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Message Communications Section 8-2
Note 1. If the CPU Unit attempts to send another message or receives a message
from another node within the message communications time, the second
message or the message being received from another node may be de-
stroyed. Never execute a second communications instruction before the
message communications time has elapsed and never send messages to
any one node at intervals less than the message communications time.
2. If send or receive messages are destroyed, error records 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 Configura-
tor.
3. The above equations can be used to find the approximate message com-
munications time, but this is a typical time, not the maximum time. The
message communications time will vary depending on the frequency of
message communications, the load on the remote node, the communica-
tions cycle time, and other factors. For any one Master Unit, the message
communications time can be greatly increased due to heavy loads and the
user program must be written to allow for this.
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 completes sending the
response message to the client.
The message response times described below are based on the following
conditions:
1,2,3... 1. Only one DeviceNet Unit is mounted.
2. The I/O memory read request is returned from CS/CJ-series PC in two cy-
cle time.
3. Baud rate: 500 Kbps
4. Communications cycle time: 3 ms (no remote I/O communications)
5. PC’s cycle time: 10 ms
6. DeviceNet Unit’s refresh time: 2 ms (See note.)
Note The PC’s cycle time will be delayed as shown below for one De-
viceNet Unit. For details, refer to Refresh Time on page 192.
0.7 ms + (number of occupied words × 0.001 ms)
Request from One Client (BYTE DATA READ)
PC
Instruction
execution
Instruction
execution
BYTE DATA READ processing time:
PC's cycle time x 2
Shared memory
DeviceNet internal buffer
Communications cycle
Client
Read
request
Read result