T
RM
+ T
PC
x 8 + T
RM
x 35 + T
RF
x 2 x 8 =3+80+105+32
= 220 ms
Note The above equations can be used to find the approximate message response
time, but this is the typical time, not the maximum time. The message
response time will vary depending on the frequency of message communica-
tions, the load on the remote node, the communications cycle time, and other
factors. For any one DeviceNet Unit, the message communications time can
be greatly increased due to heavy loads and the user program must be written
to allow for this.